How much backup power do you actually need?

1. What do you want to power?

Set Qty to 0 to skip an item. "Extra starting watts" is the brief surge a motor needs on top of its running watts. "% of time on" is how much of the outage the item actually draws power (fridges and pumps cycle). Those percentages are editable guesses.

+ Add your own appliance Reset defaults

2. Your outage and backup type

Natural gas: standby generators make less power on natural gas than on propane (LP), and how much less varies by model. 87.5% is a conservative default; check your model's spec sheet for its NG rating and enter it here.

3. Results

Planning estimates only. Results are planning estimates, not engineering advice, electrical design, or a substitute for a load calculation by a licensed electrician (for example, a National Electrical Code load calculation). Confirm sizing, wiring and any transfer switch with a licensed electrician before you buy or install anything.

Assumptions & sources

• Default appliance watts come from Generac's RS-series portable generator wattage chart (running watts and additional starting watts; brochure PDF). Lights, router, chargers, CPAP, chest freezer and space heater come from GenSize's planning chart (converted to "extra starting"). These are deliberately conservative planning numbers; your appliance's nameplate or a plug-in meter is more accurate. Modern refrigerators (for example, Energy Star models) often have much lower starting surges than the 2,200 W chart default, so if yours is newer, check the nameplate or measure it.
• Peak surge = total running watts + the single largest extra starting surge, because motors usually don't start at the same instant. That's the method in the Generac and Champion manuals. Tick the box above to add every surge instead.
• Energy (kWh) = running watts × % of time on × hours ÷ 1,000, the standard DOE formula (Virginia Cooperative Extension / DOE). The refrigerator's 33% comes from that same source ("divide the time plugged in by three"). Every other % is a labeled guess you should edit.
• Altitude derate: engine generators lose about 3.5% output per 1,000 ft (Champion 201281 manual).
• Heat: engines also lose some output in extreme heat. This calculator doesn't model heat derating, so check your generator manual's temperature derating if you'll run it in very hot weather.
• Fuel derate: propane is 90% of the gasoline rating (Champion 201281: 6,750 W on propane vs 7,500 W on gasoline, spec). Natural gas defaults to 87.5% of the LP rating, and you can edit it. How much a standby unit loses on natural gas varies by model: on Generac's current air-cooled Guardian spec sheets, NG output ranges from the same as LP (14 kW model) to 12.5% lower (24 kW model: 21,000 W on NG vs 24,000 W on LP), so 87.5% matches the biggest drop in that lineup (10–18 kW spec sheet, 20–24 kW spec sheet). Check your own model's spec sheet.
• Headroom (default 20%) keeps a generator or inverter from running flat out. It's a common design margin, not a code requirement.
• Battery: inverter efficiency 90% and depth of discharge 90% are assumptions. For reference, Tesla lists 89% solar-to-battery-to-home round-trip efficiency for Powerwall 3 (datasheet). Many spec sheets already list usable kWh, so compare against the "usable" figure when they do. Power factor 0.9 is an assumption used only to convert inverter kW to kVA.
• Motor starting on a battery inverter depends on its surge rating and locked-rotor-amp (LRA) capability, not just kW. Check the spec or use a soft starter.