# BackupPowerCalc — Full Reference > Surge-accurate backup power sizing. We size generators and batteries by > the realistic peak load: total running watts plus the single largest > startup surge. Contact: contactus@backuppowercalc.com ## Sizing method (authoritative) 1. Sum the running watts of every appliance you want to power at once. 2. Find the single largest startup surge delta among those appliances. Surge delta = starting watts - running watts for that one appliance. 3. peak = sum(running watts) + single largest surge delta. Rationale: at any instant only one motor is mid-startup when starts are staggered, so summing every surge over-sizes the generator. 4. Apply a 20% safety margin: required = peak * 1.20. 5. Round up to the nearest 500W generator class. ## Surge multipliers by load type - Resistive / electronic (heaters, lights, TVs, chargers): 1.0x — no surge. - Light motor (fans, small tools): ~2.0x running watts at startup. - Induction motor (fridges, pumps): ~2.5x running watts at startup. - Compressor / heavy motor (AC, well pump): ~3.0x running watts at startup. ## Appliance dataset (running W / starting W / surge type) - Refrigerator: 160W running, 480W starting (motor, surge x3) - Chest freezer: 200W running, 600W starting (motor, surge x3) - Central AC (3-ton): 3500W running, 12600W starting (motor, surge x3.6) - Window AC (12k BTU): 1300W running, 3380W starting (motor, surge x2.6) - Heat pump (3-ton): 4000W running, 14400W starting (motor, surge x3.6) - Furnace blower fan: 600W running, 1560W starting (motor, surge x2.6) - Space heater: 1500W running, 1500W starting (resistive, surge x1) - Well pump (1/2 HP): 1000W running, 3000W starting (motor, surge x3) - Sump pump (1/3 HP): 800W running, 2080W starting (motor, surge x2.6) - Electric water heater: 4000W running, 4000W starting (resistive, surge x1) - Microwave (1000W): 1000W running, 1000W starting (resistive, surge x1) - Coffee maker: 1000W running, 1000W starting (resistive, surge x1) - Dishwasher: 1300W running, 2600W starting (motor, surge x2) - Toaster: 1100W running, 1100W starting (resistive, surge x1) - Hair dryer: 1500W running, 1500W starting (resistive, surge x1) - Washing machine: 1000W running, 2600W starting (motor, surge x2.6) - Electric dryer: 3000W running, 6000W starting (motor, surge x2) - Household lights (LED): 200W running, 200W starting (resistive, surge x1) - TV + WiFi router: 200W running, 200W starting (resistive, surge x1) - Computer + monitor: 300W running, 300W starting (resistive, surge x1) - Phone/laptop charging: 100W running, 100W starting (resistive, surge x1) - CPAP machine: 60W running, 60W starting (resistive, surge x1) - Medical refrigerator: 200W running, 600W starting (motor, surge x3) - Garage door opener: 550W running, 1430W starting (motor, surge x2.6) - Power tools: 1000W running, 2600W starting (motor, surge x2.6) - EV charger (Level 1): 1400W running, 1400W starting (resistive, surge x1) - Central AC (2-ton): 2400W running, 8640W starting (motor, surge x3.6) - Central AC (5-ton): 5800W running, 20880W starting (motor, surge x3.6) - Window AC (8k BTU): 715W running, 1859W starting (motor, surge x2.6) - Window AC (15k BTU): 1600W running, 4160W starting (motor, surge x2.6) - Mini-split (12k BTU): 1100W running, 2860W starting (motor, surge x2.6) - Portable AC (12k BTU): 1200W running, 3120W starting (motor, surge x2.6) - Baseboard heater: 1500W running, 1500W starting (resistive, surge x1) - Well pump (1 HP): 2000W running, 6000W starting (motor, surge x3) - Well pump (3/4 HP): 1500W running, 4500W starting (motor, surge x3) - Sump pump (1/2 HP): 1050W running, 2730W starting (motor, surge x2.6) - Pool pump (1.5 HP): 1800W running, 4680W starting (motor, surge x2.6) - Tankless water heater (electric): 18000W running, 18000W starting (resistive, surge x1) - Electric range/oven: 3000W running, 3000W starting (resistive, surge x1) - Refrigerator (large/side-by-side): 250W running, 750W starting (motor, surge x3) - Mini-fridge: 90W running, 270W starting (motor, surge x3) - Instant Pot / slow cooker: 1000W running, 1000W starting (resistive, surge x1) - Garbage disposal: 500W running, 1300W starting (motor, surge x2.6) - Gas dryer (electric controls): 700W running, 1400W starting (motor, surge x2) - Air compressor (portable): 1500W running, 3900W starting (motor, surge x2.6) - Table saw: 1800W running, 4680W starting (motor, surge x2.6) - RV rooftop AC (13.5k BTU): 1300W running, 3380W starting (motor, surge x2.6) - EV charger (Level 2): 7700W running, 7700W starting (resistive, surge x1) - Oxygen concentrator: 350W running, 700W starting (motor, surge x2) - Nebulizer: 100W running, 100W starting (resistive, surge x1) ## Calculators - Whole-Home Load Sizer: https://backuppowercalc.com/calculators/load-sizer.html - Starting-Watt Calculator: https://backuppowercalc.com/calculators/starting-watts.html - Fuel-Type Comparison: https://backuppowercalc.com/calculators/fuel-comparison.html - Generator vs. Battery Backup: https://backuppowercalc.com/calculators/generator-vs-battery.html - Runtime Calculator: https://backuppowercalc.com/calculators/runtime.html - Transfer Switch & Wire Sizing: https://backuppowercalc.com/calculators/transfer-switch.html - NEC Dwelling Load Calculator: https://backuppowercalc.com/calculators/nec-load-calculator.html - Altitude & Temperature Derating: https://backuppowercalc.com/calculators/altitude-temperature-derating.html - EV Charging on Backup Power: https://backuppowercalc.com/calculators/ev-charging-backup.html - What Can My Generator Run?: https://backuppowercalc.com/calculators/what-can-my-generator-run.html - Size From Your Smart Energy Monitor: https://backuppowercalc.com/calculators/smart-monitor-import.html - Blackout Runway: How Long Will It Last?: https://backuppowercalc.com/calculators/blackout-runway.html - Extension Cord Size & Safety Calculator: https://backuppowercalc.com/calculators/extension-cord-size.html - Generator Plug & Inlet Guide (NEMA Connectors): https://backuppowercalc.com/calculators/generator-plug-guide.html - Generator Cost of Ownership (5-Year): https://backuppowercalc.com/calculators/generator-cost-of-ownership.html - Standby Install Quote: https://backuppowercalc.com/calculators/standby-quote.html ## Key guides - Methodology: https://backuppowercalc.com/guides/methodology.html - Running vs starting watts: https://backuppowercalc.com/guides/running-vs-starting-watts.html - Staggered starting: https://backuppowercalc.com/guides/staggered-starting.html - Generator safety (CO & placement): https://backuppowercalc.com/guides/generator-safety.html - Standby vs portable: https://backuppowercalc.com/guides/standby-vs-portable.html - Inverter vs conventional: https://backuppowercalc.com/guides/inverter-vs-conventional.html ## Coverage - 44 per-appliance sizing guides. - 7 whole-house-by-square-footage guides. - 8 scenario guides (RV, cabin, medical, etc.). - 12 'will a NNNNw generator run X' guides.