Can I get some last minute advice before signing contract?

[mikedunkel]

Hello and thanks for looking,

I got quotes from 4 companies (3 local and Tesla) and selected a local company to do my grid-tied system and I'm trying to determine if I need to go with a single or 2-inverter build. My top priority is performance and my 2nd is cost and as everyone knows these two priorities are in direct conflict with each other. I'm willing to spend what it takes to achieve my goals but I don't want a system that has so much excess capacity that I'm constantly selling power back to the utility at pennies on the dollar (my EMC pays like 2 cents a KWH vs charging me 10 cents/KWH).

My desired system performance is to basically run the entire house during a power outage as if there wasn't a power outage, assuming daytime and full system performance. Last year my very highest hourly usage was 10.24 KW, the year prior it was around 12 KW. Most of the time my usage is significantly lower, and in addition to adding solar I'm getting new windows, going from blown to spray-in foam in the attic, and filling-in my 30,000 gallon swimming pool (2 pumps) so there is going to be an unknown but fairly significant decrease in my consumption.

The 2 systems my installer has proposed are a 13.1KW system with 2 StorEdge SE7600 inverters and 2 RESU16H batteries, and an 11.3KW system with a single SE7600 inverter and 2 RESU16H batteries. His initial recommendation was for the smaller system and only a single battery but I told him that having power in the event of an extended grid failure was more important to me than ROI so this company is not trying to gouge me, the extra battery and a quote for a bigger system were at my request and not their prompting.

So, will the single inverter be able to handle my desired output or will I need to spend another extra $9k (pre tax credit) for the additional inverter and 1.8kw of panels?

BTW, I am NOT interested in what brand of panel, inverter, or battery you think is better than what I've selected. I've gotten my quotes and my decision is final regarding the company and products. The only question is how big I need to go .

Thanks for your time.

[sabersix]

I think the smaller system. My reasoning is that a long power failure event is usually associated with some sort of weather event or fire that will inhibit your solar production. You will be relying on your batteries which are the same size for both systems. Recharging those batteries can be done by either system with little difference.

The question to me is do you need a 13.1kW system for your normal day to day usage? As you noted, you will only get pennies on the dollar for oversizing.

[mikedunkel]

I think the smaller system. My reasoning is that a long power failure event is usually associated with some sort of weather event or fire that will inhibit your solar production. You will be relying on your batteries which are the same size for both systems. Recharging those batteries can be done by either system with little difference.

The question to me is do you need a 13.1kW system for your normal day to day usage? As you noted, you will only get pennies on the dollar for oversizing.

Its more about the inverters than the system size. The difference in cost between panels in 11.3 and 13.1 isn't that large but the 2nd inverter adds quite a bit of cost. I'm just not convinced that a single 7.6 KW inverter can handle 2 3-ton air conditioners, 2 refrigerators, freezer, well pump, microwave, and all the miscellaneous lights, fans, and outlets my family uses on a day to day basis. Obviously I'll be able to manually manage some of this if push comes to shove.

A weather event can certainly take down power lines, but those lines often stay down longer than the weather event. A thunderstorm or tornado can be over in 30 minutes and the remains of an ice storm can melt the next day (I live near Atlanta) but power has been down for as long as a week (heck, I think parts of TX were down for 2 weeks this past winter). The batteries will get me through the initial event and night time and depending on WX forecast I can prioritize the food storage over creature comforts as required.

[sabersix]

If the power is out, you won't be able to run your A/C as that is a huge load. Your house will be powered from your batteries until they are depleted. Running on battery provides a constant level of power until you are on solar only. Solar power can vary significantly during the day so any dips below your demand is going to put strain on all your running electronics. Solar alone is just not a cost effective backup. You have the battery capacity. Why not add a generator instead of the second inverter. The generator can work in with your solar to recharge your batteries and keep the house going.

StorEdge with Generator.PNG

[mikedunkel]

1) Your house will be powered from your batteries until they are depleted.../...

2) Why not add a generator instead of the second inverter. The generator can work in with your solar to recharge your batteries and keep the house going.

StorEdge with Generator.PNG

I'm glad you brought these points up.

1. The reason I passed on Tesla is that once I lose the grid their panels completely stop producing electricity and I'm down to whatever my powerwalls have stored. The local company said that even with the grid out, their panels continue to produce power -- I'm not sure of the specifics; whether the panels power the house or they power the batteries which power the house, but the end result was I'd have an indefinite source of power albeit perhaps not enough to run everything at once. Are you telling me my local company is lying about what their system can do in the event of a grid outage?

2. In a system with a generator, are they smart enough to prioritize using excess solar power over the generator? Are the generators variable speed so they aren't going full-bore if I only need a few extra KW to git'er done? The cost of a 5-7kw generac is less than the excess cost of the 2nd inverter and additional panels so this could be a viable option.

[sabersix]

1. I don't think they are lying, but once your battery is depleted you will be left with a highly variable power source. The key will be managing your load so as not to fully deplete your battery. Do you have a critical load panel as part of your home electrical system? Here is an interesting article that discusses your system in real use. https://willardm22.medium.com/my-exp...e-3b306348a233

2. The generator will be whatever you purchase. The Backup Interface seems to be the smart piece of the system that manages where and for how long the power is drawn.

[J.P.M.]

Its more about the inverters than the system size. The difference in cost between panels in 11.3 and 13.1 isn't that large but the 2nd inverter adds quite a bit of cost. I'm just not convinced that a single 7.6 KW inverter can handle 2 3-ton air conditioners, 2 refrigerators, freezer, well pump, microwave, and all the miscellaneous lights, fans, and outlets my family uses on a day to day basis. Obviously I'll be able to manually manage some of this if push comes to shove.

