This column isn’t about our blooming white hibiscus, but I was looking at it from the hot tub while thinking about this topic, and I didn’t want the visual to go to waste.
Ideally, an off-grid solar plant should be as convenient as grid power, minus the outages and hefty bills. We’re pretty close, in spite of the fact that we have an electric car (a huge load), an electric clothes dryer, three electric hot water heaters, a hot tub, and use electricity for our cooking (oversize toaster oven, a microwave oven, and multiple induction cooktops). It couldn’t happen, though, without some sophisticated load management.
So how do we do it?
inverter
Our inverter is a Sol-Ark 15K which I like, but despite it’s 15,000 watt rating I have found that it cuts out when the power draw exceeds 12 kW. I don’t know why. I’ve written to the installer but haven’t yet received any useful information. That 3 kW turns out to be pretty important, but we can get by without it. First, you have to replace the inverter’s WiFi connection with a Solar Assistant. It gives you much more detailed and timely information about the inverter’s operating parameters and exposes them to your local network via MQTT, a standard protocol for controlling devices over the internet. That allows me to interact with the inverter via Home Assistant.
Home Assistant
If we had done the usual thing and just hooked up our house and garage and car to the inverter and left it at that, we would have run into problems. Even if the inverter put out its full rated 15 kW, the car draws 8 kW, the dryer 6 kW, and the hot tub and each of the three water heaters 1.5 kW each: that’s 20 kW right there, without even doing any cooking. Still, it seems that 15 kW, or even 12 kW, ought to be enough. In fact, 12 kW is plenty as long as we don’t want to do everything all at once. At the same time, though, we don’t want to have to even think about whether we’re trying to do everything all at once. That’s where Home Assistant comes in.
Home Assistant is a free, open-source home automation system. You might know people who have their lights and stereo and motorized curtains and HVAC all connected to the internet so they can set a scene (close the curtains, dim the lights, put on an Isaac Hayes playlist, turn on the gas fireplace…) at the touch of a button. Perhaps you’re one of those folks, or perhaps you think that’s a bit over-the-top. But this isn’t just about automation for its own sake; this is about being able to turn on an induction cooktop without worrying that you’ll overload the inverter and cause power to fail to the entire house for two minutes until the inverter resets. Ugh. Do you want to have to look at the current load, add 1.5 kW, make sure you’re still under the limit, and then hope a water heater doesn’t fire up while you’re still trying to cook the pasta? No, absolutely not, even with a really cool dashboard from which to get the information.
Not everything has to be on all at once. The electric car (in my case, a vintage Tesla) can be controlled over the internet. No need to charge it at its full rate if we need the power elsewhere. The hot tub works just fine if it only heats when there’s excess power available. Heating water, in general, can be deferred. Let’s look in detail:
implementing Home Assistant
Home Assistant runs on a server you own, unlike most Internet of Things (IoT) services, so it can’t be bricked by a manufacturer who has lost interest in the product they sold you, and they can’t decide to start extracting rent for a device you thought you owned. Unless you’re very like Twoprops, however, you probably don’t have spare servers kicking around your garage. You can buy a Raspberry Pi (a computer about the size of two decks of playing cards) or other tiny computer with Home Assistant pre-installed from several vendors linked on the Home Assistant site. Plug it in, connect it to the internet, then you can access it through any web browser on your home network.
Home Assistant offers integration with (tens of?) thousands of IoT and home automation devices. Of course, many of the greedier companies have locked out any kind of third-party access, but it’s surprising how many you can control and bypass the risks, limitations, and costs of using the manufacturers’ controlling apps and web sites.
charging the electric cars
Teslas can be controlled over the internet. Starting January this year (2025), you have to pay for access to the APIs to do so (I thought I owned this car; fool that I am) and hooking up can be a bit tricky. I did it, but ultimately decided to take an easier route. The same guy who developed the Home Assistant “integration” for the Tesla APIs also sells packaged access though another commercial (but low cost) integration called Teslemetry. In any case, Home Assistant can now turn charging on and off, check whether the car is at home and plugged in, monitor the battery state of charge, and control the charge rate.
I’ve written software to check the car status every ten minutes. If the solar panels are generating more power than we need, it starts charging the car at an appropriate rate. It also watches closely, because the car has over four times the battery capacity of the house. If you don’t monitor, you could easily completely drain the house batteries into the car. Since Teslas don’t natively support vehicle-to-home power, you could get stuck with a dark house and a charged car.
If you start using a lot of power elsewhere (say, turning on the clothes dryer or a number of cooking appliances) the car charging routines will see that lack of surplus power and back off on charging the car.
There’s a lot of nuance here, and I’m still filing off some rough edges, but the bottom line is that our single biggest power draw will automatically back off if we need the power elsewhere or if there isn’t enough sunshine and the house batteries are getting low.
water heaters
If we’re doing an unusual amount of cooking, and we’re trying to charge the car, and we’ve used a lot of hot water, we can approach the 12 kW limit of the inverter. Since cooking isn’t usually a long-term project, often creating a few minutes of extra capacity will be enough. The water heaters are all equipped with remote smart switches (by Kauf, in my case) and Home Assistant can turn them off for a few minutes. The water is still hot — chances are they’ll be back on before anyone notices.
dehumidifiers
We have four dehumidifiers scattered around our gated compound. They use over a hundred watts each. Dumping them wouldn’t really help mitigate too much demand, but sometimes we get low on battery power. It’s unusual, but for example two days ago the weather forecast called for sun all day, so early in the day I did a couple of loads of laundry. The electric clothes dryer is an energy hog, and it drained our batteries a lot. Shortly after I started the second load, an unpredicted cloud bank moved in and covered the mountain for the rest of the day. As a result, come sunset our batteries were low. We had enough to get through until sunrise, but just barely. Dumping the four dehumidifiers (which otherwise would have consumed 500 watts all night long) made a significant difference.
hot tub
The hot tub is an unusual design. It’s extremely well insulated and designed to run from 110v power. It only has to run about 2-3 hours per day to stay at a good temperature. Just as with water heaters, unless someone is actually in the tub they won’t notice if the power is off for minutes (or even hours) as long as it’s up to temperature in the evening when it usually gets used.
Using these load management strategies, we’re pretty much freed from having to worry about our power. We just have to limit how much we drive, and sometimes defer laundry to a sunny day (just as we’d do if we didn’t have a dryer at all).
—2p