I have a smaller, but similar set up, and asked the same questions for 2 years before finally pulling the trigger on an install.
I have 12.8kw across 32 panels, and will have about 20 kw between 4 batteries of battery backup (Batteries have been backordered since September, waiting to be installed any day now). I also have a 3 ton central A/C unit, and I live in South Georgia near the Florida border; 80+ degrees most of the year, with cold snaps of 30 degrees for a few days between december and march.
I agree with Paul, there is a lot that goes into answering this question. What I was able to do, is my power company allows me to download my usage, broken down by 15 minute intervals. I was able to look at my highest usage day each month of the year, and break down how much energy I used during the day and during the night on those days using the data from my power company. This helped me get a better understanding of my day vs night energy use, which for me turned out to be fairly consistent 60/40 split (60% of my energy consumption was during daylight, 40% was at night). I was told by my installer that's a fairly common breakdown. You really need to know this information to be able to determine how much battery capacity you need. Mine was available online, but one of the techs was also able to send it to me in a huge spreadsheet. Capacity alone isn't the issue though.
I wish I had my batteries to give you a better answer, BUT, I will say with my 12.8kw of panels I am overproducing electricity by a lot. Now, I went through and changed EVERY single light in my house, including microwave, fridge, exterior perimeter lights...EVERY light is now LED in my house. I bought a $2,000 heat-pump water heater that I program to only turn on twice a day, which has been a HUGE energy saver (was using 15kw daily just on my hot water heater, now I use about 1.5kw daily). I bought a better, wifi thermostat I can control and program better to not waste energy during the day. I also installed energy monitors on almost my entire panel box and can monitor real time and record energy by each circuit.
I'm averaging using about 10-15kw during the day to power my house, which is significantly less than I have ever used. My inverter is capped right now at allowing my panels to only produce about 9.5kw at any time (when my batteries are installed it will allow the full use of the 12kw I'm told). We did not change our lifestyle, just changed 1 major appliance to be much more energy friendly and we have been slightly more mindful about our energy consumption: We try to wash/dry clothes during the day, cook a little earlier so we are on solar power. But, nothing major. If I need to cook or wash clothes at night, so be it. I'm only monitoring my consumption during the day when my inverter is active right now (I really should use my energy monitors to get a better idea of night use, but i havent yet), but I'm over- producing 45-60kw that I am selling back to the grid daily, on top of powering the other 10-15kw my home needs throughout the day all through solar. Even if I consume 20kw of electricity from the grid at night, I am still way ahead (My co-op buys energy back at 1/2 the price it sells, so I need to over-produce 2x the energy I consume at night to break even). I probably only use 10kw at night, meaning i would only need to sell back 20kw each day to break about even, and I'm doing way more. Even on cloudy days I'm able to over-produce by 20kw+. I use 20kw because that is my battery size, so kind of what I am eyeballing as my target for making sure i stay under each night, and making sure i can over-produce each day to re-charge my batteries when I get them.
This has actually changed my entire mindset about how I will use my batteries when they are installed. My plan was similar to yours, use the batteries at night until they are depleted and only use the grid as a last resort, then re-charge all day instead of selling back to the grid. That has been my plan for 2 years while I did research. Now however, I think I am going to keep my batteries as a back-up, and only use them a few times a month to move energy through them...Assuming when the temperatures rise and we are using our central A/C more I'm able to continue to over-produce enough to cover night time usage. I was told that during the winter would be my least cost effective time of year with my panels, and when I may still have an electric bill...but I'm absolutely killing it right now with over-production. If this keeps on trend, I'll have no problem over-producing when the days get longer and i'll have no reason to use my batteries every night.
I'll update in the next week or 2 when I get my batteries installed. The biggest problem with batteries is running too much at 1 time: stove, central AC, water heater... You will need 2 inverters to be able to run it all at 1 time off batteries. Example, I believe my solark 12k inverter allows 9kw continuous power from battery. BUT, if you're just using the grid at night and selling back during the day, that isn't an issue. The problem isn't battery capacity, it's the inverter. You would need 2 inverters to run more than 9kw at any 1 time, and at that point you're going to be depleting your batteries very quickly. It's just easier (I think, this will be tested in the next few weeks for me) to over-produce and sell back during the day, and only worry about the batteries if the grid goes down or SHTF scenario. But, you would have to make sure you can sell back 19.8kw at a time. My co-op is limited to selling back 10kw at a time, so 19kw of panels would be a waste of money. I'm not even certain my 12.8kw wasn't a waste. I'll know once i get my batteries if I can actually tap into all 12kw at any given time like my inverter says.
Hopefully that makes sense.
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Robert Edwards
b1edwards@bop.gov------------------------------
Original Message:
Sent: 01-11-2022 01:20 PM
From: Barry Rosenberg
Subject: How much can you realistically power from solar panels?
We are looking at a Sun Power 19.8kW system with a 26kW battery back up. How much could we realistically power at night with this system. We have two regular heat pumps with out heating strips and one mini split heat pump without heat strips. Can't get actual numbers from sales person on what we can run at night.
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Barry Rosenberg
British Car Service
britcars@bellsouth.net
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