Dual Inverters for low and high wattage

[Inq720]

I'm doing a small off-grid system for a small sailboat. I want to utilize every amp-hour I can generate and store efficiently. Most things like lighting, marine electronics and even USB charging will be on 12 VDC. For large, temporary loads like an inverter microwave or blender I will be using the GoWISE 1500 watt inverter which maxes out the Overkill BMS and I'll have the remote switch in the boat's galley.

In this thread I want to explore my options for low wattage items that may run for hours. I'm considering things like laptops and LED TV's. I'm thinking a 100-200 watt inverter will cover all these items... even simultaneously. This inverter would also have to be a pure sine wave version. I would keep any outlets segregated from each other.

This thread is similar, but not quite what I'm asking.

1. First, is there enough efficiency gain by having a second inverter over only using say... 65 watts out of the 1500 watt inverter?
2. What parameters (and where can I find them) do I need to quantify the savings on the 1500 watt vs 100-200 watt inverters? Even the manual didn't have any numbers that seemed to shine light on this.
3. Do you have any suggestions for good quality 100-200 watt inverters?

 

[Hedges]

Look up the no-load power draw ratings of the inverters.

Also look for efficiency curves and you can figure out power draw for 100W, 1000W, whatever load.

My 6kW Sunny Island says 25W idle. 6W in sleep mode, which wakes up periodically to see if anything got turned on (which doesn't help for your need powering small things, but would be an alternate to remote switch)
Some brands are much worse, like 150W.

Your planned 1500W inverter is probably enough only for smaller microwave, maybe 700W. It is possible a 900W or larger microwave draws closer to 1800W, so check it out. A few microwaves are inverter based and actually reduce consumption at lower power settings rather than just toggling on and off.

Some laptops, TV, and others use a wall wart to produce 12V. Laptops often communicate with the 12V (or higher voltage for higher wattage laptops) and don't draw full current unless they recognize it. If all your stuff is 12V, could wire a custom cord or get cigarette lighter adapters. Then no inverter used.

 

[Inq720]

Thanks, but I have not been able to find "no-load" even in the inverter's manual or on-line.

As far as equipment, I was just using examples to give people a reference point,

• Microwave - I'm aware of the microwaves having large run and in-rush current. I have access to one of the Panasonic 950w inverter microwaves and will be doing a test on it to see if it'll trip the inverter. I'll report back to the forum what I find.
• TVs - Most of the 12 VDC TVs and wall wart versions (frankly) suck and are twice the price and only have 720p. There may be exceptions, but I can get the TCL 32" 1080P with Roku for $170. I can hook it up to a Raspberry Pi setup as a PLEX media server and have hundred of hours of movies/shows available, plus use the TV as a monitor for the Raspberry Pi to host marine chart software for radar, sonar, and navigation... all for about 60 watts.
• Laptop - I have an Intel i7 based one now and I'll definitely have one of the more powerful ones using the latest AMD chips. It'll be a hoss... performance and power wise... say about 80 to 100 watts. Even though I might find a 12 VDC adapter for it, I'd still be wanting the general case if guests want to use such laptops or any other equipment in the sub 200 watt range.

Point being - I definitely have use cases that need near 1500 watts and other cases that need less than 100 watts. I am researching here options that might make this more efficient. Since inverter company's seem to shy away from publishing such data AND watching Will's videos and how quantitative they are, I was hoping someone has done a test on their inverters, say:

1. Use 1500/2000/etc watt Inverter (preferably a GoWISE) and get the DC wattage draw for running a 1500 watt AC device
2. Same Inverter, but for running a 100 watt AC device.
3. Use a small sub 200 watt Inverter and driving a 100 watt AC device.

I will certainly be doing these tests and report back. However, I don't plan on purchasing high-end gear that people on the forum might already have for testing. I'll simply be using the BMS's usage output for the DC side and a cheap Wemo Insight plug to measure AC watts used. I'm hoping someone already has done these test with good test gear and would be willing to share their results.

