Oversizing is the expensive mistake in home backup, and it happens because people size for a worst case they've never actually experienced. A battery built to run everything for three days costs many times one built to carry the essentials overnight, and the second is what most households need.

These guides work through the decision in the order that keeps the cost honest.

What I look at, in this order

  1. What exactly you're backing up. Write the list of appliances before looking at any product. This is the step people skip, and it's the one that determines everything downstream.
  2. Kilowatt-hours per day, honestly counted. Real consumption over 24 hours, accounting for the fact that fridges and furnace fans cycle rather than run continuously.
  3. How the power reaches those circuits. Cords, inlet box, transfer switch or automatic panel. Often the largest single line in the budget, and the one no product page mentions.
  4. The expandability path and what it really costs. If the plan is "start small, add batteries later", price the expansion modules now — on some systems they cost nearly as much as the base unit.
  5. The recharge path during a multi-day outage. A battery with no way to refill is a one-shot device. Solar recharging while disconnected from the grid is a specific capability worth confirming rather than assuming.

Three budget bands, and what changes between them

Essentials. A single plug-in unit covering fridge, internet, phones and lights through an overnight outage. No electrician. The band that quietly satisfies most people who came in asking about whole-home backup.

Essentials, wired. The same class of battery, plus the electrical work to feed real circuits — so hard-wired lights and the furnace fan come back too. One electrician visit, and the largest jump in usefulness per dollar in this category.

Whole-home. A permanently installed system with automatic switchover, sized to carry large appliances. Permits, panel work, professional installation. Genuinely right if you own the house, stay put, and outages are long or frequent.

The jump from the first band to the second buys capability. The jump to the third mostly buys convenience and the ability to run the loads most households can do without for a day.

Which guide you want

If you need to turn a list of appliances into a battery size without overpaying, that's the home battery backup sizing guide — the complete version of the method above, including how to count loads that cycle.

If you're still deciding between a plug-in unit and a wired system, size later and read home battery vs portable power station first. For a verdict on a specific system, my home backup reviews. To put numbers on your own outage, the home backup calculators.

Frequently asked questions

How many kWh do I need for home backup?
It depends entirely on your appliance list and how long you want to carry it. The reliable method is to add up what those specific appliances consume over 24 hours, remembering that fridges and furnace fans cycle rather than run constantly, and then multiply by the number of days you want to cover. Work through the method →
What is the oversizing trap?
Sizing for a scenario you have never actually faced, usually several days of running everything including air conditioning. Each extra kilowatt-hour costs real money and mostly sits unused, while the thing that limits you during a genuine outage is more often which circuits you can reach than how much energy you stored.
Should I buy a battery that can expand later?
Only after checking what the expansion modules cost. Expandability is a genuine advantage when the modules are reasonably priced, and a marketing point when they cost nearly as much as the original unit, which is common enough to be worth verifying before you commit.

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