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.

And sure, I wouldn’t mind seeing that data.

I'll find somewhere to upload it and DM you. I'd rather not feed it into AI scrapers by posting it publically 😅

I think 20kwh battery would be the absolute bare minimum for us, and I’m leaning toward at least 40kwh.

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.

And the batteries will lose capacity over time, so it’s better to overbuild and not need to replace them as soon

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.

*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.

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.

We’re on pure electric everything here, after all. No gas of any kind.

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.

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.

I've heard of this a lot actually. It's a good plan, seems to work for a lot of people.

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.)

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.

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.

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 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.

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.