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Construction

Generator Sizing

Sizing a backup generator means handling two different load numbers: <strong>running watts</strong> (steady-state continuous draw of everything you have on) and <strong>surge watts</strong> (the brief 2-3x spike when motors start up — refrigerator, AC, well pump, sump pump). Generator must handle both, so you size for running watts + the single largest motor's surge contribution. This calculator lets you pick the appliances you want to run during an outage, totals running watts and adds the worst surge, recommends a generator size with 25% safety margin, and estimates fuel use. Portable inverter generators (3-12 kW) for partial home backup; standby whole-home generators (10-26 kW) for everything.

Pick what you want to run

Portable vs standby generators

Portable inverter generators (3-12 kW): $400-2,500. Roll out from garage, plug in extension cords or hardwire to inlet box on house. Manual start. Need fuel (gas/propane). Quiet (50-65 dB at distance). Best for short outages (1-3 days) and partial home backup (critical loads only).

Standby whole-home generators (10-26 kW): $5,000-15,000+ installed with auto-transfer switch, concrete pad, gas line. Auto-start on grid loss (~10-30 sec delay). Quiet running. Use natural gas (unlimited fuel) or propane (tank size determines runtime). Best for whole-home coverage, frequent outages, or remote properties.

Inverter generators: clean sine wave power (safe for electronics). Standard portable generators may damage sensitive electronics; inverters are required for modern home use.

The sizing math

Generator wattage = (sum of running watts of everything you want to run) + (extra surge watts of largest single motor). Add 25% safety margin, then round up to standard generator sizes.

Worked example: fridge (700W run, 2200W surge) + furnace blower (800W) + lights (100W) + internet (30W) + sump pump (600W run, 1800W surge). Running watts: 2,230. Largest surge: fridge (+1500). Peak: 3,730W. Add 25%: 4,663W. Recommend 5kW generator (closest standard size up).

How to use this calculator

  1. Check boxes for each appliance you want to run during the outage.
  2. Calculator totals running watts + largest motor's surge contribution.
  3. Output: recommended generator size in kW (with 25% margin), running watts, peak surge, fuel use at 50% load.
  4. Add 1-2 kW extra for future loads or comfort margin. Generators run more efficiently at 50-75% load.

Common scenarios

Essential loads only (fridge, freezer, lights, internet, sump pump, gas furnace blower): roughly 3-4 kW running, 5-6 kW peak. Recommended: 5-7 kW portable inverter generator. Powers critical loads through a 5-15 hour outage on a tank of gas.

Essentials + window AC: add 1,200W run + 2,400W surge for window AC = ~5-6 kW running, 8-9 kW peak. Recommended: 8-9 kW portable or 10 kW standby.

Whole-home including central AC and electric range: 8-12 kW running, 15-20 kW peak. Recommended: 17-24 kW standby generator. Necessary for true whole-home automatic backup.

FAQ

Running vs surge watts? +
Running watts: continuous draw during normal operation. Surge watts: brief spike (1-2 seconds) when motors start. Refrigerators surge 2-3x running watts. AC compressors surge 3-4x. Well pumps surge similar. Generator must handle the combined running watts + the worst surge of any single motor starting.
Portable vs standby — which is right for me? +
Frequent outages (5+/year), remote properties, need whole-home backup, can absorb $5K+: standby. Occasional outages (1-2/year), partial loads OK, prefer lower upfront cost: portable. For grid-tied homes in areas with reliable power but occasional storms: portable is fine.
Fuel cost during operation? +
Portable gas (5 kW at 50% load): 0.4-0.6 gal/hour = $1.50-2/hour at $3.50/gal = $35-50/day continuous. Standby propane: similar gal/hour, $50-90/day at $2-3/gal propane. Standby natural gas: cheapest fuel cost long-term, ~$30-60/day.
Can I run a generator continuously for days? +
Portable generators can run continuously but need fuel refills + oil changes every 50-100 hours of operation. Standby generators are designed for continuous duty up to ~500 hours before service. For 1-3 day outages, both work fine. For week-long outages, standby is much more practical.
Do I need a transfer switch? +
For portable generator backfeeding: required by code in nearly all jurisdictions. Connecting a portable generator to your house via dryer outlet (backfeeding) is illegal AND can kill utility workers via the unisolated grid connection. Use a portable inlet box ($300-500) and manual transfer switch, or an interlock kit on the panel. For standby: automatic transfer switch is included.
What about installation cost for standby? +
Generator: $3,000-8,000. ATS (transfer switch): $500-1,500. Concrete pad: $300-700. Gas line install (NG or propane): $500-2,000. Electrical install: $1,000-2,500. Total turnkey: $5,000-15,000+. Many manufacturers (Generac, Kohler, Briggs) have authorized dealers who handle the whole project.
Will it power my electric stove / dryer / EV? +
Electric stove (8,000-12,000W) and electric dryer (5,000W) require 8+ kW generators just for that appliance. EV Level 2 charging (10,000W): needs 12+ kW generator. Most portable generators can't power these AND also run other essentials. Standby 20-26 kW handles it; portables typically can't.
Inverter generator vs conventional? +
Inverter: clean sine wave power (safe for laptops, smart appliances, sensitive electronics), quieter, more fuel-efficient at partial loads, more expensive (50-100% premium). Conventional: cheaper, louder, dirty power that can damage sensitive electronics. For home backup powering modern appliances and electronics, inverter is the safer choice.