diy solar

diy solar

Solar Power with Grid Backup

hill37588

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Feb 19, 2021
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I was wondering if it is possible to have a solar system with batteries and use grid power as a backup instead of a generator? If I loose my solar panel/battery energy source and the inverter no longer produces power, can an auto transfer switch to the grid as a backup source? I not interested in net metering so it would not be a grid tied system so to speak. I would like for this to power the entire home all 120 and 220 outlets.
 
Welcome to the forum.

Yes. AC-input doesn't matter whether it's grid or genny.

The selection of inverter is key. You would want one that puts preference on solar and battery power and stays out of the AC-in until the batteries drop to a certain point.
 
I can think of a few ways to do this. How grid independent do you want to be?

Using grid power, only if you don't produce enough solar.
A normal off grid system with DC charge controller, battery bank, and an inverter, but then instead of generator start, if the battery runs too low, it kicks on a grid powered charger?

More efficient, so maybe less solar panels needed, but may use more grid power from time to time.
Use a battery inverter charger, and AC couple micro inverters. Solar to AC power is very efficient, when the power is used while the sun is up. But it is less efficient when you use power at night as the power has to convert 3 times instead of 1. DC from solar panel to AC, the AC back to DC to charge the battery, the DC back to AC to run your appliances. And if the battery runs down, just like above, have it use the generator start function to connect the AC grid to the system, and it can charge up the batteries and run your loads.

Maybe the best overall performance, but more expensive and complex. Mix the two systems. And if you have roof sections at different angles and such, you can optimize battery charging and device power. Have a few panel DC coupled with a charge controller to charge batteries, but also AC couple some micro inverters. This way, when the sun is up, you have both efficient AC power from the micros, and efficient DC charging of the battery bank. The battery inverter is running to produce the reference AC wave for the micros, but if your micros are making more power than your are using, the extra can still help charge up the batteries. When the sun goes down, it goes back to just running on the battery inverter. If the battery gets too low, connect to the grid, maybe charge a little until the sun comes up.

To design the system, it is basically the same as an off grid to start. You need an energy audit to determine how much power you need. Find the peak at any given time and the total used for a day. Work from there.
 
I like the idea of micro inverters since there will be less clutter inside. What is needed between the micro inverters and the house breaker box? Can the micros produce 120 and 240? What is needed to trigger the backup A/C source when batteries reach a critical point? I would still need an inverter to convert battery to A/C right?
 
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There are several different systems. I like my Schneider XW-Pro unit, but the software has a few odd issues that I hope they will eventually fix. It can supply 6,800 watts. It can accept up to 6,800 watts of AC coupled solar. To add DC solar, you would need to get a separate DC charge controller. I only have my Enphase AC coupled micro inverters. My system is not off grid.

If I was building from scratch again, I think I would use an Outback Skybox as the main battery inverter. Then you need a 48 volt battery bank of about 400 amp hours for a typical house. Have you estimated your power needs? The skybox can provide 5,000 watts of power from the batteries. It outputs 120/240 split phase power. It can handle up to about 7,000 watts of grid tie type solar inverters on it's output. So while the sun is shining, you could pull over 10,000 watts. The Skybox also includes an MPPT DC solar input that will accept solar panels directly to charge the batteries. The DC solar input is a high voltage string, 200 to 600 volts, so that does need at least 6 panels in series to work well. You can find over 350 watt panels now, for under $200 each. For the AC coupled solar, I think it is hard to beat Enphase micros. I have 16 of their iQ7's on 300 watt panels. The 7A version will take up to 450 watt panels. With Enphase, they have a combiner bow where you can bring in up to four 20 amp strings of micros. I only have 16 300 watt panels, so it is just one 20 amp string. The combiner has the Envoy monitoring hub in it. It is how the system reports the production and also handles software updates and configuration. It should have an internet connection. It can do WiFi or ethernet cable. They also have a cellular option, but then you have to pay for a service plan for it.

There is also the Sol-Ark which is higher power, but cost a bit more. From what I can tell, it requires some DC coupled solar before it will use micro inverters. They have an 8,000 watt or a 12,000 watt.