 

[Mike Jordan]

I have very similar thoughts for a travel trailer. I don't know what is the right route to go, so following. My thought is, that the main inverter, or larger size inverter, is only used infrequently and for shorter periods. So I could turn it on/off as necessary. For smaller TV, laptop, etc uses that will be more frequent and for longer periods, I was thinking about very small single use inverters at the appliance use area. Then I could just use the standard 12v power ports, or maybe use the smaller Anderson connectors, and completely unplug the inverter, or maybe switch the 12v power off at that point, when not in use

I like your budget plan. So post usage results when you have them

 

[Inq720]

While trying to find my own answers... I ran into this Amazon review for the GoWISE 1500 from a retired EE.

" I measured the conversion efficiency at 1500W, 104W, and 52W and the calculated conversion efficiencies were 90%, 78%, and 70% respectively."

Considering the 300W inverter I'm researching claims to 94% efficiency in the sub 100 w range... sounds like it would provide a significant energy savings worth the extra trouble adding it to my boat instead of running the 1500W unit at 70% efficiency... especially over many hours of use!

 

[Hedges]

Consumed 22W making 100W, 15W making 50W.
If you can afford 15W x 24 hours = 375 Wh/day, just run the 1500W inverter continuously.
That much power costs me $0.07 to buy from utility, $0.02 to make with PV panels ($7/year assuming 10 years operation.)
Takes 70W of PV panel, a fraction of a 250W, 350W, 500W panel you might install.

 

[Mike Jordan]

Consumed 22W making 100W, 15W making 50W.
If you can afford 15W x 24 hours = 375 Wh/day, just run the 1500W inverter continuously.
That much power costs me $0.07 to buy from utility, $0.02 to make with PV panels ($7/year assuming 10 years operation.)
Takes 70W of PV panel, a fraction of a 250W, 350W, 500W panel you might install.

Wait.... isn't 15w from a 1,500w inverter 99% effective? 150w would be 90% effective. 150w x 24 hours is 3,600w. Thats a whollotta watts to over come in the small space of a mobile system

 

[Hedges]

My calculation of 15W dissipated was from delivering 52W output with 70% efficiency.
Not 15W dissipated while delivering 1500W

52W/0.70 - 52 = 22.3W wasted (I got 15W doing it in my head)

Efficiency as reported, and corrected dissipation

Wattage ... Efficiency ... dissipation
1500W ... 90% ... 167W
104W ... 78% ... 29.3W
52W ... 70% ... 22.3W

22.3W is 535 Wh/day

 

[Inq720]

Consumed 22W making 100W, 15W making 50W.
If you can afford 15W x 24 hours = 375 Wh/day, just run the 1500W inverter continuously.
That much power costs me $0.07 to buy from utility, $0.02 to make with PV panels ($7/year assuming 10 years operation.)
Takes 70W of PV panel, a fraction of a 250W, 350W, 500W panel you might install.

If you would spend half as much of your time trying to answer constructively the questions posed versus telling me I'm wrong, your posts might actually be useful. You did not even try to answer any of the three questions in the OP. You should not assume that the OP is ignorant. You should also not assume that the OP gave all the details. I used examples to illustrate the usage patterns. If I were to give full disclosure, the OP would be pages long and no one would read it.

The very first line of my OP fully explains why your Math is invalid. There is no utility to plug into with a boat that may be weeks or months between docking at marinas that might have shore power. First, basing a argument on a commodity that is not available. Second, assuming when I said efficiency, that I was even considering money to be the guideline for efficiency.

Here is the Math that is of value to this thread. If the review in post #5 is accurate and marketing data for these 300W inverters is to be believed, we have for watt-hours consumed with an 80 watt load. Interpolating the data, we get:

1. 1500w Inverter @ 74% efficiency: 80 / 0.74 = 108 watts
2. 300w Inverter @ 95% efficiency: 80 / 0.95 = 84 watts

For my theoretical battery bank of 3400 watt-hours, that usage will last:

1. 1500w Inverter: 3400 / 108 = 31.5 hours
2. 300w Inverter: 3400 / 84 = 40.5 hours

That is 9 more hours of getting weather reports of an impending hurricane or if money is your only yardstick, I may gain or keep from losing tens-of thousands of dollars by making a wise investment trade in those 9 hours.

