I’m planning to cut my grid connection and go full off-grid. Which raises the question of how much capacity I really need. And what you see here is me trying to answer that question based on ~51,000 5-minute data points of usage data my power company provided to me, complete with custom-made python script to parse all that data and pretty charts! (Yes, I have 6 monitors. And this kind of shows why.)
And what I’m really torn about now is how much battery capacity I need. Should I get enough to cover the absolute worst case scenario (~66kwh) or is something closer to my average overnight usage (~20kwh) acceptable?
Not the absolute end of the world if the batteries run dry and I need to temporarily supplement with generator backup, I suppose. Nobody is life-or-death depending on any electrical appliances here, and even on the coldest days, waiting a few minutes to go start a generator isn’t a huge deal.
But definitely would like a word from the wise about how much battery capacity I really need. And also how much solar panel wattage I need, I suppose.
A) Ah ok, well good that you’ve thought it through. I had disconnecting from the grid in mind when we fist did ours, and it turned out to be the wrong approach, but our situation was quite different.
B) That seems weird but ok, some power companies here don’t offer anything related to solar but we have a national spot price buy/sell system so almost anyone can start a power company selling the grid power, and you can use almost any power company from almost anywhere, but I know that we’re lucky with that. We are limited (everyone is limited) at how much we can sell at a time in order to keep the power line voltage within tolerance, which I guess your power company handles in another way.
C) Ah that sucks. We get paid 2/3 of the value of buying when selling to the grid - though it is in credit. I haven’t worked out what happens when we never use the credit but it doesn’t seem to get arbitrarily reset!
Yeah it sounds like you’ve got some strong arguments for it!
Well, I like my power bills because most of the year they are credits 😆
It seems like most of my experience might not be helpful to you. I’m willing to send you a year of power generation data from Home Assistant if you think it would be helpful? It would show you the variation day to day and might help you see what kind of impact there is with poor weather.
At a guess I’d say I don’t think one 10kWh+ battery would be enough to go off grid. Two might cover most situations if you have enough panels, don’t live in a super cold area (or have non-electric heating), and do power hungry things only in the daytime (charge car, run dishwasher, run clothes dryer, etc). Also an electric stove can use more power than you might think, cooking a roast dinner can chew through half our battery no trouble. Plus it will work out better if you shower in the morning (so heating mostly happens when the sun is shining). Three batteries still won’t cover every day but I think if you have a generator available and are careful about timing during the winter period then it should get you through most of the time.
Solar panels are pretty cheap now, so don’t under do it. Load up on your daytime capacity and use the power then, to reduce your reliance on the still expensive batteries.
Maybe make the call based on the lead time to order the components. If you find you didn’t get enough capacity, it’s easier to solve if you have a 2 week wait to get some more panels and an extra battery vs if you have to wait 6 months.
Sorry, not sure if I’m helping 😅
Nah, don’t worry – that’s helpful! And sure, I wouldn’t mind seeing that data. Though our situations are pretty different, more data couldn’t hurt, right?
Heh, yeah. I think 20kwh battery would be the absolute bare minimum for us, and I’m leaning toward at least 40kwh. (Especially when you factor in other things – the conversion from battery power to 120v/240v is only ~80% efficient, depending on inverter.* And the batteries will lose capacity over time, so it’s better to overbuild and not need to replace them as soon.)
*Another upgrade path for the future is to run a DC power circuit through the house, at least for some lights and little stuff like phone/tablet chargers. DC power will be significantly more efficient because you don’t have to go through that 80% efficient inverter to get it out of the batteries. Maybe I can even manage to find a heat pump that runs directly on DC.
We’re on pure electric everything here, after all. No gas of any kind.
For the water heater in particular, I’ve already got a plan: going to put that sucker on a timer, so it can only run during the daytime. Actually already got a $1 thrift store timer, big old electro-mechanical thing, that should be able to do that job. A decent hot water heater will stay hot for days without power, and it’s just two of us here, so it won’t run out too quickly. Let it heat up during the day and then disconnect at night, and the water heater won’t be a drain on the batteries at all. (That works well with our existing routine of showering in the evening, too. It should be nice and hot after running all day, and if it stays cold most of the night, no big deal. In fact, after some experimenting to see how quickly it heats up from dead cold, I might not even have it run during all daylight hours, just start it up a few hours before we usually shower.)
To some degree, I might try to do the same thing with heat, using programmable thermostats to raise the temperature during the day and then reduce heating demand during the night. Thermal storage is a lot cheaper than battery storage, after all. Who knows, might even go super crazy and actually build in some water tanks that can be heated during the day and then release that heat at night. Got a bunch of old 55 gallon drums sitting around I’m not using for anything else … just need some heating elements and timers… (But, yeah, that part would definitely be a project for later. And still not sure I want big drums of water sitting around in our living space, especially because they’ll be nearly useless in the summer.)
If it gets really desperate, I might even put the refrigerators on timers. They should be able to stay cold overnight okay, and then they can run all they want during the day.
I had an electric water tank for some years, it did only heat during 4h a day splitted in 2 morning and 2 afternoon, thats more than enough. Just install a temperature logger so you keep track of the temperature. In terms of batteries, they are too expensive for a normal use, invest on a few more panels and try to reshape your consumption.