Inverters - whats the inside track?

As will all Web info, you have to spend a disproportionate amount of time scraping off the hype/bs/rubbish speculation etc to get to what you really need to know - deep down and dirty - as I found out building my own Prismatic 24V bank - the devil is in the detail.
OK I have little experience in large inverters and only used a small 500W - $50 one as a UPS for my PC/wifif during covid. I found out that these cheap ones are woefully over stated in power delivery and volt control also reliability/ will shutdown at the slightest hiccup - etc.

However listening to you good buddies here I find there's a lot to consider if you want say a reliable 3kVA inverter for continuous domestic use.
For example you have to consider what loads you have vs their inrush currents. The humble fridge/freezer can have alarmingly high inrush which is why it uses a special relay.
Starting up an inverter needs special care as it has large input caps that need to be precharged or you will damage/trip your bms and so on.
But you guys know all this Im sure
What is not so obvious is what quality/design needs for the MOSFETs, heat dissipation/ reliability. Protection against EMI/spikes ?

So - as with LFPs - the quality can vary wildly.

What are the issues to consider? I am surprised there isnt a separate category here addressing inverters - given their importance in a solar grid

You will probably get a plethora of responses to this along the lines of "I have a [insert make/model here]. It's been great! No issues."

That is fine, but what you REALLY need is the experiences of users who have had to operate their inverters under adverse conditions. How did they do when repeatedly overloaded? Overheated? Other stuff that could happen? Etc. I think that may be a better indicator of quality/longevity.

You are probably well aware that these days, there is often a "race to the bottom" in terms of price. That will translate to engineering and constructing stuff with components sized just enough to get the job done with no "margin". Everybody is worried that a competitor will be a couple bucks cheaper.

I suppose that an inverter with a VERIFIABLE large surge capability that lasts for a usable amt. of time (not some made-up spec pulled out of thin air) could indicate some overrated components were used internally.

Boron said:

... I am surprised there isnt a separate category here addressing inverters - given their importance in a solar grid

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There are sub Forums on inverters. Off grid, Hybrid and Grid tie and AIO. Also several discussions about purpose, trouble shooting and use can be found in other places.

I could suggest a couple of quantitative parameters that will help you weed out the trash. Since you mentioned the importance of starting surge, let's start with that.

Don't focus too much attention on how much an inverter can surge to, but instead focus on how LONG the inverter can surge! Cheap inverters will claim they can surge to 200%, but diving into the details, they can only maintain that surge level for 8-16 milliseconds, about 1 AC cycle. Quality inverters (iron-core low-frequency) mention they can surge for between 5-60 seconds (not milliseconds), and that's what you need to start heavy-draw electric motors.

Cheap inverters will not even mention how long they can surge, because they know their numbers are pathetic. So, if they won't mention the time, I'd suggest forgetting about them.

A second criteria that does not get as much air-time is total harmonic distortion, or THD. A quality inverter will be <5%, and some even <3%. Cheaper budget inverters have a THD ~10%. Don't get one of those. The THD strongly effects the running of electric motors, and a motor on a quality inverter will run more smoothly with lower power draw, than with dirty power. I myself have seen this.

Third is background idle draw, which is directly related to the quality of design. More poorly designed inverters of a certain size will have higher background power requirements. More well-designed inverters will have lower background current draws.

Lastly, I'd suggest a review published by another member RCinFLA, that I've cut and pasted a link to. Very good reading.

Issues to consider? If you want an inverter to last, buy one with a PROVEN track record for longevity. You get what you pay for.

Q-Dog said:

... If you want an inverter to last, buy one with a PROVEN track record for longevity. You get what you pay for.

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Sunday when I was fumbling through my storage building for a part that I was just sure I had I stumbled on an old inverter. I bought from Walmart back 26 years ago when I moved to my unimproved land and camped on it. It is a modified square wave and IIRC it cost about $25 at the time. Had all of 300w output from a 12vDC battery. Used it for a few small AC loads (1 I had hoped to use it for was my battery powered drill but the charger for it freaked at being plugged into the inverter).

Just for grins I clipped it to a battery I had and tested it. Still works! Now that is longevity. Can't recommend it though since I doubt you could find the same model.

