Starting from nothing, but going big
- Thread starter TRW
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Not to beat a dead horse here, but this is THE major reason to size the battery first. Unlike some people, I'd say it isn't wise to design based on the sunniest day. I know that isn't what you are saying, but you need to realize you will have days (probably multiple) with not much sun. If you are putting the emphasis on the array, it isn't doing much for you.
Turning to some of your other points: If the grid is unavailable and you don't have fuel for the generator, what happens on those days that are not real sunny?
Also, saying you will be able to run the fridge on cloudy days with the limited sunshine might be a bit optimistic. A fridge is a "medium" load. Not as big as a well pump or microwave, but bigger than most of the other "necessary" loads.
I think that especially for a truly off grid situation (like mine), you should put the emphasis on the battery, knowing that oversizing or adding to the PV array is easy and inexpensive. That array does no good for you if you don't store it up for the nights and cloudy days.
In the end, it's up to you. Know that for suitcases, paychecks, and batteries you eventually use all you have and wish you had more.
Well, I think what may be being lost in these exchanges, as with most debates that become ultra-polarised these days, is that thinking about one end of the stick first doesn't exclude the other end. Anyone who first thinks about the panels, is still going to subsequently think carefully about the battery also. One doesn't exclude the other.
I may not have emphasised this enough, but planning for the worse case scenario where there is no grid, and no petrol/diesel/propane... also entails facing the fact that panels outlast batteries. And the shallower the discharge of the battery bank the longer it lasts. We can and should exploit this fact by oversizing the battery, but that's much more expensive than minimising how much we draw from the battery and looking at a larger solar array so everything that can be done during sun hours... is. Which leads me back to first wanting to get a ballpark array size to make that possible.
Concerning the fridge, a 4kWh array, even if cloud cover has reduced its output to 10%, is still going to generate 400w, so roughly 2kWh in a day. A modern, efficient fridge, might use about 1kWh in 24 hours. Even after accounting for energy loss within the system, the fridge isn't really ever in jeopardy. An 8kWh array? Roughly 4kWh on a bad day, leaving power for a few other applications.
That's where I started my process because I'm thinking about this differently than you. That does not mean I'm right, or that you are wrong. I'm not interested in convincing you to think about it differently: your situation is probably different, and likewise, your philosophy.
Now, after realising that my site can only comfortably accomodate a 5.5kW array at present, I then turned my attention to what can be accomplished with that capacity, and how sizing a battery might optimise my situation. I was reassured those 5.5kW would be enough in a survival situation with minimal battery capacity, but then it was time to consider how big a battery I could reasonably afford, how many wants I would add to my needs during this second step, and how much I wanted to minimise the depth of discharge and extend lifespan by oversizing.
If you are truly able to shift perspective away from your own, you will understand that this order of approach makes perfect sense for me.
Hedges
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I agree. If the cost is affordable, covering 100% (or up to whatever code allows while leaving fireman access) is the thing to do.
Isc of the array has to be within limits of the inverter or charge controller.
If you have batteries, charge current has to be kept within the limit it accepts.
Some systems will regulate charge current to a target rate (e.g. 0.2C) even though PV array can deliver much more.
If you have a 10kW PV array, it could make 55 kWh on a good day. Even on a quite poor solar day it would power critical loads like a refrigerator (1.5 kWh/day.)
The utility may never want to pay individuals for power, but virtual power companies may buy on the spot market with individual systems bidding or responding to bids. SMA has mentioned something like that, "Provides a gateway to the rapidly changing energy market of the future"
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Thanks for the input, I am glad to hear your perspective. What are your thoughts on these big high output panels?I agree. If the cost is affordable, covering 100% (or up to whatever code allows while leaving fireman access) is the thing to do.
Isc of the array has to be within limits of the inverter or charge controller.
If you have batteries, charge current has to be kept within the limit it accepts.
Some systems will regulate charge current to a target rate (e.g. 0.2C) even though PV array can deliver much more.
