Heat pumps and time-of-use rates: does the plan that suits your car suit your heating?

What a heat pump does to the load curve

A heat pump moves heat rather than making it, which is why it uses a third of the electricity of resistance heating for the same warmth. But it moves heat against the outdoor temperature, and it works hardest when that gap is widest. In winter that is the pre-dawn hours, with a spike around 6am to 8am when thermostats come off setback and the house is coldest. In summer it is the late afternoon, 3pm to 7pm, when the sun has been on the roof for hours. Our modelled family home has central air with its cooling load peaked at 3pm; an electrically heated version peaks at 7am. Neither of those is an hour you can change with a timer.

The summer question: how wide is the peak?

Nearly every time-of-use plan has a summer peak that overlaps the cooling load. The plan is not necessarily wrong for a heat-pump home because of that — the same house pays the same peak price for cooling on a flat plan, in the sense that the flat price is set high enough to cover it. What matters is how much of the household’s use lands in the peak block and how much cheaper the rest of the day is. A 4pm-to-9pm peak catches most of the cooling load; a noon-to-6pm peak catches it too but leaves the evening cheap. The ratio matters as much as the hours: the median plan in our index charges 2.6× more at peak, and at that ratio a house that cannot move its cooling and has nothing else to shift comes out close to even or slightly behind.

This guide goes through what does move cooling load: pre-cooling the house before peak, setting the thermostat back during it, and letting a well-insulated house coast. That is worth a few degrees of comfort for a few hours a day, and on a wide-gap plan it is worth real money.

The winter question: is there a morning peak?

This is the part of the tariff to read carefully, because plans differ sharply. Some have a single peak block year-round, usually in the evening, which a heat pump barely touches in winter — the house is warm by then. Some have no winter peak at all, which makes the plan pure upside for a heat-pump home with a car. And some, particularly in cold states and at utilities with a lot of electric heating, add a winter morning peak from about 6am to 9am. That block lands exactly on the heating spike, and there is no timer that moves it.

Every time-of-use plan on this site is drawn month by month, so a winter morning peak shows up as a second dark block on the left of the grid from November to March. If your plan has one, the cost is roughly the morning heating load multiplied by the gap, and the savings have to come from somewhere else to cover it.

Where the savings come from

• An electric car is the usual answer. A car charging overnight is the largest movable load in the house, and it is what makes most heat-pump households come out ahead on a time-of-use plan. Without one, the case is thinner.
• A heat pump water heater can be scheduled off-peak like a resistance tank, though it needs a longer window.
• Thermostat setbacks that respect the peak. Warm the house up before a morning peak rather than during it. Let it drift during an evening peak. A heat pump hates large setbacks — it may call in backup resistance heat to recover — so keep them modest, two or three degrees.
• Backup heat lockout. Resistance backup strips draw three to four times what the heat pump does. If they run during a peak block, they wipe out a month of savings. Set the thermostat to lock them out above whatever temperature the heat pump can handle alone.

The short version

Look at the winter grid first. If there is no morning peak and you have a car, switch. If there is a morning peak and no car, do not. In between, run the household through the calculator on your utility’s page with electric heat turned on; it adds the morning spike to the model and prices every plan against it.