A weather event can certainly take down power lines, but those lines often stay down longer than the weather event. A thunderstorm or tornado can be over in 30 minutes and the remains of an ice storm can melt the next day (I live near Atlanta) but power has been down for as long as a week (heck, I think parts of TX were down for 2 weeks this past winter). The batteries will get me through the initial event and night time and depending on WX forecast I can prioritize the food storage over creature comforts as required.

I'd start by removing Tesla from consideration. Sounds/Reads to me like even though cost may not be your primary consideration, I bet most bang for the buck is, and that includes equipment and installation quality considerations. Staying with local, quality, established vendors is as important a consideration as equipment quality. You'll get neither with Tesla.
After that, and before I made any decisions on which of the remaining vendors to choose, and even though it'll be a bit of a SWAG, I'd get a better handle of expected annual load, and if you havn't done so already, as well as getting a plan for how to reduce your emergency power outage load, starting with living without or greatly reducing the big draws for A/C while the power is out. Doing both those things will perhaps reduce not only your array size but also maybe reduce your excess generation which seems to be a somewhat secondary goal.
Your goal of full power outage operation is certainly a choice, but it'll likely increase system acquisition costs by a significant amount. Depending on how often, or your angst for power outages, my guess the cost of perceived safety may be higher than you might anticipate.
I'd size and price a system with and without the emergency load considerations and decide if the cost differential is worth it to you. If not, I'd consider getting serious about your push come to shove plans and revise your emergency load criteria.
One usually cost effective approach and an adjunct to serious emergency load reduction is a generator.

[mikedunkel]

I'd start by removing Tesla from consideration. Sounds/Reads to me like even though cost may not be your primary consideration,../...
Your goal of full power outage operation is certainly a choice, but it'll likely increase system acquisition costs by a significant amount. Depending on how often, or your angst for power outages, my guess the cost of perceived safety may be higher than you might anticipate.
I'd size and price a system with and without the emergency load considerations and decide if the cost differential is worth it to you. If not, I'd consider getting serious about your push come to shove plans and revise your emergency load criteria.
One usually cost effective approach and an adjunct to serious emergency load reduction is a generator.

Not sure if you saw my OP but Tesla is already out of the discussion, the only reason I mentioned them was to show that 1 system turns off when the grid goes down and a 2nd does not. I agree 100% with your sentiments about local companies and those were a significant factor in my decision. I asked the local vendor to requote me the smaller system, eliminate one of the RESU16 batteries, and replace it with an LP generator that could run all my big ticket items. Since they have to do all the electrical rewiring anyway, I suspect the generator will be a lot less expensive (and probably more effective) than the 2nd inverter and battery.

[J.P.M.]

Not sure if you saw my OP but Tesla is already out of the discussion, the only reason I mentioned them was to show that 1 system turns off when the grid goes down and a 2nd does not. I agree 100% with your sentiments about local companies and those were a significant factor in my decision. I asked the local vendor to requote me the smaller system, eliminate one of the RESU16 batteries, and replace it with an LP generator that could run all my big ticket items. Since they have to do all the electrical rewiring anyway, I suspect the generator will be a lot less expensive (and probably more effective) than the 2nd inverter and battery.

Understood. I missed the Tesla removal. My bad.

Good luck.

[theoak]

I'm not a fan of 11.3 kW of panels on a single 7.6 kW inverter.

[mikedunkel]

I'm not a fan of 11.3 kW of panels on a single 7.6 kW inverter.

Can you elaborate? The local company said the inverter is speced to handle 150% of its rated power.

[sabersix]

Can you elaborate? The local company said the inverter is speced to handle 150% of its rated power.

You will get some clipping with the smaller inverter as it will not go much above 7.6 kW. You system is effectively a 7.6 kW system. There is really no point to 11.3 kW worth of panels unless you are in a sub-optimal solar location (which doesn't seem to be the case for GA). You should reduce the number of panels and save even more money.

On the other hand, two 7.6 kW inverters is way overkill for 13.1 kW of panels. You will probably only get 11 kW peak out of those panels. Have you run a solar calculation using PV Watts https://pvwatts.nrel.gov/ for your area?

[solarix]

I'm not a fan of 11.3 kW of panels on a single 7.6 kW inverter.

If you put at least a third of the 11.3kW array at an orientation that is at least 90deg different in azimuth from the main array, there will only be a few days of the year when "all the stars align" and you will get a bit of clipping. The inverter is always going to protect itself from an oversize array by setting the operating voltage to something non-optimum - so you are not going to hurt the inverter no matter how big the array is. I design a lot of systems to be larger than 10kW on a 7.7kW inverter with no problems. With the utility going away from net metering, we have to go with bigger arrays to compensate (no problem now that PV panels are so affordable), but the 20% backfeed rule in the Code is a real hindrance to large systems. By "overpanelling" the inverter, large systems can be done without having to do dual inverters and expensive service panel upgrades.

[theoak]

Typically you oversize in the 1.2 range. This is over 1.5. Anything over 1.3 and you essentially hit diminishing returns. For a 7.6 kW inverter I would be looking at around 9.1 kW worth of panels. Now, unless because you are doing a battery, there is something special that the battery needs that can justify that oversizing. Shading or odd roof peaks may justify this too. Otherwise, you could save a good chunk of money.

I think SolarEdge allows a max of 155%. It almost seems like your installer is just going for the max here???

As noted though, if you have some sort of split array in multiple directions, then I would not worry about it.

[mikedunkel]

Thanks for these inputs, they are very helpful. He came back with the generator quote and something is amiss -- $15k for a generac generator!!!! The most expensive residential application is only $5700 and the 10KW which should be sufficient for my needs is about half that. The 48kw commercial generator is about that price so I think there was an error in the quote.