If you have a 10kW PV array, it could make 55 kWh on a good day. Even on a quite poor solar day it would power critical loads like a refrigerator (1.5 kWh/day.)
The utility may never want to pay individuals for power, but virtual power companies may buy on the spot market with individual systems bidding or responding to bids. SMA has mentioned something like that, "Provides a gateway to the rapidly changing energy market of the future"
Discover the Data Manager: Monitoring & Control | SMA America
Find out about the Data Manager M: Professional monitoring and control for decentralized energy systems up to the megawatt range ► Discover now!www.sma-america.com
Hedges
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Big commercial panels are likely the most cost-effective and should have good quality/durability, but there have been bad ones.
Big can mean 2-man installation. So far all of mine I've carried up ladders with one hand because I do everything myself. Doing 2-man is probably faster and safer. On a sloped roof it is effort to keep panel from sliding off before being bolted.
To avoid excessive bending, might need 3 rail for support rather than two.
Small panels (100W) will require extra hardware and labor per panel, but above some size the benefit from going larger may level out.
A few of the panels have clever layout of series/parallel internal wiring and no diodes, intended to handle partial shading that could occur in a large commercial installation, with each row partially shading the next. Those could be OK on a roof that will never get shaded, otherwise I would avoid them.
Some people here were ordering 500W panels direct from China. I don't know if that is still cost effective.
I think what is most important is quality. Some panels have started dying early. I found articles that talked of hot spots visible in commercial arrays and loss in power due to failing panels. I've linked reports on HAST testing of panels, with some brands performing well and some showing high degradation rates. In the reports only the high performers were named; they were silent about which showed failures. Apparently the commercial industry deals with this by manufacturers showing HAST test results so 3rd parties will guarantee the performance warranty for decades. That way someone spending $millions on a plant has assurance of financial return on investment.
If you aren't too price sensitive, I would say buy the best you can find, a top brand. But price has come down so much it isn't as critical as before. When I got mine in the early 2000's, a 10kW (STC) array would cost $50k to $60k; today that can be had for $5k to $10k retail, maybe as little as $3000 or so. If there is a problem in 5 or 10 years they can all be replaced. You've probably broken even with utility rates in 3 or 4 years.
Again lots more valuable info, I will study this a bit and as always have more questions. Thanks a bunch!
GXMnow
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The question of how much battery is a tough one. If you are off grid, you do need to use a lot of battery. Two day of battery only is probably a good place to start. On grid can be so much more cost effective. When I did my initial design for adding the storage battery, I thought 10 KWH would be plenty. I ended up with nearly 18 KWH because I found a great deal. And now I wish I had more still.
Currently batteries are still expensive and certainly cost more than the solar panels. My current project is to add 6 more panels for DC charging of my battery bank. The 6 solar panels are such a small part of the cost. I wish I had room to put up more, because the balance of system cost will not change much. Where I am, I do not have many choices. We have a lot of codes here to deal with, and there are only so many places I could put panels without running into compliance issues. So I totally agree, if you have room for a larger array, just do it. I would have put 4 more panels on my original Enphase system, but I would have needed to also change out my main breaker panel, which would not only have been more cost, it would have caused issues with getting the permit and inspections also.
Having my Solar for almost 2 years, and now the battery inverter for 6 months, I have a much better idea of my real loads. One thing I stumble on, is that I really like the idea of having BOTH microinverters on some panels, and a DC charge controller on the rest. A system like a Sol-Ark or Outback Skybox does sort of work like that, but being all in one, it is still a common failure point. Here is my thinking.
Any power that you use while the sun is shining is going to be most efficient coming from microinverters. If they do make more power than you need, the extra can either back feed the grid, or be fed back into the battery through the battery based inverter.
Charging batteries will be the most efficient from a good MPPT DC charge controller. But if you are also feeding an inverter at the same time, it is less efficient at making the AC power than the microinverters. The battery power should only be used when the sun goes down and the need to use the battery power. So the DC array is sized for the overnight loads, and the AC array is sized for the daytime loads. And having a bit more than you need is not a bad thing. You can toss out power you don't need and can't use, but if you don't make enough power due to clouds etc., you are going to have to buy it from the utility, and put more fuel in the generator.
