Backup Power Sizing Calculator

Quick answer: most homes need 1,800–4,000W continuous and 2–4kWh of battery to keep essentials (fridge, Wi-Fi, lights, phones, one pump or CPAP) alive through a 12–24h outage. Your exact number depends on the surge of your largest motor — select your appliances below and we'll compute it live.

Start from a scenario (or build your own below):

1 · What needs to stay on?

2 · How long is the outage?

Your numbers

Select appliances to see your continuous watts, surge, battery size, and which units fit.

Assumptions: 20% sizing headroom · 85% inverter efficiency · measured duty cycles per appliance (shown in each wattage guide) · surge = largest single motor-start. Guidance, not electrical design — 240V circuits need a licensed electrician.

Frequently asked questions

How do you calculate what size backup battery I need?

Three numbers: continuous watts (sum of running watts of everything selected), surge watts (running total plus the single largest motor-start bump), and watt-hours (each appliance's watts × its duty cycle × outage hours, divided by 85% inverter efficiency, plus 20% headroom).

What is a duty cycle and why does it matter?

Most appliances cycle on and off — a fridge compressor runs ~35% of the time, a well pump ~10%. Sizing at 100% duty overbuys by 3-5x. Our calculator applies measured duty cycles per appliance automatically.

Why do I need 20% headroom?

Batteries lose usable capacity in cold weather and over their cycle life, inverters run less efficiently near their limits, and outages run longer than forecast. 20% is the industry-standard safety margin.

What if no portable unit fits my load?

Loads above ~7kW continuous or multi-day whole-home needs point to installed batteries (Tesla Powerwall 3, FranklinWH aPower 2) — see our comparison and check your state's incentives, which can cut thousands off installed systems.