Ways a House Can Store Energy

What are the main types of home energy storage?

There are three practical families. Electrical storage means batteries, which hold power for later use or for backup. Thermal storage means keeping heat or cold, most often in a hot-water tank. Thermal mass means letting the building's own materials, such as a concrete slab or masonry wall, soak up warmth and release it slowly.

A fourth family exists at neighbourhood scale: seasonal heat stores in the ground or in large insulated tanks. Homeowners rarely buy these on their own, but they are worth knowing about because they answer a problem the other three cannot.

How do home batteries work?

A home battery stores electricity chemically and gives it back as electricity. Capacity is measured in kWh, and a separate power rating in kW says how fast it can deliver. Owners use batteries for backup during outages, to shift use from expensive hours to cheaper ones where rates vary by time of day, and to soak up surplus from rooftop panels. Whether any of those pays is a case-by-case question; see solar batteries in Canada.

Cold affects batteries, and placement matters. The details are in solar batteries in cold weather.

Round-trip losses are worth knowing about. A battery returns somewhat less electricity than it takes in, and the shortfall appears as waste heat. A tank loses heat to the room at a rate that depends on its insulation and temperature, so a hotter tank holds more energy but leaks it faster.

How can a house store heat?

The hot-water tank is the everyday example. A heat pump water heater, for instance, can heat the tank when electricity is cheap or when the house has spare solar, then hold the heat for the evening; heat pump water heaters in Canada covers the basics. Some homes use electric thermal storage heaters, ceramic or brick cores that charge on off-peak power, though availability and rate structures vary, so check with the local utility.

Thermal mass works passively. A well-placed concrete floor in a sunny room takes in afternoon sun and returns it at night, as discussed in passive solar design. Mass smooths a day's swings. It cannot carry heat through a cold week.

Which storage suits which job?

A small comparison shows why heat storage is attractive. A 180 litre tank warmed by 50 C holds about 180 kg x 4.18 kJ/kg-K x 50 K, or roughly 37.6 MJ, which is about 10.5 kWh of heat. A typical home battery is in the same energy range, and the tank usually costs a small fraction as much per kWh stored. The catch is that the tank's energy can only be used as heat, while the battery's can run a fridge, a car charger or a heat pump.

So the matching rule is simple. Store heat if the demand is for heat, and store electricity if the demand is for electricity. Converting between them loses energy. What thermal energy storage is explains the physics behind that tank.

What about storing heat for months?

Drake Landing Solar Community, in Okotoks, Alberta, stored summer solar heat in the ground. Its 144 boreholes, about 35 m deep, formed a thermal battery under a park, and the core approached 80 C by late summer before the heat was drawn back through the winter. The field is described on the borehole storage page, and the general idea in seasonal thermal energy storage. No single house has enough room or money for this, which is why such stores serve groups of buildings.

Scale is the reason. The bigger the store, the smaller the share of heat that leaks out through its surface, so seasonal storage tends to make sense for groups of buildings rather than single homes.

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