Mattb4 said:

Sunday when I was fumbling through my storage building for a part that I was just sure I had I stumbled on an old inverter. I bought from Walmart back 26 years ago when I moved to my unimproved land and camped on it. It is a modified square wave and IIRC it cost about $25 at the time. Had all of 300w output from a 12vDC battery. Used it for a few small AC loads (1 I had hoped to use it for was my battery powered drill but the charger for it freaked at being plugged into the inverter).

Just for grins I clipped it to a battery I had and tested it. Still works! Now that is longevity. Can't recommend it though since I doubt you could find the same model.

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Well something hardly ever switched on/rarely used is often reliable? Would the same on 24/7 near max load last?

Boron said:

Well something hardly ever switched on/rarely used is often reliable? Would the same on 24/7 near max load last?

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Likely not. Most items operating 24/7 max load have limited lifespans.

Similar story-

I've got a 1kW "WaganTech" (Fry's Electronics store brand) MSW inverter that I've had for years and use for camping, and at my shop daily as part of my small "evaluation" system here.

It's been connected/disconnected/whatever bazillions of times. Couple years ago, connected backwards (stupid) while camping in the desert west of Phoenix. Big sparks. "Oh, great", I say, no power for the rest of the weekend...

Well, figure no harm, no foul and took it apart. Wonder of wonders, it has three 45A automotive blade style fuses. They are all blown. Even better, they aren't soldered into the board. And I never travel without a small box of those.

Replace them, and gingerly connect to the battery. Power LED lights up, fan comes on, Woo Hoo!

Using it right now to run the network and compose this post.

I wonder does a soft start module make sense for a domestic fridge ca 150W.

OBTW can anyone suggest a piece of kit to log mains starting/stopping V+I conditions on domestic appliances, like ridges, freezers, microwaves, lighting (fluorescent tubes, incandescent) say <1kW max (not going for megamongrel wall ac units - what a pig they are).

obviously one could string together various meters etc - bit messy to lug around the house? I think I heard of a recording clamp meter?

Sounds like you're not a fan of Kill-A-Watts, but they do record enough data that you can backtrack and calculate the duty cycle of anything connected. All you need is total kWh, elapsed time, and instantaneous power, which is something they do. And they are pretty accurate, at least in my experience.

v_green57 said:

Sounds like you're not a fan of Kill-A-Watts, but they do record enough data that you can backtrack and calculate the duty cycle of anything connected. All you need is total kWh, elapsed time, and instantaneous power, which is something they do. And they are pretty accurate, at least in my experience.

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Oh yes I have a few of those types, plug in energy meters - good kit. I didnt know they could log inrush conditions. IIRC they will store a max power value. I wonder if they are quick enough to capture peak conditions? Certainly cheap enough $10. But an inrush spike is what can kill an inverter

Ah, inrush current...no they won't do that. Peak/hold is the only way there.

Mattb4 said:

Likely not. Most items operating 24/7 max load have limited lifespans.

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Yes the old chestnut MTBF bath tub model - doesnt take into account sitting on a shelf for years. I was surprised at failure rates for different types of capacitor in electronic ccts - an eye opener. There is also the issue of how many power cycles are done/year

Posters make big noise about Solar panel life, LFP cell etc. AFAIK old school OGs using tubular LABs expected 10 years use before getting to 60% SOH but their battery banks were a huge heavy investment. It seems inverters have a notably limited lifespan. I wonder if we are focussing in the right areas of a modern og Solar/Lithium system.

I have ordered a battery power meter clamp attachment (ca£20) - which is purely the hall effect ac/dc current C clamp which produces 1mV/Amp which you can plug into your multimeter. Warning for in rush purposes this wont work on cheap <£20 meter with a "Hold" feature which only freezes the skittish reading inherent in digital displays. What is needed is a "Max/Min" feature which holds a peak value over time (till reset). This can be found in more quality meters ca £50+ (like my Owon B35T+ which has BT display also - there are other quality meters out there - cheap Charlie wont suffice)
FWIW herewith - I will do some testing on a chest freezer and post results here soonest

No affiliation btw and I am not a front line leccy who probably knows all about this. Its just what I found to address the inrush issue wrt inverters.

