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.
I’ll find somewhere to upload it and DM you. I’d rather not feed it into AI scrapers by posting it publically 😅
Don’t underestimate how expensive that will be! Ripping all your electricals out and re-doing it to code will start to look like the cheaper option! It’s not unusual for one battery to be more than half the cost of a solar system. If you have the money it will give you peace of mind, but you don’t need the capacity for daytime usage as you’ll just go straight from solar panels through inverter to what you’re powering.
Batteries are a quickly evolving technology, you may well save money or gain higher capacity by only buying as you need the capacity (as the capacity of your batteries falls over time). Also, there is evolving technology in relation to cars too. A mid-range EV may have a 60kWh battery. Tesla is testing technology where you can plug your car in and have the car power the house overnight then charge in the the day when you have excess solar production. I would never suggest buying a Tesla but the technology sounds like a good idea and in 5-10 years it might be available across a wide range of cars.
Unless you have incandescent lights, I think that’s the wrong thing to focus on. Lights use hardly any power compared to heating. Heating air, heating water, heating your over, heating a clothes dryer. AC can use a lot too. If you can get those via DC that might help, but honestly that sounds expensive compared to just throwing on some more panels and using them in the day.
You can get heat pump hot water heaters, which will save energy. Ours also allows control for excess solar production, heating to a higher temp when solar production is high. By heating to 75 Celsius (the max) it will go all night without needing to heat again, and be about 65 in the morning. It can get to 65 with the heat pump, then has an element to get to 75 (less efficient but it’s only used for the excess solar anyway). It still comes out of the tap at 50 even if the tank is 75 so I don’t have to worry about scalding the kids or anything. I do this control via Home Assistant and a wifi connection but it is advertised as “solar ready” and has a cord that apparently can connect to a solar system to control that excess usage. It’s an all in one unit, the whole thing sits outside.
You can also get heat pump clothes dryers, and of course a heat pump for heating/cooling.
Depending on climate (I keep saying that as I don’t want to be responsible for you freezing to death), that could make a big difference. We are a family of 5, but the house is insulated in ceiling/external walls/under floor and it has double glazed windows. Yours is 100 years old. So I’m thinking that 40kWh may actually be a good amount of battery capacity for you.
I’ve heard of this a lot actually. It’s a good plan, seems to work for a lot of people.
If it’s a modern hot water heater, like you say it will stay hot easily. Let it turn on some time in the morning once the sun is up and strong, that way you have it hot if you need it and it’s unlikely to make too much difference in the power - especially since most of the year you’ll be making more than you can use.
This sort of thing used to be what people would install in their houses. Older houses around here often have “night store” heaters, which is where they are on timers for the cheap night electricity and they heat bricks within a metal cover that then leak that heat out the rest of the day. That sort of thing but heated in the day would be ideal, I have no idea if you can still buy that sort of thing off the shelf but your hot water idea seems like the same thing - just that you need that water stored inside, and one day (whether next year or in 30 years) they will rust through and make a big mess! If you keep an eye on it, then it seems like it would work. After all, it’s basically a radiator.
If you use one of the many online tools to mark your location and roof angle, it can tell you your estimated production over the course of a year. Inverters tend to only handle 10kWh production at a time (with short term spikes above that), but if you can generate even an average of 5kWh over 8 hours in winter that’s 40kWh of power. It’s hard to imagine 2 people using more than that outside of very specific events, in which a backup generator will likely be cheaper than another battery (if you don’t have one already) and can be filled up again even if the sun isn’t shining (which isn’t the case for an extra battery).
Right, I’ll find that solar generation data and send it through.
The Ford Lightning does that, too, without having to support the Muskrat. But a vehicle that new would be pretty far outside the budget right now.
Anyway, you’ve got me a bit curious now about the possibility of finding an EV battery in a junkyard and maybe using that as solar battery storage. Though … those usually run at pretty high voltages. Finding both charge controllers and inverters capable of dealing with those voltages might be difficult and expensive enough to negate any possible cost savings on the battery itself. Plus, then I’d be kind of locked into that type of battery when the time eventually comes to replace it, since that high voltage equipment probably won’t easily convert to a more usual 48V system.
Don’t worry, we’re hardy folk! Having parts of the house freeze over is actually kind of routine for us at this point. We only heat small rooms within the house, and through much trial and error, we’ve found freeze-resistant plumbing options. (We used to have basement floods every winter, but now we’ve converted the whole house to pex plumbing, which turns out to have good freeze resistance. Copper always developed cracks or pinhole leaks, steel would always burst a fitting, PVC would absolutely shatter, but pex has enough flex to it that it has been able to survive a few good freezing events without causing any leaks – just a temporary lack of water. And in those temporary freeze-ups, we have an outdoor frost-free faucet run directly off the supply line that always works, and we can fill from that for a day or two until the cold snap ends and the plumbing gets above freezing again.)
And there’s always the backup generator, which will cost a fortune to run, but will prevent anybody freezing.
Though it may worry you even more, I actually think I’m planning on putting that backup generator in the basement (with tightly sealed and double-wrapped exhaust pipe running outside, air intake routed from outside, a remote shutoff switch, and a dozen carbon monoxide alarms scattered around, of course!) But the advantage there (besides not needing to build and outdoor weatherproofing enclosure for it) is that most of the generator’s waste heat would then help heat the house, at least enough to keep the basement above freezing and keep most of the pipes from freezing. Since most of the times we’d need a generator are when it’s very cold, that could actually make it way more efficient, since most of its energy would be used for heating anyway.
Don’t worry about that, they’re plastic drums. The biggest concern would be that they might at some point freeze and burst … but that would never happen if I fill them with an antifreeze solution. The real biggest concern is just how much space they would take up … and possibly how much weight they’d be adding to the floors.
Yeah, may end up having multiple charge controllers in addition to the inverter’s built-in solar charge controller(s).
Personally, I’d prefer to have those systems be entirely separate in the first place, so each one can be sized according to need and each one can be independently replaced if it breaks. But it seems that most inverters capable of handling our ~6.2kw absolute peak usage already come as ‘all in one’ inverters with solar charge controllers built in … I guess I might as well use the built-in charge controller, at least for part of the solar array.
Yeah. Honestly, I have no idea how we ever got up to 6.2kw that one time… Our average usage never even gets close to 2kw. We’d definitely have to be running the oven at full blast, probably and a couple stove burners, and possibly the microwave/tea kettle at the same time … and probably some other big loads to even get that high.
But the nice thing about having all this data to look at is being able to see that those peak usages are very rare. Out of ~51,000 five minute data points, only 70 five minute segments had usage above 4kw, and never more than about 15 minutes in a row.
Even if we changed absolutely nothing about our usage, if we built the system to be capable of providing 4kw reliably, we’d only need to use the generator ~6 hours per year.
And, honestly, It just has to be a massive cooking task contributing to those absolute peak usage moments. Nothing else we have could possibly come close to using that much power. If we simply avoid using the stove/oven at night in the winter, that’s probably enough to drastically reduce our peak demand and significantly reduce our need for battery capacity.
Thanks!