Home battery backup: what it can and cannot power
A home battery backup system stores electricity and feeds selected circuits in your house when the grid goes down, switching over automatically in a fraction of a second. How long it keeps the lights on depends on the battery's usable energy in kilowatt-hours and on how much power the connected loads draw.
How does a home battery backup system work?
A battery pack, an inverter and a switching device sit between the grid connection and a set of protected circuits. In normal times the battery charges from the grid or from solar panels. When the grid fails, the system disconnects from the utility, which is a safety requirement so that no power flows back to workers on the lines, and then supplies the protected circuits from the battery.
Most homes do not back up everything. An electrician moves the circuits that matter into a separate backup panel: the fridge, some lights, the furnace blower, internet gear, perhaps a well pump or a sump pump. This is called critical-load backup, and it keeps the battery small and the price lower.
How long will a home battery last in an outage?
Two numbers decide it. Energy, in kWh, is the size of the tank. Power, in kW, is how fast you can draw from it. A battery with plenty of energy but a small power rating still cannot run an electric stove and a dryer at the same time.
A simple example, with assumptions stated. Say the battery has 10 kWh of usable energy, and the protected circuits average 0.5 kW over a day: a fridge cycling on and off, some LED lights, a gas furnace fan, a router and device charging. Runtime is roughly 10 divided by 0.5, or about 20 hours. If a deep freezer and a sump pump are added and the average rises to 1 kW, runtime falls to about 10 hours. Actual loads vary widely, so a quote should list every circuit and its draw.
Solar changes the picture, because panels can recharge the battery in daylight during a long outage. That works only if the system is built to operate while the grid is down; see will solar panels work in a power outage for the details.
What can a battery power and what is hard for it?
Think in terms of what is easy, what is possible and what usually is not worth it.
- Easy. Lighting, refrigeration, phones and computers, a gas furnace blower, a sump pump.
- Possible with a larger system. A well pump, a microwave, a short run of an induction cooktop.
- Hard. Electric heating, electric water heaters, central air conditioning and heat pumps in cold weather, all of which draw several kilowatts for hours.
Heating is the key winter question. If the house is heated electrically, a battery that suits a gas-heated house will drain in a few hours. Cold also affects the battery itself, which is why placement matters; the cold-weather battery guide explains why. Households with long rural outages sometimes pair a battery with a generator, and the solar generator versus battery backup guide compares the options.
What does installation involve?
A licensed electrician installs the system, and in most areas it needs an electrical permit, an inspection and, if it connects to solar, approval from the utility. The battery goes in a location with limited temperature swings and the clearances required by the local code. Garages and basements are common. Rules on location and fire safety vary, so ask the installer to show where the requirements come from.
Costs depend on capacity, the number of protected circuits, the inverter type and how much electrical work the panel needs. Prices vary by region and year, and written quotes are the only reliable figure. The solar battery guide lays out a worked cost example, and energy storage types for homes shows how batteries compare with other kinds of storage.
Is battery backup worth buying?
It is mainly a resilience purchase. In places with frequent or long outages, medical equipment at home, or a freezer full of food, the value is easy to see. In places where the grid is steady, the battery may sit idle for years unless it also earns savings through time-of-use rates or self-use of solar power, and those depend on how your utility bills.
Drake Landing Solar Community took a different route to resilience: it stored summer heat in the ground for winter. The borehole storage page shows how heat storage and electricity storage solve different problems.
Quick answers
Can a home battery run my whole house?
Only if it is large enough and has a high enough power rating, and most homes back up selected circuits instead. Electric heating and air conditioning are the loads that most often exceed a battery.
How long does a home battery last in a power outage?
Divide the battery's usable kilowatt-hours by the average kilowatts drawn by the protected circuits. Ten usable kWh and an average draw of 0.5 kW gives roughly 20 hours.
Does a home battery need solar panels?
No. A battery can charge from the grid, though pairing it with solar lets it recharge during a long outage if the system is designed for that.
Do I need a permit for a home battery?
In most areas an electrical permit and inspection are needed, and a utility may need to approve the connection. Ask your municipality and utility, and hire a licensed electrician.