Home Battery Backup

A backup battery for home use stores energy and releases it when the grid fails. Sizing it comes down to two separate figures: watt hours, which decide how long it lasts, and watts, which decide what it can run at all. A battery with enough capacity and too little output will not start your fridge, and no amount of extra capacity fixes that.

That distinction is the whole subject. Almost every disappointing backup purchase is someone who bought a large number of watt hours and discovered the unit could not carry the load they bought it for.

Capacity and output: the two numbers

Every battery on the market is sold with two headline specifications, and they answer completely different questions.

A unit advertised as “2,000W” may mean 2,000Wh of capacity or 2,000W of output. Those are not the same product and the difference matters enormously. Read the specification, not the model name.

Surge is the third number and the one that catches people out. Anything with a motor or a compressor — a fridge, a freezer, a well pump, a sump pump, a furnace blower — draws three to seven times its running wattage for a fraction of a second when it starts. A fridge that runs at 150W can spike past 1,000W. Size the capacity on the average draw and the output on the surge, and be generous on the surge, because a marginal inverter fails in the worst way: it appears to work, then trips hours later when the compressor restarts against a warm load.

The three kinds of home backup

“Home battery backup” covers three genuinely different products that get discussed as if they were one.

Most people asking about a backup battery for home use want the first or second row and are being shown the third. A portable unit that carries a fridge, a router, phone charging and some lights through a twelve-hour outage solves the problem people actually have, at a tenth of the cost of a whole-home system. Start by deciding which of these three you are buying, because everything after that follows from it.

Sizing it to your essential circuits

Work from the loads, not from a product. Write down what genuinely has to keep running, find each item’s average draw, and add them up.

A typical “keep the food cold and stay online” set — fridge, router, lights, charging — averages around 150–250W and consumes 2,000–3,000Wh a day. That is the arithmetic behind the common advice to buy something in the 2,000Wh class: it is roughly one day of the essentials.

Then check the output separately. The same set peaks when the fridge starts, so you want at least 1,000W continuous and meaningfully more surge headroom. Add a sump pump or a furnace blower and the output requirement roughly triples while the capacity requirement barely moves. That asymmetry is why the two numbers have to be sized independently.

What each tier costs

These are market ranges for the tier, not quotes for any particular product, and hardware prices in this category move quickly.

Expandable systems change this calculation. Several manufacturers sell a base unit that accepts add-on battery modules, so you can buy 2,000Wh now and double it later without replacing the inverter. If you are unsure how much you need — and most people are, until they have lived through one outage with a meter running — buying an expandable base is usually cheaper than guessing high.

The comparison tool lists capacity, output and expandability side by side across the catalogue, which is the fastest way to see what a given budget actually buys.

Transfer switches and when you need an electrician

A portable unit needs no installation: you run extension cords to what you want powered. That is genuinely fine for a fridge and some lamps, and it is what most people should start with.

It stops being fine when the thing you need is wired in — a furnace, a well pump, a sump pump, hardwired lighting. You cannot plug those into anything. The connection is a transfer switch: a device that disconnects selected circuits from the grid and connects them to your battery instead.

• A manual transfer switch is a small sub-panel carrying a handful of circuits. You throw it by hand when the power goes. Cheapest route, and adequate if you are usually home.
• An automatic transfer switch senses the outage and switches within milliseconds, so a desktop computer or a sump pump never notices. This is what you want if the load is a sump pump and the outage is a storm.
• An interlock kit is a mechanical plate on your existing panel that makes it impossible to have the grid breaker and the backup breaker on simultaneously. Cheaper than a sub-panel where code permits it.

How the main brands differ

The four brands that dominate this category have genuinely different characters, and the right one depends on which of your two numbers is binding.

Beyond those four, OUPES, ALLPOWERS and Mango Power compete mainly on capacity per dollar, and Renogy comes at it from the off-grid and RV side. Head-to-head comparisons are in Bluetti vs EcoFlow and Bluetti vs Anker SOLIX.

Adding solar, and whether it is worth it

A battery with solar input becomes a solar generator: it can recharge itself without the grid, which turns a one-day backup into an indefinite one. For a multi-day outage that is the difference between managing and not.

The arithmetic is less generous than the marketing. A 400W array in decent conditions replaces roughly 1.6–2 kWh a day — about what the essentials consume. That is enough to hold steady, not enough to run a house. Sizing an array to a daily consumption figure is what the solar battery calculator is for, and how to size a solar system works through the panel side.

If your interest is cutting a bill rather than surviving an outage, that is a different product entirely — see plug-in solar, which reduces what you import but gives you nothing during a cut.

When a battery is the wrong answer

• You need to run heat. Electric resistance heating is 1,500W per space heater, all day. No portable battery does that, and a 400W array replaces about a tenth of it. Space heater wattage works the numbers; the answer is a fuel-burning heater, not a battery.
• You need central air conditioning. Same problem, larger. A window unit on a large battery is feasible for a few hours; a central system is not.
• You lose power for a week at a time, every year. At that duration a fuel generator is cheaper per kilowatt hour, and a battery is a companion to it rather than a replacement. Battery or gas generator compares them honestly.
• You want to go off grid entirely. That is a different design problem with different hardware. Start at off-grid and RV.

If none of those is you — you want the food to stay cold, the internet to stay up, phones charged and the sump pump running through the kind of outage that lasts hours rather than weeks — a battery in the 2,000Wh class with 1,800W or more of output is the answer, and it is a much smaller purchase than the whole-home marketing implies.

Frequently asked questions