It was my desire that someone on this forum might have better insight to the comparison between a low and high wattage inverter since available numbers from the Internet are either absent or marketing data.

 

[Dzl]

If you would spend half as much of your time trying to answer constructively the questions posed versus telling me I'm wrong, your posts might actually be useful. You did not even try to answer any of the three questions in the OP. You should not assume that the OP is ignorant. You should also not assume that the OP gave all the details. I used examples to illustrate the usage patterns. If I were to give full disclosure, the OP would be pages long and no one would read it.

There is a more constructive and respectful way to express this. Hedges may not have answered your specific questions, but their contributions appear to me to be constructive and in good faith. I did not interpret their responses as assuming you were ignorant or wrong, but rather, articulating an alternative line of thinking. Which is not a bad thing. I learn a lot from people challenging my mindset, sometimes it misses the mark, sometimes it lets you see things in a different light. In either case, try to be more constructive/respectful.

One thing to be aware of and get used to here, many forum members, especially many of the more experienced members, are coming from a residential / fixed structure background, or a hobby system background. And sometimes don't have a full awareness of the different mindset and design-goals of mobile (or especially marine) system design. So there is a bit of unconscious bias towards that mindset here (I would encourage people to try to be conscious of context). We are all shaped/conditioned by our experience.

What makes sense for a stationary system does not always translate to a marine or mobile system. Especially for marine or mobile systems operating in very remote or harsh conditions. But take the opportunity to explain how your goals/priorities differ, or just be patient and rephrase/reiterate the question rather than getting upset. I know Hedges to be a helpful and knowledgeable person, I am sure their purpose in commenting was not just to show you you were wrong. And be understanding that your intent and context is not always clearly understood from a short post.

In this thread I want to explore my options for low wattage items that may run for hours. I'm considering things like laptops and LED TV's. I'm thinking a 100-200 watt inverter will cover all these items... even simultaneously. This inverter would also have to be a pure sine wave version. I would keep any outlets segregated from each other.

A further consideration. Running as many devices off DC as possible. Laptops, TV's, phones, etc are all DC devices that use AC-DC chargers/power supplies. If you can find reasonably efficient DC-DC chargers for these devices you could avoid the small inverter altogether. It may or may not be worth the trouble for you. But its an option.

1. First, is there enough efficiency gain by having a second inverter over only using say... 65 watts out of the 1500 watt inverter?

In many cases I suspect, yes. Assuming the small inverter is reasonably efficient and the large inverter has a non-negligible idle consumption.

1. What parameters (and where can I find them) do I need to quantify the savings on the 1500 watt vs 100-200 watt inverters? Even the manual didn't have any numbers that seemed to shine light on this.

Good question. I don't have enough experience to answer this with confidence. What I look for: (1) no load consumption (2) conversion efficiency (3) efficiency curve if available. The first two are usually found in the datasheet. The latter is harder to find even with many reputable inverters.

1. Do you have any suggestions for good quality 100-200 watt inverters?

Victron Phoenix -- The smallest is ~250W for under 100 dollars. Efficiency in the low 90's, no-load is pretty low, no-load in power save mode is near zero, and its a reputable brand.

 

[Inq720]

Dzl, I have found you to be very thoughtful, respectful and fair even when someone (in this case, me) deserves a "talking to". Even though I felt the same affront in his first post, I tried the constructive route in #3 to be more detailed. Mike confirmed that there are others with a similar problem/need as I have. I fully understand that there is a huge diversity here... from theoreticians to electrical technicians with extensive practical training to those without any formal training, yet years of DIY experience. All can contribute constructively one way or another. I suspect that most all are borderline genius or better. I'd bet Hedge is also. Everyone here, can and should be able to think outside of their comfort zone, but they first must take the time to consider the questioner's need. If I had responded to this thread, I might also have pointed out alternative devices, but I still would have given an honest attempt at a useful answer to the direct question. The fact that he responded with an on-grid solution in the Off-Grid Inverter forum AND my post explicitly stated off-grid AND boat really smacked me that reading the question is important.

