Size your generator, wire and battery backup — with the code tables shown.

Backup power

Generators, portable power stations and home batteries: how big, how long, how much fuel.

• Generator size calculatorAdd up running and starting watts for the appliances you want to keep on.
• Power station runtime calculatorHours of runtime from a portable power station or battery for any load.
• Home battery backup calculatorkWh and number of battery units to ride out an outage.
• Generator fuel consumption calculatorGallons or cubic feet per hour, per day and per tank.

Wiring

Wire gauge, voltage drop and breakers by the NEC tables, including the cable-type rules most charts skip.

• Wire size calculatorAWG for a circuit by amps, cable type and run length, per NEC 310.16.
• Voltage drop calculatorVolts and percent lost over a run, and the gauge that keeps it under 3%.
• Breaker size calculatorBreaker amps from watts or amps, with the 125% continuous-load rule.
• EV charger circuit calculatorBreaker, wire and charging speed for a Level 2 charger.

Energy use and cost

Watts, amps and what an appliance costs to run at your state’s actual electricity price.

• Appliance electricity cost calculatorWhat any appliance costs per hour, month and year at your state’s price.
• Watts to amps calculatorConvert watts and amps at any voltage, with power factor.

Electricity prices by state

Average residential price, January–July 2026, from the U.S. Energy Information Administration. The cost calculators use these figures.

All 50 states and DC, with year-over-year change

Guides

One question each, answered with numbers from the calculators.

• How long will a 13.5 kWh home battery last in an outage?A 13.5 kWh battery (the size of a Tesla Powerwall) runs essentials for about a day and an average home for about 10 hours. Runtime by load, and how to stretch it.
• Can a portable power station run a CPAP?How many nights a 256 Wh to 2,048 Wh power station runs a CPAP, with and without the heated humidifier, on AC and on DC. Calculated runtimes and tips.
• What size breaker for a water heater?A 4,500 W, 240 V water heater needs a 25 A minimum breaker on 10 AWG; most are wired at 30 A. Breaker and wire for 3,000–6,000 W, and why the 125% rule applies.
• What size wire for a 20 amp circuit 100 feet away?12 AWG drops over 5% at 100 feet on a loaded 20 A, 120 V circuit. Voltage drop by wire size and distance, and when 10 AWG or 8 AWG is worth it.
• What size wire for a 30 amp breaker?10 AWG copper for a 30 A breaker, 8 AWG aluminum, and when a long run needs 8 or 6 AWG. NEC table, distance chart and the 240.4(D) rule explained.
• How many amps does a refrigerator use?A modern refrigerator draws 1–2 A running and up to about 10 A for a second at start-up. Running and starting amps, what circuit it needs, and what it costs a year.

All 17 guides