If you want to truly run off grid(ish) you may need a bigger inverter, or stack 2. But most of us that do have grid access find it is cheaper to use the grid for heavy loads like whole house air conditioning, an electric clothes dryer, or if you have a welder. So I run 2 breaker panels. One is just on the grid, and the other is on the battery inverter. When there is a power failure, the stuff in the grid panel does stop working. But I still have the essentials. The batteries and large inverter and huge number of soral panels needed for me to go off grid is just more expensive than buying a bit of electricity for the heavy short term loads. My electric bills average about $50 now. Last month was just $35 and the summer with A/C on all the time just topped $130. Before solar, I was topping $300 a month all summer. The biggest cost is the battery bank. My system is using rejected Chevy Bolt batteries. They were new, but didn't meet GM's specs. I got 18 KWH of battery for only $2,000 but I had to build the system. The fuses, buss bars, cables, disconnect switch, steel cabinet, etc. probably added up another $1,000 by the time it was ready to run. And it has no warranty at all. I figure by the time the cells croak, there might be better tech available. 18 KWH of battery, is a little more than a Tesla Powerwall2. It's not quite enough battery for off grid, I save about 40% capacity incase of a power outage overnight, so by 10 PM I am back on grid until I have sun the next day at 7:30 am. There are companies that pre build battery bank systems. Many have been doing lead acid based battery banks for years, and they are still cost effective, but lithium is coming down in price, and they last longer and don't need constant maintenance.

All of the decent battery inverters that can run off grid will be able to trigger a generator start. This could be programmed to trigger a relay to connect the grid instead of starting a generator. Set it to cut in at a safe low battery level. With a little logic, it could still start a generator if the grid is also down. In a system like mine, I don't bother doing this as I have the grid connected all the time and just pull or export power as needed. And then the unit will isolate from the grid and run off grid only when the grid fails.

I think I hit all your questions. Don't be afraid to ask questions. The more you know, the better you can make decisions. Building a large solar power system is a big expense, so figure out what you want up front. In my case, I put in the Enphase solar first. I knew it would not provide power in a blackout, but I wanted to get the major expense in when I could still get the federal 30% tax credit. I think it is down to 15% or even 10% now, but it still helps. It was going to go away completely, but that may change now. Enphase did come out with their own battery backup system, but it is expensive.
 
I not interested in net metering so it would not be a grid tied system so to speak.

Net metering would mean cycling surplus power into the grid whenever PV production exceeds consumption, and cycling power out of the grid whenever consumption exceeds PV production. In this situation, PV inverter will be 97% to 99% efficient, and all power available from PV will always be captured.

Instead using a battery inverter and only drawing from grid when battery low as you propose ... power stored in batteries for later use will generally pass through an MPPT SCC of about 98% efficiency and a battery inverter of about 95% efficiency, so loss of 7%. With lithium battery storing power for later use, maybe 2% loss. With LA battery, maybe 30% loss. Once battery is full, excess PV production capability is wasted.

Lead-acid battery cost and cycle life imposes a price in $/kWh that is on the order of what buying power from utility would have cost. Commercial lithium batteries have a similar price per kWh, but cost more and last longer. DIY lithium can be fairly attractive, a fraction the cost of utility power in some locations and about the same cost as PV panels and inverters (on a $/kWh basis over lifespan)

I would recommend net metering if available and terms are not onerous. Using batteries when you could have used net metering is more about making a statement. Deliberately cycling batteries and connecting to grid only when low may mean unnecessary wear on batteries. I think a grid-tie system set up for zero export would be preferable; possibly with batteries and possibly without. Assuming lithium (which unlike LA isn't harmed by low SoC), the ideal system would only charge from PV, and that only enough to take power from PV in excess of consumption.
 
Hedges, on your last paragraph, what do you mean by onerous? Was that a typo?
 
Hedges, on your last paragraph, what do you mean by onerous? Was that a typo?


Some utilities and localities charge large fees, up front and ongoing, for net metering users. In which case they may be better off not participating.
 
Learn something new everyday. Surprised that local utilities would “penalize” you for going green and taking some stress off the infrastructure.
 
Learn something new everyday. Surprised that local utilities would “penalize” you for going green and taking some stress off the infrastructure.
They still want to make money.