Back to constructive...

For me and other small boaters and I suspect for Mike and ALL small*, off-grid users (US), we do go out of our way to use DC directly. In my case, I'll have more than twenty devices that will be used most of the time while on the boat. All are 12 VDC based. Most all are trivial low wattage digital electronics. The biggest users in this class are radios, radars and autopilots. The biggest around the clock user will undoubtedly be a Refrigerator which I extensively studied and will also be DC based. Even for my laptop, I'll look for a DC converter.

For (US) using an Inverter at all will be for un-planned cases or when DC versions of a product are not available or are prohibitively expensive. I do not consider idle current to be an issue as I will never have an Inverter on that is not being used. The convenience factor of it instantly working once plugged-in is simply not worth the idle current. I'll will be wiring the remote Inverter switch. At the high-end, I'd like to use a Microwave. At the low-end, a guest might need to plug in their laptop. I need both extremes, I'd like to handle it efficiently... as pointed out above it might be a life saving decision.

As no one had helped to point US in a direction, I continued my stumbling research alone which added to the frustration of marketing data. I finally decided to test a Giandel 300 watt unit because they claimed up to 95% efficiency. I probably would have considered the Victron, but that model does not show up. Even now, doing a search on Amazon for "victron 250 watt inverter" does not get a hit. ONLY if you select the 1200 watt version are you then able to see the 250 watt version. Bad search algorithm or bad product definition!

Continuing... the Giandel only showed 85% efficiency in my crude testing at my desired wattage case. I still have hopes this thread's premise will be valid and useful... a user post in that Giandel thread indicated a high wattage inverter was only 65% efficient at that low output. Once I get my battery (still on slow boat from China) and large inverter, I will run the same efficiency test on it and we'll have the answer to this thread. And those of (US) that are concerned about off-grid efficiency, will be able to make an informed decision whether it is worth having Dual Inverters for low and high wattage or not.


* Small is defined here as those with less than 5000 watt-hours of battery storage and less than 300 watts worth of solar panels.

 

[Hedges]

The fact that he responded with an on-grid solution ...

If you can afford 15W x 24 hours = 375 Wh/day, just run the 1500W inverter continuously.
That much power costs me $0.07 to buy from utility, $0.02 to make with PV panels ($7/year assuming 10 years operation.)
Takes 70W of PV panel, a fraction of a 250W, 350W, 500W panel you might install.

I did consider "First, is there enough efficiency gain by having a second inverter over only using say... 65 watts out of the 1500 watt inverter?"
I tried to quantify that efficiency gain and suggest how the difference in efficiency could be addressed in a different way.
I thought there was not enough efficiency gain; instead, add about 70W more PV panel at very small cost.

I didn't mean to offer an on-grid solution, just point out how little power is wasted if you simply left the 1500W inverter (for which consumption was quoted) running continuously.
Using a smaller inverter would save part of that consumption.

375 Wh/day.
For my particular grid-connected rates, that would be worth $0.07/day, but using PV panels just $0.02/day.
Capital cost to produce this power is about $10 to $30, 70W worth from one 250W to 500W panel out of several you install.
With your 12V bank this would be 30 Ah/day if no sunlight - not insignificant but presumably you have several days worth.
Compared to the 65W load you are considering, the 15W standby consumption of the large inverter is relatively insignificant.

Looking up the small Victron inverters others suggested,

https://www.victronenergy.com/upload/documents/Datasheet-Phoenix-Inverter-VE.Direct-250VA-1200VA-EN.pdf

Looks like that reduces standby power to 4W or 8W.

So my suggestion was rather than trying to save power with a second inverter, just use the big one and put in more PV panel.

Few inverter vendors give all the data needed to to calculate consumption at no-load and at any amount of load. Often they just quote "peak efficiency", which doesn't help you determine battery drain and PV requirement. So I couldn't come up with a precise figure for power savings, just an upper limit.

