The All-Electric House, Eight Months On
by Heather Phelps, KEEP Resource Specialist and Kit Coordinator
Turning a 92-year-old house into a modern, efficient, all-electric house hasn’t been a quick or easy task – I started back in September with electrical work, a whole-house air source heat pump, and the solar installation. With utility schedules, it took another month for that to be properly inspected and turned on, and the battery still hasn’t been activated while the installation company tries to figure out why it’s having issues. There are definitely the occasional hazards of being an early adopter of some of these technologies! Still, I’ve been persevering, and in March I got new insulation in my house and also swapped the gas water heater for an electric heat pump one, meaning there’s now no more fossil fuel combustion on my property!
As an energy geek, I’m naturally very fascinated by how much electricity all of these technologies use, and one of the cool features of the inverter in my solar array is that it gives me real-time data on how much power my house is using, along with how much is coming from the solar array vs. the grid. (Interested in getting this type of data for your school? Sign up to compete in Renew our Schools and get a free eGauge installation!)
One thing that’s immediately clear in the data is that, the moment you install a heat pump or electric car charger, all other electricity uses become nearly negligible.
This graph shows what my house’s electrical usage looks like on a typical morning when I don’t have the heat pump turned on. I don’t have a smart electrical panel that can tell me exactly what appliance is drawing how much power, but I suspect that the smaller spikes are the fridge, and the larger ones are the heat pump hot water heater. The “baseload” is everything else plugged in – my computer, some vampire loads from my printer and microwave, and the inverter itself.
Meanwhile, a few days prior, it was still cold enough in the mornings I had the heat pump on. You can see that up until about 3:30 in the morning, the house was just coasting, but eventually it cooled far enough down for the heat pump to kick on. The heat pump doesn’t stay on consistently, but instead makes the spiky pattern you see above. Once the scale of the chart adjusts for a heat pump, as you can see, the difference between a 100 watt load and a 300 watt load, that in the previous chart looked so striking, have nearly vanished. Instead, the main electricity consumer is the heat pump, which usually uses 2-3 kW of power when it turns on. However, around 6 am, I have the schedule set to bump the temperature up a notch before I get up. With the outside temperature below freezing, it decided to draw on the back-up heat for support, and spiked all the way to 6 kW, before going back to regular heat pump mode.
Finally, this day was one I’d been out of the house all day, so just having the heat pump keep the house temperature stable. And then I came home and plugged in my car – I suspect you can guess when I did that! Unlike the heat pump, a level 2 EV charger draws a steady 7.5 kW charge until the car is finished charging. In this case, I’d just driven back from Madison, so I needed around 20 kWh to fully recharge, which took about 4 hours.
To go back to the heat pump for a bit, it has been an intensely fascinating experience. I went for a Mitsubishi cold climate heat pump, because I knew I didn’t want to keep my gas furnace for winter heating. Still, even a heat pump designed for cold climates like ours still starts to struggle when the outside temperatures are below zero. And while climate change is warming our winters alarmingly fast, we still do get periodic polar vortexes to prepare for. So in addition to the heat pump, I also got a back-up electric resistance heater – the same concept as a space heater or baseboard heater, but just built into the existing duct system.
Well, we had a few cold snaps this winter, and I discovered there are occasional downsides to being an early adopter – I called the installer back after our first super cold weekend in December, and asked if they could come and check why the back-up heater hadn’t turned on. Turned out they hadn’t completely connected it in the first place! Still, it was a great example of what a heat pump can and can’t do – at temperatures below zero (and in a pretty poorly insulated house), the heat pump was losing ground, which meant my house temperature was dropping. But it only took the outside temperature getting as warm as 3° F before my heat pump could start warming my house back up again!
It was definitely nice after that, when the back-up heat was able to turn on during the colder days, but it is amazing how much power it takes to create heat, rather than moving it like a heat pump does. One change I’m going to make before next winter is making sure it’s set to only turn on the back-up heat when it really needs to – I’d rather take a few minutes longer to warm the house via the heat pump than jack up my electric bill with a bunch of electric resistance heat!
Ultimately, though, electric heat does require a lot of electricity. The graph below startled me in just how well it tracks – the teal bars are my daily electric usage. The red line on top is shows heating degree days – the number of degrees below 65 the temperature was each day. It’s pretty stark how strong the correlation is – the colder the day, the more power I used heating the house.
But that was a graph from over the winter. Now it’s spring, the temperature is finally warming up, and my solar panels are clear of snow and doing good work! More of my days than not I’m selling more power back to the grid than I’m pulling from it, and it’s fun to see day after day setting new records for solar production. I can’t wait to see what this array will do during the summer!