You use less power, they charge you more for what you do use, and more taxes even if you use nothing. My bills are less, but not by anywhere near as much as how much less power I am using. Back when I had tiered power, my lowest rate was just $0.15 per KWH for the first 400 KWH in a month. Then it would go to $0.37 for the next 400 KWH. So if I used 800 KWH in a month, it cost me $60 + $148 = $208 for 800 KWHs.

Now the lowest rate is $0.22 per KWH and it jumps to $0.43 from 4pm to 9pm even when I use very little power. When I installed the Solar, I expected to only use that first 400 KWH at 15 cents = $60. But NO!!! Now I use 250 KWH at 22 cents, and 150 KWH at 43 cents. So now I use half the power, but it costs me $55 + $64.50 = $119.50 So I pay nearly double for every KWH I use compared to the old first tier. But yeah, before Solar, I had $200 electric bills for half the year, now they average under 100, and in these cool but sunny months, I do get as low as $35 and if my trend keeps up this month, I may actually have a zero bill, with just taxes on it. But that is using the battery bank to not have to buy any of the 43 cent power.
 
That does look like a very good return on your system. To truly zero out my electric utility usage, I would need nearly double the solar panels. I don't quite have that much room. I am hitting close to zero use in the colder but sunny days, but my heat is natural gas.
 
I put in excess PV capacity, and get credits/charges that vary with time of use. I have enough surplus to run electric heat rather than gas.
At most recent annual true-up, there was still a credit of $1500 based on retail rates. For a time I avoided electric heat during peak hours, until I realized there would be unused credits. I also had surplus kWh (production minus consumption) that paid a $60 cash credit at $0.025/kWh, which covers 6 months connection fees.

Better utilization would come if I had more off-peak loads, like charging EV. Used to be that off-peak could give me more kWh than I produced, but hours of peak time changed last year. Not sure what surplus credit I'll have with a full year on the new schedule. This past year's unused credit would have bought 10,000 kWh at off-peak, enough for 30k miles of Tesla charging.
 
I put in excess PV capacity, and get credits/charges that vary with time of use. I have enough surplus to run electric heat rather than gas.
At most recent annual true-up, there was still a credit of $1500 based on retail rates. For a time I avoided electric heat during peak hours, until I realized there would be unused credits. I also had surplus kWh (production minus consumption) that paid a $60 cash credit at $0.025/kWh, which covers 6 months connection fees.

Better utilization would come if I had more off-peak loads, like charging EV. Used to be that off-peak could give me more kWh than I produced, but hours of peak time changed last year. Not sure what surplus credit I'll have with a full year on the new schedule. This past year's unused credit would have bought 10,000 kWh at off-peak, enough for 30k miles of Tesla charging.
You obviously aren't on PGE
 
Hedges, Can u give a brief summary of your PV setup?

I started with SMA SWR2500U inverters fed by either 24s1p AP120 or 9s2p NE-Q5E2U, for grid-tie.
The inverters combine in a 225A QO breaker panel, go through a 100A visible-blade disconnect, and land on a 70A breaker in another 225A panel with 200A main breaker.
That was a grid-tie only system for 17 years.

I've recently inserted SI-6048 2s2p with 8x 6V 405Ah SunXtender (20kWh). Those have 2-pole Schneider 63A DIN rail breakers on input and output.
Replaced GT inverters with 10000TLUS for compatibility with Sunny Island. Replaced again with 5000US due to issues driving a VFD.
Added SPR-327NE 8s, with different orientation, to make power for more hours.

House is fed output of Sunny Island as UPS, with a load-shed relay to disconnect at 70% DoD.
An interlocked breaker allows bypassing Sunny Island to put house direct on grid.

Another interlocked breaker disconnects grid from main panel and lets Sunny Island feed garage as well.

I have an electric duct heater so I can throw a switch to use electric vs. gas.
I need to wire a signal so that automatically deselects when grid down, so I don't drain battery in 80 minutes.

When running off-grid, A/C and all other house loads are fed direct from PV --> AC
Battery charging is set to 0.2C
Battery barely lasts the night with multiple refrigerators/freezers, yard lights etc. Auto disconnect of those loads would help.
 
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