If you are considering solar, the odds are good that you are also interested in being energy efficient in general. The most efficient way to heat a home is with a heat pump (aka “mini-split”), and the most efficient way to heat hot water is with a heat pump water heater.*
What exactly is a heat pump?
A heat pump is a mechanism that captures energy from one place, concentrates it, and delivers it as heat to another place. A window air-conditioner is a familiar type of heat pump, which captures energy from your indoor air and moves it to the outdoors. The result is a cooler indoor space and a (very slightly) warmer outdoors.
When people use the term “heat pump,” they are usually referring to a system that runs in the opposite direction: it captures energy from the outdoors and uses it to warm the indoors. “Cold-climate heat pumps” are versions specially designed to operate down to very low temperatures. Depending on the model, they can capture usable heat from the outdoors even when outdoor temperatures drop as low as -18°F.
Similarly, a heat pump water heater captures heat from the air in your basement and uses it to heat water for your shower and sinks.
The nature of the heat pump cycle means that heat pumps deliver useful heat far more efficiently than systems that generate new heat directly. This translates into energy savings and associated monetary savings.
If you start heating your home and/or water with heat pumps, this will reduce the amount of propane or heating oil you were using previously, while increasing the amount of electricity you are using. In almost all cases, what you spend on the electricity will be a good bit less than what you would be spending on fossil fuels. This is, of course, especially true if your electricity is generated with a PV Tracker.
So what will happen to your electrical usage if you go with heat pumps? According to Green Mountain Power, use of a cold climate heat pump of the following sizes will result in approximately the following change in electric usage and cost. Naturally, the exact electricity use will vary from home to home based on many factors.
Electric usage from a cold climate heat pump (Green Mountain Power)
|Heat pump BTU rating||Avg. monthly bill increase||Avg. monthly kWh increase||Annual bill increase||Annual kWh increase|
|9,000||$26||163 kWh||$312||1,950 kWh|
|12,000||$37||231 kWh||$444||2,775 kWh|
|15,000||$47||294 kWh||$564||3,525 kWh|
|18,000||$63||394 kWh||$756||4,725 kWh|
Source: Green Mountain Power presentation at Montshire Museum of Science, June 7, 2016.
As for a heat pump water heater, here’s what that looks like.
Electric usage from a heat pump water heater
|# people||Gallons used per day||Average electricity usage to heat water||Cost @ GMP residential rates|
|1||19.5||716 kWh/yr||60 kWh/month||$10 per month|
|2||35.8||1,315 kWh/yr||110 kWh/month||$18 per month|
|3||52.0||1,910 kWh/yr||159 kWh/month||$26 per month|
|4||68.2||2,505 kWh/yr||209 kWh/month||$34 per month|
|5||84.4||3,101 kWh/yr||258 kWh/month||$42 per month|
Sources: Parker & Fairey, “Estimating Daily Domestic Hot-Water Use in North American Homes” ; 10 CFR 430, Table III.3 Medium-Usage Draw Patterns ; calculations based on energy guide label.
* True, a solar hot water system can be even more energy efficient, but I defer to Martin Holladay at Green Building Advisors that “solar thermal is dead.”