It’s been two weeks since the hurricane passed by and screwed up my analysis of the new solar system. Let’s look at the numbers from the past two weeks’ production:
Overview
The house used 621.4 kwh during the period, or an average of 44 kwh per day. The lowest demand day was 36.2 kwh, and the highest was 60.9 kwh. The system generated a total of 663 kwh, which works out to an average of 47 kwh per day. The low was 36.6 kwh, the high was 57.6 kwh.
Overall, it appears as though my system is properly sized as far as generating capacity. How well will we do for the main intended use? That depends on storage capacity. As I said, I have been using the power company as a battery of sorts, in that I send power to the grid during the day, then take power from the grid at night. That begs the question: What if the grid is down? To answer that, we have to dive a bit deeper into the numbers.
A Closer Look
Our highest use days are when we are both home, and when it is hottest outside. This isn’t surprising, since the air conditioning runs more on hot days, especially when we are home. On top of that, the clothes dryer uses a HUGE amount of power, and laundry days are also heavy draw days (the dryer uses more power than both air conditioners combined).
The average solar intensity at my location for any 24 hour period is 200 watts/square meter for an average production of 47 kwh. The best day for production was 60.9 kwh, with an average solar intensity of 232 watts/ square meter. No surprises there- 16% more sun gets you 30% more power.
The good news here is that the hottest days also tend to be the sunniest days, so heavy use days caused by the cooling system are also the highest production days, with the result being the largest daily deficit was only 7.4 kwh. The largest surplus was 13.4 kwh.
What this means is that, should the grid go down, this surplus is likely wasted power. In our case, we would be wasting about 3 kwh per day. Saturday, August 10, and Sunday, August11 were our worst two days:
Saturday, August 10
39.9 kwh produced, 45.9 kwh used, 6.0 kwh deficit
The lowest outdoor temperature that day was 77 degF, with the high being 92 degF. Average solar intensity was 166 watts/square meter.
The air conditioner compressors ran for 6.5 hours
During the day on Saturday, we did 2 loads of laundry.
Sunday, August 11:
44.2 kwh produced, 51.6 kwh used, 7.4 kwh deficit
The lowest outdoor temperature that day was 77 degF, with the high being 93 degF. Average solar intensity was 190 watts/square meter.
The air conditioner compressors ran for 6.6 hours
During the day on Sunday, we did 3 loads of laundry. One of them was bedding, which takes more time and power to dry.
What about the batteries?
The day that saw us import the most from the grid was Sunday, August 18. We imported 25.9 kwh from the grid on that day, with most of that being the hours of 6 pm to 10 pm. This was while we were doing laundry.
My Analysis
We are producing plenty of power for our needs. We are keeping the batteries charged at 85% so that there is plenty of power available for a grid outage, and we are using the grid like an extra battery, storing our excess production during the day in the grid for use at night. If the grid were to go down, we would likely have enough battery power to run the house as normal, but we would likely have to restrict the activities that use the most power (laundry, cooking) for daylight hours. Additionally, there would have to be a restriction on the amount of laundry- no more than one load a day.
Other than that, we appear to have sufficient total capacity, and sufficient battery storage to run the household without interruption.
Interestingly, as a side note: The area near my house has a lot of solar power. Doing the area survey with my drone, I noted that 23% of the homes within a mile of my house have solar systems installed.
15 Comments
Grumpy51 · August 20, 2024 at 11:29 am
Good information.
and interesting use of a drone….. expect others to use it in “grid-down” situations to determine where potential power is…….
lynn · August 20, 2024 at 2:12 pm
Knowing you, you probably wrote an analysis of the materials, controller, and software you used. These are at:
https://areaocho.com/designing-our-system/
https://areaocho.com/panels/
Did the electric company ever start paying you for your excess power ?
Divemedic · August 20, 2024 at 2:23 pm
How that works is that you sell back your excess for 1:1 credit. So let’s say that I send back 100 kwh in a month, but I used 120 kwh from the grid, that 100 that I sent in would offset my bill so that I only pay them for 20 kwh.