As for the battery size, I think most of us here are limited by budget more than what we would like to have. If you can afford it, it would be great to have enough battery to run everything you want to for 2 full days. If you don't have good sun, that give you time to get ready to fire up the generator. This is not practical in all cases. In the heat of summer hear, I run my A/C and I just know I need to buy some power from the grid for that. If my system runs for half of the day, that will need 12 hours at 15 amps x 240 volts = 43.2 KWH's just for the outdoor compressor and condenser fan. That would need 3 times the battery I have, and a LOT more solar panels. Without my A/C running, I use just 25 KWH a day to run everything else in my house. My Enphase solar array has 4,800 watts of panels, limited to 3,900 watts of inverter, and makes about 25-30 KWH's a day for 9 months of the year, when it is sunny. My proposed DC array will add about 2,100 watts of panels, and produce 10-15 KWH's per day to charge the battery bank. On most days, the DC array alone should be able to top up the battery.
Sizing a system to minimize the cost can be tricky. Making a system that can make enough power in a limited area can also be a problem. Battery storage is still expensive. There is no one rule that works in every case. Having plenty of room to use a lot of cheap solar panels is a very good place to start from though. If I was starting over, I would probably reverse what I have. Use 1/3 of the array as microinverters to run the day time loads, and 2/3 of the array DC coupled to charge the batteries. I look at it this way. You have less than 1/3 of the day to make all of your power. 1/3 of the time with sun, and 2/3 of the time, running off of the batteries. You might use a bit less power over night, but you also get just 5-7 hours of sun. And you need to use more lights at night. I think this is a fair trade off.
Currently batteries are still expensive and certainly cost more than the solar panels. My current project is to add 6 more panels for DC charging of my battery bank. The 6 solar panels are such a small part of the cost. I wish I had room to put up more, because the balance of system cost will not change much. Where I am, I do not have many choices. We have a lot of codes here to deal with, and there are only so many places I could put panels without running into compliance issues. So I totally agree, if you have room for a larger array, just do it. I would have put 4 more panels on my original Enphase system, but I would have needed to also change out my main breaker panel, which would not only have been more cost, it would have caused issues with getting the permit and inspections also.
Having my Solar for almost 2 years, and now the battery inverter for 6 months, I have a much better idea of my real loads. One thing I stumble on, is that I really like the idea of having BOTH microinverters on some panels, and a DC charge controller on the rest. A system like a Sol-Ark or Outback Skybox does sort of work like that, but being all in one, it is still a common failure point. Here is my thinking.
Any power that you use while the sun is shining is going to be most efficient coming from microinverters. If they do make more power than you need, the extra can either back feed the grid, or be fed back into the battery through the battery based inverter.
Charging batteries will be the most efficient from a good MPPT DC charge controller. But if you are also feeding an inverter at the same time, it is less efficient at making the AC power than the microinverters. The battery power should only be used when the sun goes down and the need to use the battery power. So the DC array is sized for the overnight loads, and the AC array is sized for the daytime loads. And having a bit more than you need is not a bad thing. You can toss out power you don't need and can't use, but if you don't make enough power due to clouds etc., you are going to have to buy it from the utility, and put more fuel in the generator.
As for the battery size, I think most of us here are limited by budget more than what we would like to have. If you can afford it, it would be great to have enough battery to run everything you want to for 2 full days. If you don't have good sun, that give you time to get ready to fire up the generator. This is not practical in all cases. In the heat of summer hear, I run my A/C and I just know I need to buy some power from the grid for that. If my system runs for half of the day, that will need 12 hours at 15 amps x 240 volts = 43.2 KWH's just for the outdoor compressor and condenser fan. That would need 3 times the battery I have, and a LOT more solar panels. Without my A/C running, I use just 25 KWH a day to run everything else in my house. My Enphase solar array has 4,800 watts of panels, limited to 3,900 watts of inverter, and makes about 25-30 KWH's a day for 9 months of the year, when it is sunny. My proposed DC array will add about 2,100 watts of panels, and produce 10-15 KWH's per day to charge the battery bank. On most days, the DC array alone should be able to top up the battery.
