big loads offgrid with a small battery (sidelined)
No matter that the hype cycle wants you to think, the renewable energy<br>transition is the biggest thing happening in tech and it's happening faster<br>and faster. Despite being neck deep in it personally with offgrid solar<br>projects, most recently solar hot water, increasingly it becomes clear I'm<br>watching from the sidelines.
In Australia,<br>everyone gets 24 kwh of free daytime electric power now.<br>That's without installing any solar panels of their own, the grid just has<br>that much excess capacity. All it takes to save $thousands per year (and<br>avoid emissions) is to schedule some big loads like the hot water heater<br>and EV to charge during the day. To save more, drop in a home battery<br>that charges for free and powers the home through the evening.
In Germany, a 2 kwh plug-in home battery costs $350 and the electric<br>company will pay you<br>$130 per year to plug it into your wall.<br>There are similar offers throughout Europe.
In Cuba something something geopolitics, oil blockade, belt and road =><br>suddenly 1GW of solar farms with another gigawatt on the way.
I'll soon visit South Carolina where with no subsidies whatsoever from a<br>decidedly renewable-unfriendly government, it made sense for my dad's house<br>to get a whole home battery and double the solar array. The resulting<br>system will be able to power the well pump and probably also the whole<br>geothermal HVAC system through the kind of month-long grid down events that<br>happened in Hurricane Helene.
Myself, well, I've got a by modern standards small 4 kwh home battery that<br>powers my house offgrid, and I've recently installed a heat pump hot water<br>heater. That's after about a decade pondering what solution to use for<br>solar hot water, to replace an aging and horrible propane instant water<br>heater. I've in the past considered everything from evacuated tubes to<br>special direct drive inverters to DC resistive MPTT dump loads. The solution<br>turned out to be just a big enough solar array, and plugging in a 120v hot<br>water heater that needs only 500 watts in heat pump mode. Plus a small<br>amount of code to manage when it runs.
In the time I was thinking about that, economies of scale and tech<br>improvements just wiped all those other possibilities off the map, it's not<br>economical to install and maintain a separate evactuated tube heat<br>collector when a pile of solar panels costs so little and when electric<br>hot water has gotten more than 200% efficient.
I also recently completed my permanant EV charger installation, with a new<br>inverter and conduit and proper wiring, and increased the car's charge rate<br>to 2 kw. Eliminating the need to charge anywhere except at home except<br>on road trips.
Coordinating when these two big loads run, to maximize solar production and<br>ensure that the house battery is full at the end of the day was ... not<br>hard at all actually? The car charger amps can be dialed up and down to<br>match incoming solar power fairly well, and leave some room for the hot<br>water heater. They both operate as more or less dump loads. More or less<br>because neither one can be cycled on or off very fast (to avoid wear and<br>tear on the car's contactor and the heat pump's compressor), so it makes<br>sense to leave them on and skate through short cloudy sections of the day,<br>as long as the house battery doesn't get too low.
How low is too low for the house battery? Depends on the time of day. The<br>code it's currently using, which may get tweaked over winter:
-- When the battery is charged enough to run major loads that may prevent<br>-- charging it further.<br>-- This varies with the hour of day. Early in the day, the battery does not<br>-- need to be as full to be considered well charged, since there is<br>-- still plenty of time for it to charge up. Later in the day, with less<br>-- time to charge, it needs to be more full.<br>wellCharged :: Hour -> Percentage<br>wellCharged (Hour hour)<br>| hour 9 = Percentage 90 -- night<br>| pmhour 0 = Percentage 50<br>| pmhour 1 = Percentage 60<br>| pmhour 2 = Percentage 70<br>| pmhour 3 = Percentage 80<br>| pmhour 4 = Percentage 90<br>| otherwise = Percentage 95<br>where<br>pmhour = hour - 12
More complicated is, what to do it there's solar power to run one or the<br>other, but not both? This is starting to get into the territory of<br>microgrids now, or of demand response programs, so there's a whole industry<br>or three out there doing industry things geared at the kind of no-brainer<br>solutions I mentioned earlier. From what I've gathered, all of them<br>involve proprietary protocols and gear.
What I've done is to read the state of the hot water heater and car, and<br>prioritize hot water over the car. Except, if the car is below 10% it<br>urgently needs to charge.
And I found a really simple way to decide when to run the low-priority<br>load: Just check if the house battery's current charge will be considered<br>wellCharged in an hour. So if it's 2 pm, the battery needs to be 80%<br>charged to run the lower-priority load, and if it dips below that, that<br>load will turn off but...