If I sent back more than I used, that carries over from month to month. If, at the end of a year, I still have credits left, the power company buys them at the wholesale rate, which is currently 3 cents per kwh. That’s why it isn’t good to have a system that is far too powerful, or else you paid for capacity that you don’t use. Waste of money.
The sweet spot is to generate slightly more than you consume, and have enough battery power to last you through the night. I think I am there now.
lynn · August 20, 2024 at 2:32 pm
My neighbor two streets over has not seen a penny from the electric company in two years since they installed their $120,000 system with custom batteries. We are in the Houston, Texas area. Our electric distributor is claiming that they do not have the engineering experience to analyze his system so he is just dumping excess power to the grid and getting nothing for it. Sounds like a scam to me. He did have power all during hurricane Beryl while the rest of us were running our generators.
Divemedic · August 20, 2024 at 3:38 pm
I don’t know what to tell you about this situation. This is an anonymous guy from 5 states away and the story has no real details.
IcyReaper · August 20, 2024 at 5:09 pm
Forgive me if this was in a previous post but where are your powerwalls located?
lynn · August 20, 2024 at 5:44 pm
Nah, I still am comparing a whole house generator to a solar panel / battery system. Almost five years ago, I was quoted $65K for solar power / battery system. We never got past that figure because I was quoted $25K for a 38 kW natural gas fueled liquid cooled generator, installation included. I judged the two prices and jumped on the natural gas generator. I figure that I have five year of life left in it.
I use about twice as much electricity as you do, here in south Texas, mostly on air conditioning. It hit 104 F here today. I have a 4 ton and a 3 ton a/c units in a 3,300 ft one story. But I also have to plan for 6 F (low temperature on Christmas Eve, 1989 for 48 hours) so we have need for heating for about 90 days per year. I use natural gas for my air heating, water heating, kitchen five burner cooktop, and whole house generator. We were on the generator for three days after hurricane Beryl which used $100 of natural gas.
So I keep on watching the solar systems and waiting for the prices to come down. I figure that I would need 20 kW of panels and four Tesla Powerwalls. That is a lot of cash but I am not sure what the cost would be here.
Divemedic · August 20, 2024 at 7:54 pm
That system you talk about is double the size of mine, so you are looking at $80k worth of solar. You would get $24k of it back as a tax credit, making the final cost at $56k. One thing to consider is that you won’t get an electric bill with a solar system, so it does offset the cost somewhat.
Some Random Guy · August 20, 2024 at 10:17 pm
As far as laundry goes..most of the usage is the dryer. Since you live in a sunny state, maybe put up a clothesline to skip the heavy usage part when there ia an outage?
Southern NH · August 21, 2024 at 7:27 am
My two cents…. Dry laundry outside. Takes about 10 minutes to hang. Will dry in several hours, or overnight, depending on conditions. I haven’t used a dryer in 20 years. We don’t have anything thing that needs immediate wash and dry requirements.
Divemedic · August 21, 2024 at 7:54 am
You don’t live in the humid south. The Dewpoint in southern NH right now is 54 degF. Here in Florida, the dewpoint is 76 degF right now. That has a huge effect on drying time, which means that clothes hung out to dry are often stiff, crusty, and smell moldy.
That doesn’t even mention the amount of insects here. Clothes left hanging outside, especially overnight, will be filled with bugs as well.
Southern NH · August 21, 2024 at 9:14 am
Yeah humidity is a problem here, but not as bad as yours. We also don’t have your bugs. Bugs here don’t ever end up on the laundry. In the winter I usually have to hang inside, in the basement or on a couple of wooden racks in the bedroom. A covered screened breezeway or porch might work for drying, if the humidity comes down.
Daniel K Day · August 23, 2024 at 1:19 am
Thank you for all these details. This is very interesting information.
Jim_R · August 21, 2024 at 6:54 pm
We line dry jeans, wool socks, bath mats, and new/nice shirts (for shrinkage) in the basement, but we have a dehumidifier because our basement would be humid anyway. It wouldn’t be worth it for your everyday, but you could dry a second load in a grid down situation, if you really needed to.
Jim_R
Divemedic · August 21, 2024 at 7:06 pm
I could. I figure that, should the grid go down, one load of laundry a day will suffice.
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