Sizing a system to minimize the cost can be tricky. Making a system that can make enough power in a limited area can also be a problem. Battery storage is still expensive. There is no one rule that works in every case. Having plenty of room to use a lot of cheap solar panels is a very good place to start from though. If I was starting over, I would probably reverse what I have. Use 1/3 of the array as microinverters to run the day time loads, and 2/3 of the array DC coupled to charge the batteries. I look at it this way. You have less than 1/3 of the day to make all of your power. 1/3 of the time with sun, and 2/3 of the time, running off of the batteries. You might use a bit less power over night, but you also get just 5-7 hours of sun. And you need to use more lights at night. I think this is a fair trade off.
Sanwizard
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DMI just added a device to his Trane HVAC that dropped the load to 8 amps while running. Pretty cool.
Do you have a link?
I am likely with you on your feelings on government, but until the Chinese take over, we will need to play by what we are told the rules are today!
Thanks a bunch for your thoughts and interesting ideas. If I don't tie to the grid, I will have a generator to back things up. I love that independence, but my wife loves convenience and comfort.
Sanwizard
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Not a link, but google "Micro air easy start". I am going to get two, one for each of my 3.5 ton units.
So I looked this up and love the concept, but where can I find info on the reliability of the unit and the compressor?
Hedges
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(not saying the device does this but ...) if an induction motor partially loaded draws excess current out of phase (poor power factor), then optimum operation could be to vary amplitude of sine wave such that current drawn and watts delivered is just sufficient. The PWM of a VFD could do that.
Of course, it would need PF corrected front end to not be as bad itself. Running line synchronous without a rectified DC rail, a circuit could do that with AC in, AC out. A single pulse per phase (like a dimmer) might reduce out of phase current but would still have poor PF. The 2 hp pool pump with VFD that I use needs 20A breaker vs. 15A with original single phase motor, presumably due to the diode/capacitor front end of VFD.
Not something you're going to find in a couple hundred dollar consumer easy-start, though. I think the easy-starts chop AC current to make pulses of narrower width during start-up.
Do VFDs really improve the power factor of AC motors?
Using a VFD to drive an AC induction motor can improve power factor – but it’s not the panacea that some manufacturers suggest.
www.motioncontroltips.com
As a kid I discovered I could vary speed of a DC motor by rotating the brush holder. Apparently just making poor PF (which I didn't know about at the time) with out of phase current (which I did.)
While I know enouph to underst
While I know enough to understand what you are saying and I have seen enough to agree, the claim was to drop the amperage close to 30 percent or more. I don't think they can do that. I will say that any start reduction device is a good thing especially if it proves reliable for the device and the motor. A compressor builds load fast when they start, so a slow ramp up makes me curious to how well it starts. But I don't know what I don't know! I assume this ramp up time is less than 3 seconds.(not saying the device does this but ...) if an induction motor partially loaded draws excess current out of phase (poor power factor), then optimum operation could be to vary amplitude of sine wave such that current drawn and watts delivered is just sufficient. The PWM of a VFD could do that.
Of course, it would need PF corrected front end to not be as bad itself. Running line synchronous without a rectified DC rail, a circuit could do that with AC in, AC out. A single pulse per phase (like a dimmer) might reduce out of phase current but would still have poor PF. The 2 hp pool pump with VFD that I use needs 20A breaker vs. 15A with original single phase motor, presumably due to the diode/capacitor front end of VFD.
Not something you're going to find in a couple hundred dollar consumer easy-start, though. I think the easy-starts chop AC current to make pulses of narrower width during start-up.
Do VFDs really improve the power factor of AC motors?
