Calculate the total wattage needed to size your backup generator correctly based on the appliances and devices you need to power during an outage.
Last updated: March 2026 | By Patchworkr Team
This accounts for starting surge and provides safe operating headroom
Generator sizing involves calculating the total electrical load (in watts) that your backup generator must supply to power your essential appliances and devices during a power outage. Proper sizing ensures your generator can handle both the continuous running load and the higher starting surge that occurs when motors and compressors kick on.
Running watts (also called rated watts) is the continuous power an appliance needs while operating. Starting watts (also called surge watts) is the brief spike of power needed when motor-driven appliances like refrigerators, air conditioners, and pumps first start—typically 2-3x the running watts. Your generator must be sized for the highest starting surge, not just the total running load.
An undersized generator will overload, trip its breaker, or even suffer damage. An oversized generator wastes fuel running inefficiently at low loads and costs more upfront. This calculator helps you find the right size by accounting for both running and starting loads, then recommending a generator with adequate capacity and safety margin.
Generator sizing considers both running and starting loads:
Essential appliances for power outage:
Note: Only one motor starts at a time, so we use highest single surge + remaining running loads
Running watts is the continuous power an appliance uses while operating. Starting watts (surge watts) is the higher power needed for a few seconds when motor-driven appliances first start. Electric motors need 2-3x their running watts to overcome inertia and start spinning.
No! This calculator shows maximum load if everything runs simultaneously. In practice, you can manage loads by not running high-draw appliances (microwave, electric heater) at the same time. Stagger starting large motors to avoid simultaneous surges.
Add 10-20% safety margin if possible. A 4,000W requirement is well served by a 4,500-5,000W generator. This provides headroom for future needs, accounts for generator efficiency loss over time, and ensures you never push the unit to its absolute limit.
Well pumps (1500-3000W starting) and furnace blowers (600-875W) are common high-load items. Add their wattages to your calculation if needed during outages. Furnaces also need ignition and control power even if they're gas-fired.
Portable generators (1000-10,000W) are mobile, cheaper ($400-$2000), require manual starting and fuel refilling. Standby generators (7000-20,000W+) are permanently installed, auto-start during outages, and cost $3000-$15,000+ including installation.
Check the manufacturer nameplate/label on each appliance. Look for "Watts" or calculate from Volts × Amps. If only amps are listed: Watts = Volts × Amps (120V for standard outlets, 240V for large appliances like dryers and ranges).
Unlikely with portable generators. Whole-house power requires 15,000-20,000W for central AC, electric range, dryer, etc. Most portable generators (3500-7500W) run essentials only: fridge, lights, furnace blower, few outlets. Standby generators offer whole-house coverage.
Inverter generators produce cleaner power (safer for electronics), run quieter, and are more fuel-efficient than conventional generators. They're rated the same way—just ensure the surge watts meet your needs. They're excellent for sensitive electronics like computers.
Related Tools
Calculate battery capacity.
Calculate charging duration.
Calculate runtime.
Calculate battery pack size.
Calculate appliance costs.
Calculate single device cost.