What if a home ran on a big battery?

Most homes get electricity through wires from far away.

But some homes have a big battery too. What if a home ran on its battery alone? How long would it last?

Most homes get their electricity through power lines. Some also have a big battery, often charged by solar panels on the roof, so they can store electricity for later.

So what if a home had to run on its battery alone, for example during a power cut? Would it last an hour, a day or a week? The answer depends on two numbers: how much energy the battery stores, and how fast the home uses it. In this lab you can change both.

EIA reports that many small-scale battery systems are owned by electricity customers who charge them with solar photovoltaic systems; in 2021, utilities in 42 states reported 1,094 MW of small-scale battery capacity linked to customers' PV systems.

This lab asks how long such a battery can carry a home on its own. It uses a simple energy model: a battery of fixed capacity in watt hours feeding a steady load in watts, and compares a light load with a heavy one.

In Olly's lab, the home battery is full.

The home uses only a little electricity. The battery lasts more than a whole day and night!

Our model battery stores 13,500 watt hours, which is 13.5 kilowatt hours. A watt hour is the energy of using one watt for one hour.

Start with a light load: the home uses a steady 500 watts. The battery lasts 27 hours, more than a whole day and night. Watch the charge meter fall slowly from 100 percent to empty.

The model divides deliverable energy by load: t = (C × usable share × efficiency) ÷ P. With C = 13,500 Wh, 100% usable, 100% efficiency and P = 500 W, t = 27 h.

EIA defines the units: watts measure power at a moment, watt hours measure energy used over time, and one kilowatt hour is one kilowatt for one hour. Its example: a 40-watt bulb for five hours uses 200 Wh. In the model, that bulb alone would run for 14.1 days on this battery.

Now someone turns on a big heater.

The battery empties much faster. It lasts only part of a day.

Now raise the load to 2,000 watts, the model's setting for a heater. The battery is just as big as before, but now it is empty after 6 hours and 45 minutes.

Four times the power means the energy runs out four times as fast. That is why the biggest loads matter most when a home runs on battery power.

At P = 2,000 W the runtime drops to 13,500 ÷ 2,000 = 6.75 h (6 h 45 min). Runtime is inversely proportional to power: quadrupling the load quarters the time.

At the slider's top load of 3,000 W the battery lasts 4.5 h. Doubling the capacity doubles every runtime, so a battery bank holding 27,000 Wh would carry the 500 W load for 54 h.

A battery is like a full water tank. The things in your home drink from it.

A heater drinks fast. A lamp sips slowly. Fast drinking empties the tank sooner.

Energy is how much the battery holds; power is how fast it is used. Divide one by the other and you get the time: 13,500 watt hours shared out at 500 watts lasts 27 hours.

Our model is simple. The home draws exactly the same power the whole time, and the battery gives every bit of its energy, whatever the weather. Real homes switch things on and off, and real batteries slowly lose some of their capacity as they age.

Energy E (Wh) = power P (W) × time t (h), so t = E/P. The model adds two advanced controls: a usable share (depth of discharge) and a conversion efficiency for wires and inverters. At 90% efficiency the 500 W runtime falls to 24.3 h; with 90% usable as well, 21.9 h.

Limits from its documentation: a steady load (real fridges switch on and off), full capacity regardless of power, temperature or age, and no voltage or current, only energy. Real batteries also lose capacity with age: the Alternative Fuels Data Center reports studies finding that an electric-vehicle battery could still have at least 70% of its initial capacity at the end of its life.

Real homes use lots of electricity. Heating and cooling use the most.

Some homes with solar panels fill a battery in the day and use it at night.

The U.S. Energy Information Administration says an average U.S. home bought 10,791 kilowatt hours of electricity in 2022, about 899 a month. Spread evenly over every hour of the year, that is about 1,230 watts. In our model, the 13,500 watt-hour battery would run that average home for about 11 hours.

EIA also says heating and cooling are the biggest uses of electricity in homes, with air conditioning the largest. So a home running on a battery would last longest by keeping those big loads small.

EIA: in 2022 the average U.S. residential customer purchased 10,791 kWh (about 899 kWh a month), ranging by state from 14,774 kWh in Louisiana to 6,178 kWh in Hawaii; purchases can understate use in homes with rooftop PV. Dividing by 8,760 hours gives a mean load of about 1.23 kW, for which the model battery lasts about 11 h.

EIA identifies heating and cooling as the largest annual residential uses, with air conditioning consistently the largest. Real household demand varies by hour and season, so the steady-load runtime is a guide, not a forecast for any particular home.