PS The Owon 35T series has 6000 counts/sec. which is average for that price point (series 41T does 22000 cps ca £65 better value IMHO)

Can anyone say what practical response time is useful when determining inrush problems. I have a DSO that does 1 GaS/s 100Mhz but is that overkill for the inrush problem - in practice.

AC inrush currents aren't a problem for a decent inverter. If you're in the UK then a low frequency Victron is probably your best option.

Precharging the DC inputs is still required, but that's only when starting up the inverter which isn't often.

hwy17 said:

AC inrush currents aren't a problem for a decent inverter. If you're in the UK then a (LOW) frequency Victron is probably your best option.

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Indeed I contacted Victron and , as in the past, I found their response to any enquiry hinting a criticism or spec query was met with arrogant dismissal. I wonder what other members think of Victron. I dislike the proprietary comms protocols locking you into their grid systems and their kit seems designed not to be repairable (eg MPPT unit as per YT vid).

I think OGs like to have more independence and choice to allow for contingencies as may arise outback.

searching around I dont have much choice other than a quality ready made inverter charger (eg Victron et al) If I want redundancy buy two and have a hot standby?

How long do these Multiplus last? any MTBF's figs assuming benign use. Anyone any experience with these softstart modules to use domestically?

Mattb4 said:

Now that is longevity

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Does it count as longevity if it was not being used?

Boron said:

Indeed I contacted Victron and , as in the past, I found their response to any enquiry hinting a criticism or spec query was met with arrogant dismissal. I wonder what other members think of Victron. I dislike the proprietary comms protocols locking you into their grid systems and their kit seems designed not to be repairable (eg MPPT unit as per YT vid).

I think OGs like to have more independence and choice to allow for contingencies as may arise outback.

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I just looked up a victron MPPT teardown on youtube.
WTF!! Why would they do this to us!!

Boron said:

I think OGs like to have more independence and choice to allow for contingencies as may arise outback.

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Only true independence is to build yourself a warpverter. Otherwise Victron is about as good an ecosystem to get locked into as any other.

But open loop brings freedom too. I don't need my charge controllers, batteries, or inverters to talk to each other at all.

Boron said:

Indeed I contacted Victron and , as in the past, I found their response to any enquiry hinting a criticism or spec query was met with arrogant dismissal. I wonder what other members think of Victron. I dislike the proprietary comms protocols locking you into their grid systems and their kit seems designed not to be repairable (eg MPPT unit as per YT vid).

I think OGs like to have more independence and choice to allow for contingencies as may arise outback.

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They don't generally deal directly with end users. That's why it's critical to find an authorized distributor who is competent and can deliver great support, they are the first point of contact if you happen to encounter an issue.If you were in the US I would recommend Current Connected but I'm guessing some of our UK members may be able to recommend someone closer.

There's a 5 year transferable warranties with the option to extend to 10 years so not being user repairable in this case is not an issue at least for me with two 150/35s. The RS450/100 and 200 models can be repaired.

Boron said:

I wonder does a soft start module make sense for a domestic fridge ca 150W. View attachment 223391

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I have a Victron 48/1200. Its running all my base load (including full size french door fridge freezer, 7 cu ft chest freezer and 21 cuft upright freezer). It will surge up well beyond 1200VA when one of the compressors kicks in but immediately drops back down to regular base load of around 200-300W.

You couldn't do this with a high frequency inverter with a similar rating. I would suggest starting with a 24/1200, you will be surprised at what it can power, for a very reasonable price. Or if your budget allows, a multiplus 2 3000VA. The 1200 can't run a microwave or power hungry coffee machine etc.

ScropusGobbleBottom said:

I just looked up a victron MPPT teardown on youtube.
WTF!! Why would they do this to us!!

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Yep thats the one - Bubba took a disc grinder to cut it open so he could repair it - Mountain folk!

The humble fridge/freezer can have alarmingly high inrush which is why it uses a special relay.

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My problem at the moment is trying to calculate the start-up current for a fridge freezer i don't yet own.I'm planning a new 24V system specifically to power this, getting rid of our propane one.I once emailed AEG to try & find out what our washing machine used on start up & I'm still waiting for a reply.....
Is using the annual Kwh rating & doing some maths a shot in the dark ? No manufacture seems to put the compressor rating in the specs.I did try the Honda Generator online calculator for this & it suggested anywhere between 1.6 and 2.8 times the running W.