 

[Mike Jordan]

Dzl, I have found you to be very thoughtful, respectful and fair even when someone (in this case, me) deserves a "talking to". Even though I felt the same affront in his first post, I tried the constructive route in #3 to be more detailed. Mike confirmed that there are others with a similar problem/need as I have. I fully understand that there is a huge diversity here... from theoreticians to electrical technicians with extensive practical training to those without any formal training, yet years of DIY experience. All can contribute constructively one way or another. I suspect that most all are borderline genius or better. I'd bet Hedge is also. Everyone here, can and should be able to think outside of their comfort zone, but they first must take the time to consider the questioner's need. If I had responded to this thread, I might also have pointed out alternative devices, but I still would have given an honest attempt at a useful answer to the direct question. The fact that he responded with an on-grid solution in the Off-Grid Inverter forum AND my post explicitly stated off-grid AND boat really smacked me that reading the question is important.

Back to constructive...

For me and other small boaters and I suspect for Mike and ALL small*, off-grid users (US), we do go out of our way to use DC directly. In my case, I'll have more than twenty devices that will be used most of the time while on the boat. All are 12 VDC based. Most all are trivial low wattage digital electronics. The biggest users in this class are radios, radars and autopilots. The biggest around the clock user will undoubtedly be a Refrigerator which I extensively studied and will also be DC based. Even for my laptop, I'll look for a DC converter.

For (US) using an Inverter at all will be for un-planned cases or when DC versions of a product are not available or are prohibitively expensive. I do not consider idle current to be an issue as I will never have an Inverter on that is not being used. The convenience factor of it instantly working once plugged-in is simply not worth the idle current. I'll will be wiring the remote Inverter switch. At the high-end, I'd like to use a Microwave. At the low-end, a guest might need to plug in their laptop. I need both extremes, I'd like to handle it efficiently... as pointed out above it might be a life saving decision.

As no one had helped to point US in a direction, I continued my stumbling research alone which added to the frustration of marketing data. I finally decided to test a Giandel 300 watt unit because they claimed up to 95% efficiency. I probably would have considered the Victron, but that model does not show up. Even now, doing a search on Amazon for "victron 250 watt inverter" does not get a hit. ONLY if you select the 1200 watt version are you then able to see the 250 watt version. Bad search algorithm or bad product definition!

Continuing... the Giandel only showed 85% efficiency in my crude testing at my desired wattage case. I still have hopes this thread's premise will be valid and useful... a user post in that Giandel thread indicated a high wattage inverter was only 65% efficient at that low output. Once I get my battery (still on slow boat from China) and large inverter, I will run the same efficiency test on it and we'll have the answer to this thread. And those of (US) that are concerned about off-grid efficiency, will be able to make an informed decision whether it is worth having Dual Inverters for low and high wattage or not.


* Small is defined here as those with less than 5000 watt-hours of battery storage and less than 300 watts worth of solar panels.

I caught that Victron recommendation from Dzl. I couldn't find it on an Amazon search either. But some off Amazon searches lead to this page where they are all at.

https://www.amazon.com/Victron-Energy-Sinewave-Inverter-VE-Direct/dp/B01NAO10QX/ref=sr_1_3?crid=21KU2LRHQPFL7&dchild=1&keywords=victron%2Bphoenix%2Binverter%2B12%2B500&qid=1610943698&sprefix=Victron%2BPhoenix%2BInverter%2Cinstant-video%2C192&sr=8-3&th=1

But the Victrons are lots larger than any other low wattage counter part. So that throws a wrench into that idea for me... I guess. (not completely taken it off of the table)

I have decided that for my low load solution, am going to make a tool box size solar generator/power station.... when my 100ah cells arrive. I am going to try to keep the tool box as small as possible, to make it easy to use where ever I want. But still fit a 300-500 watt inverter, 120v outlets, 12v outlets, and USB. Then I can use it in the RV for the small loads, around camp, or at the house for short term power outages.