Using a VFD to drive an AC induction motor can improve power factor – but it’s not the panacea that some manufacturers suggest.
As a kid I discovered I could vary speed of a DC motor by rotating the brush holder. Apparently just making poor PF (which I didn't know about at the time) with out of phase current (which I did.)
Sanwizard
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This is interesting, thanks for the info. This does say the amperage was dropped about 1.25 amps, the original claim said 8 amps, so this clears that up.
Hedges
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I've thought the same too about slow starts.
I have a Dayton compressor that now has trouble starting the second time (unloader but tank at 80 psi). I've had it connected across two 120V circuits. I think when one tripped it was experiencing brown out, often stalled and got hot. I think the starting windings could be partially cooked.
An A/C bleeds down pressure fully, unlike an air compressor. So each time it has to fill the condenser with pressurized refrigerant. How long at 3600 RPM does that take? That would say how may turns at reduced speed the soft-start could do.
I don't have anything soft-start. I do have a VFD with default setting of many seconds to ramp up a 3-phase water pump. A single phase motor wouldn't want to be started that slowly. But I would think even 1/4 second ramp up would be a big reduction in current draw.
If anyone does measure reduced running current with an Easy Start, my first suspicion would be the meter isn't true-RMS. When modified sine wave voltage is measured with a cheap meter, I think it inaccurately registers 90V or 100V instead of 120V so would expect similar from a funny current waveform.
It does appear the Easy Start make a big difference for starting an A/C when limited current is available.
If you haven't built yet, an inverter drive compressor would be even better.
I have one experience with a transformerless PV inverter apparently being upset by VFD, which is why I recommend transformer type. Any new inverter meeting latest codes for grid-tie is probably transformerless. I picked up older models.
For off-grid, there are several transformer type battery inverters with good motor starting surge. I have Sunny Island (more of them than I really need) and a small A/C, no trouble running that. Brands like Victron and Schneider have models that should work as well. Most brands will also have lightweight transformerless designs. Certainly for PV inverters, possibly for battery inverters (SMA has Sunny Boy Storage with 400V battery, and Tesla Powerwall is similar, both wimpy surge.) Most older 48V battery inverters would be transformer type, but I think newer and cheaper brands aren't.
I have a Dayton compressor that now has trouble starting the second time (unloader but tank at 80 psi). I've had it connected across two 120V circuits. I think when one tripped it was experiencing brown out, often stalled and got hot. I think the starting windings could be partially cooked.
An A/C bleeds down pressure fully, unlike an air compressor. So each time it has to fill the condenser with pressurized refrigerant. How long at 3600 RPM does that take? That would say how may turns at reduced speed the soft-start could do.
I don't have anything soft-start. I do have a VFD with default setting of many seconds to ramp up a 3-phase water pump. A single phase motor wouldn't want to be started that slowly. But I would think even 1/4 second ramp up would be a big reduction in current draw.
If anyone does measure reduced running current with an Easy Start, my first suspicion would be the meter isn't true-RMS. When modified sine wave voltage is measured with a cheap meter, I think it inaccurately registers 90V or 100V instead of 120V so would expect similar from a funny current waveform.
It does appear the Easy Start make a big difference for starting an A/C when limited current is available.
If you haven't built yet, an inverter drive compressor would be even better.
I have one experience with a transformerless PV inverter apparently being upset by VFD, which is why I recommend transformer type. Any new inverter meeting latest codes for grid-tie is probably transformerless. I picked up older models.
For off-grid, there are several transformer type battery inverters with good motor starting surge. I have Sunny Island (more of them than I really need) and a small A/C, no trouble running that. Brands like Victron and Schneider have models that should work as well. Most brands will also have lightweight transformerless designs. Certainly for PV inverters, possibly for battery inverters (SMA has Sunny Boy Storage with 400V battery, and Tesla Powerwall is similar, both wimpy surge.) Most older 48V battery inverters would be transformer type, but I think newer and cheaper brands aren't.
