Home battery backup: the complete guide

Most people buy a home battery for one reason: to keep the lights on when the grid goes down. That is a good reason. But "backup" quietly means very different things depending on how the system is set up, and the gap between what people expect and what they get is where the disappointment lives. This guide walks through what backup actually covers, how long a battery really lasts, how to size one, and when a generator or solar changes the answer.

What "backup" actually means

There are two flavors, and the difference is bigger than the words suggest.

Essential (partial) backup keeps a chosen list of circuits alive: your fridge, some lights, internet, a few outlets, maybe a furnace fan. Everything else stays dark. This is the common default because it stretches a modest battery a long way.

Whole-home backup keeps the entire panel energized, including the power-hungry stuff: air conditioning, electric heat, a well pump, an electric range, an EV charger. It is more convenient and far more demanding. It usually needs both more stored energy and more inverter power to start large motors, and it drains much faster.

Neither is "better." Essential backup gives you more hours per kWh; whole-home gives you a normal life for a shorter window. See whole-home vs essential backup for the wiring details.

How long a battery actually lasts

The honest formula is simple:

Runtime (hours) = usable kWh / average load (kW)

Two things trip people up. First, usable is not the same as the number on the spec sheet. Batteries hold back a slice to protect their lifespan, so a 16 kWh nameplate might give you closer to 14 to 15 kWh usable. That reserve is set by depth of discharge. Second, your load is not one number. It swings as things switch on and off.

Here is an illustrative walk-through. Say you have a battery with about 13 kWh usable (a typical home battery holds roughly 10 to 16 kWh usable).

• Essential loads. Fridge, LED lights, phones, router, a laptop. That averages maybe 0.4 to 0.6 kW. At 0.5 kW, 13 kWh lasts around 26 hours. Duck below that and you can stretch past a full day.
• Whole-home loads. Add central air or electric heat, an electric water heater, and cooking. Now you might average 2 to 3 kW, with spikes far higher. At 2.5 kW, that same 13 kWh is gone in roughly 5 hours.

Same battery. The difference between one day and one evening is entirely about what you choose to power. These numbers are illustrative; your appliances and climate will shift them. If you want to see the outage economics against your own rates, run the Worth It calculator.

How many kWh you need

Size backward from what you want to keep on and for how long.

1. List the loads. Write down each device and its rough draw. A fridge sips; a well pump gulps.

2. Estimate average kW. Not everything runs at once, so use a realistic average, not the sum of all nameplates. If you are unsure what a kilowatt-hour even represents, start with kWh explained.

3. Pick your window. How many hours of outage do you actually need to cover? For most people, most outages are under a day.

4. Add margin. Multiply average kW by hours, then add roughly 20 to 30 percent for depth-of-discharge reserve and inverter losses.

A worked example: essentials at 0.5 kW for 20 hours is 10 kWh, plus margin lands near 12 to 13 kWh usable, which is one typical battery. Whole-home for the same window can easily demand two or more batteries stacked together. That capacity decision drives most of the cost, so read how much a home battery costs before you commit to a size, and compare battery models on usable capacity and inverter output rather than headline numbers.

Battery vs generator for backup

This is the fork most buyers hit, and the honest answer is "it depends on your outage pattern."

A battery is silent, switches on instantly, needs no fuel, and doubles as a daily energy tool if you have time-of-use rates or solar. Its ceiling is capacity: when it is empty, it is empty until it recharges.

A generator runs as long as you feed it fuel, which makes it the workhorse for rare multi-day outages. The trade-offs are noise, fuel storage and refueling, maintenance, emissions, and a short delay before it kicks in.

Rough guidance: if your outages are frequent but short, a battery is the better daily citizen. If you face rare but long outages, a generator (or a battery paired with solar) tends to win on cost per hour of coverage. Plenty of homes run both. The full trade study is in battery vs generator.

Can solar recharge you during a long outage?

Yes, and this is the piece that changes the whole calculation. A battery on its own is a fixed tank. A battery paired with properly configured solar is a tank that refills every sunny day.

In a multi-day outage, solar charges the battery during daylight while also running your loads, then the battery carries you overnight. Done right, this is the only realistic way to ride out a long grid failure on storage alone, and it is why solar-plus-battery is the setup serious backup buyers gravitate toward.

Two honest caveats. Not every solar install can charge a battery during a grid outage; the system has to be wired and configured for islanding, so confirm this explicitly rather than assuming. And solar output collapses on heavily overcast days, so treat sunshine as a bonus that extends your runtime, not a guaranteed refill.

The bottom line

Backup is not one spec, it is a match between the loads you keep on and the hours you need. Decide whether you want essential or whole-home coverage, size the usable kWh backward from your real loads with margin, and be honest about outage frequency and length when weighing a battery against a generator. If long outages are your worry, pairing with solar is what turns a battery from a few hours of relief into genuine resilience.

One note on money: the 25D federal tax credit for cash purchases expired on December 31, 2025, so a 2026 cash buyer gets $0 federal on the purchase. Some savings still exist through lease or PPA structures under 48E. We keep incentives in their own guides so this page stays about the engineering, not the tax code.