Then it can solar charge and power devices independently of any other system

I guess I won't know how practical it will be to have the box in the RV. That will probably depend on the size. But I think I can keep it pretty compact. Regardless, the other usages will make it worth the build.

On another tangent.

I am deep into the Ryobi tool eco system. I bought a 150w inverter for my 18v One+ batt. It powered my cable modem/router/wifi for 5.5 hours and (separately) my 40" TV and cable box for 1 hour 45 min on a 4ah batt. It is not pure sine wave, which makes many folks unnecessarily in most instances (in my opinion) bat crap crazy



Ryobi also makes a 300w inverter, that is pure sine wave, for the 40v series. I don't have one yet. But based on the success of my 18v inverter tests, I'm going to buy one. So these batt/inverters are another idea I may use for low wattage intermittent usage. The good thing is the batt and inverter combo is pretty small, letting you put it where ever you need it. Then move it to the next location

They have wall chargers for these batts, but also 12v chargers. So you can charge from the main boat/RV system at a better conversion rate. Or.... like me, from the toll box solar gen

 

[Dzl]

Dzl, I have found you to be very thoughtful, respectful and fair even when someone (in this case, me) deserves a "talking to". Even though I felt the same affront in his first post, I tried the constructive route in #3 to be more detailed. Mike confirmed that there are others with a similar problem/need as I have. I fully understand that there is a huge diversity here... from theoreticians to electrical technicians with extensive practical training to those without any formal training, yet years of DIY experience. All can contribute constructively one way or another. I suspect that most all are borderline genius or better. I'd bet Hedge is also. Everyone here, can and should be able to think outside of their comfort zone, but they first must take the time to consider the questioner's need. If I had responded to this thread, I might also have pointed out alternative devices, but I still would have given an honest attempt at a useful answer to the direct question. The fact that he responded with an on-grid solution in the Off-Grid Inverter forum AND my post explicitly stated off-grid AND boat really smacked me that reading the question is important.

I appreciate and respect the well thought out and well articulated reply (also the nice clean clear paragraphs ). And I think I understand where you are coming from.

Back to constructive...

For me and other small boaters and I suspect for Mike and ALL small*, off-grid users (US), we do go out of our way to use DC directly. In my case, I'll have more than twenty devices that will be used most of the time while on the boat. All are 12 VDC based. Most all are trivial low wattage digital electronics. The biggest users in this class are radios, radars and autopilots. The biggest around the clock user will undoubtedly be a Refrigerator which I extensively studied and will also be DC based. Even for my laptop, I'll look for a DC converter.

Personally I have been able to find DC-DC chargers for my primary devices (phone, laptop, and anything else that uses the USB-C power delivery protocol up to about 60W) But I have not been able to measure efficiency but I would like to someday.

For (Us) using an Inverter at all will be for un-planned cases or when DC versions of a product are not available or are prohibitively expensive. I do not consider idle current to be an issue as I will never have an Inverter on that is not being used. The convenience factor of it instantly working once plugged-in is simply not worth the idle current. I'll will be wiring the remote Inverter switch. At the high-end, I'd like to use a Microwave. At the low-end, a guest might need to plug in their laptop. I need both extremes, I'd like to handle it efficiently... as pointed out above it might be a life saving decision.

One thing I have really started to see the logic/value of lately is USB-C PD (power delivery). Many high end low power devices (phones, tablets, small laptops, etc) are using it. The charger and the device 'negotiate' power delivery between themselves and settle on the correct profile (from I think 5V to 20V and up to 100W). Both charger and device must be compatible or it falls back to normal USB 5v/2a I think. The benefit of it is you can use one charger for many devices.

As no one had helped to point US in a direction, I continued my stumbling research alone which added to the frustration of marketing data. I finally decided to test a Giandel 300 watt unit because they claimed up to 95% efficiency. I probably would have considered the Victron, but that model does not show up. Even now, doing a search on Amazon for "victron 250 watt inverter" does not get a hit. ONLY if you select the 1200 watt version are you then able to see the 250 watt version. Bad search algorithm or bad product definition!

Continuing... the Giandel only showed 85% efficiency in my crude testing at my desired wattage case. I still have hopes this thread's premise will be valid and useful... a user post in that Giandel thread indicated a high wattage inverter was only 65% efficient at that low output. Once I get my battery (still on slow boat from China) and large inverter, I will run the same efficiency test on it and we'll have the answer to this thread. And those of (US) that are concerned about off-grid efficiency, will be able to make an informed decision whether it is worth having Dual Inverters for low and high wattage or not.

95% sounded pretty unrealistic (considering that similarly sized high end inverters aren't achieving this from what I have seen, and larger Giandel inverters are not achieving this either). 85% sounds pretty okay considering the inefficiency of using a large inverter for a tiny load and the relative low cost of Giandel inverters.

 

[WA5IDX]

I went the two inverter route for my Airstream simply because it was "pretty inexpensive" and somewhat more flexible. I have very few 120v appliances that I use in the RV. The Microwave is the biggie and I have a permanently wired 3kw inverter to drive all 120v outlets as may be necessary. I have a very portable (and cheap) 300 watt Bestek ( circa $60 US) pure sine wave inverter that I move around as needed for lighter loads. The spec says it is over 80% efficient which is fine since I have a deep Battery Bank (545AHr). It has 1 duplex outlet on it as well as two USB connectors. This set-up works for me in the RV pretty well and I have 12V outlets in most places ( and extension cords for 120v are cheap).

Yes, I did have a boat ( several in fact) and cruised extensively in the Pacific. I had a 2kw inverter and 600 AHr of battery with dual alternators on the diesel plus some solar and even wind driven generators for charging. I've spend considerable time "on the hook" away from Grid power. That's all 30 years ago now and it would be much easier to do the same today. I wish that small Inverters had been available then.

There are always many ways to "skin the cat" as it were. I've learned that most solutions are a compromise so I no longer worry about making the "optimum or best" solution and use the KISS principle most of the time.

 

[Mike Jordan]

I went the two inverter route for my Airstream simply because it was "pretty inexpensive" and somewhat more flexible. I have very few 120v appliances that I use in the RV. The Microwave is the biggie and I have a permanently wired 3kw inverter to drive all 120v outlets as may be necessary. I have a very portable (and cheap) 300 watt Bestek ( circa $60 US) pure sine wave inverter that I move around as needed for lighter loads. The spec says it is over 80% efficient which is fine since I have a deep Battery Bank (545AHr). It has 1 duplex outlet on it as well as two USB connectors. This set-up works for me in the RV pretty well and I have 12V outlets in most places ( and extension cords for 120v are cheap).

Yes, I did have a boat ( several in fact) and cruised extensively in the Pacific. I had a 2kw inverter and 600 AHr of battery with dual alternators on the diesel plus some solar and even wind driven generators for charging. I've spend considerable time "on the hook" away from Grid power. That's all 30 years ago now and it would be much easier to do the same today. I wish that small Inverters had been available then.

There are always many ways to "skin the cat" as it were. I've learned that most solutions are a compromise so I no longer worry about making the "optimum or best" solution and use the KISS principle most of the time.

I have that inverter on my short list as a cheap, small, pure sine option. It has good reviews on Amazon. Good to see a good review with very idea i had in mind

 

[Mike Jordan]

I went the two inverter route for my Airstream simply because it was "pretty inexpensive" and somewhat more flexible. I have very few 120v appliances that I use in the RV. The Microwave is the biggie and I have a permanently wired 3kw inverter to drive all 120v outlets as may be necessary. I have a very portable (and cheap) 300 watt Bestek ( circa $60 US) pure sine wave inverter that I move around as needed for lighter loads. The spec says it is over 80% efficient which is fine since I have a deep Battery Bank (545AHr). It has 1 duplex outlet on it as well as two USB connectors. This set-up works for me in the RV pretty well and I have 12V outlets in most places ( and extension cords for 120v are cheap).

Yes, I did have a boat ( several in fact) and cruised extensively in the Pacific. I had a 2kw inverter and 600 AHr of battery with dual alternators on the diesel plus some solar and even wind driven generators for charging. I've spend considerable time "on the hook" away from Grid power. That's all 30 years ago now and it would be much easier to do the same today. I wish that small Inverters had been available then.

There are always many ways to "skin the cat" as it were. I've learned that most solutions are a compromise so I no longer worry about making the "optimum or best" solution and use the KISS principle most of the time.

As a follow up, what is the highest load that you have had successfully running on the Bestek?

 

[Inq720]

...
So my suggestion was rather than trying to save power with a second inverter, just use the big one and put in more PV panel.
...

I think you have something to offer and I appreciate you trying. However, I'm just not finding anything useful from your responses and you do not seem to read the important points of the situation that prompted this thread. I will try one more time.

1. It is NOT about money. It is about having working capacity!
2. OFF-GRID - no momma to run to when I run out of power. Just solar.
3. It's a 26 foot boat. I can put (2) 100 watt panels on it. Not 3, not 4. There is no usable/clear location! I suspect a van would have similar constraints.
4. A bigger battery only works if I can re-charge it. (See 2 and 3)
5. Idle/Standby current is not the issue of this thread. Inverter(s) will not be on when not being used.

My whole motivation for this thread was reading that: Inverters are most efficient near their rated output. That a large inverter running at low output would be far more wasteful than a low-wattage inverter. That's all that prompted this thread. I was hoping that someone here could definitely validate whether this was marketing hype or marketing BS.

The marketing numbers for powering an 80 watt device were (from previous post):

1. 1500w Inverter @ 74% efficiency: 80 / 0.74 = 108 watts
2. 300w Inverter @ 95% efficiency: 80 / 0.95 = 84 watts

For my theoretical battery bank of 3400 watt-hours, that usage will last:

1. 1500w Inverter: 3400 / 108 = 31.5 hours
2. 300w Inverter: 3400 / 84 = 40.5 hours

This 9 hours of surplus capacity are worth my using a dual inverter solution! Life or death out on the ocean may literally be the difference in those 9 hours.

Since the posting of those numbers, I have found by my testing that the small Giandel 300 watt unit was only 85% efficient. However, in that thread, a reply stated the large inverter might only be 65% efficient. If so... the numbers change, but the Math and premise still hold... A significant capacity surplus is available. I will be testing a Giandel 2200 watt unit once my cells get in from China. They are literally on the slow boat.

 

[Inq720]

Personally I have been able to find DC-DC chargers for my primary devices (phone, laptop, and anything else that uses the USB-C power delivery protocol up to about 60W) But I have not been able to measure efficiency but I would like to someday.

One thing I have really started to see the logic/value of lately is USB-C PD (power delivery). Many high end low power devices (phones, tablets, small laptops, etc) are using it. The charger and the device 'negotiate' power delivery between themselves and settle on the correct profile (from I think 5V to 20V and up to 100W). Both charger and device must be compatible or it falls back to normal USB 5v/2a I think. The benefit of it is you can use one charger for many devices.

I definitely agree that if the dream can be realized it will be a great overall solution. I am just wary...

At least for AC to USB-C power supplies there seems to be many proprietary and conflicting... Just between Apple, Google and Samsung phones / AC chargers, they have different standards and playing head games with each other. A Pixel charger will charge a Pixel phone at 3 amps, but not a iPhone. Apple is the same way. I routinely used a non-Pixel charger on my Pixel overnight and it must have kept charging after being 100% full. It destroyed the Pixel's battery in short order.

DC-DC USB-C chargers seem to lag in the technology compared to their AC versions and even if current, who are they current with? I see some that claim they have electronics for multiple standards... like this one. You use one port for Pixels and one for another "standard". And heaven forbid if you don't have the right cable and have to charge your Pixel with the other standard. Does is abuse the battery... etc.?

Some day all will be great. Then another standard will come out.

 

[Inq720]

I'm sounding like a grumpy old man... snapping at other forum members, questioning marketing data, wary of "standards".
Maybe its time to unplug and sail away to tropical oblivions.