Article — Generator Wattage Calculator
A typical U.S. home backup setup — refrigerator, well pump, sump pump, lights, furnace blower — needs a generator in the 5,000–7,500 W range once a 25% safety margin is included. Add a window air conditioner and the figure climbs past 10,000 W; add central AC and you are looking at a 15,000–20,000 W standby unit. Generator wattage sizing turns on two numbers per appliance: running watts (steady draw) and starting watts (the brief surge when a motor kicks on). Sum the running, find the worst-case start, multiply by 1.25, and round up.
This guide walks through the math, the appliance figures that go into it, the fuel and safety trade-offs, and the mistakes that leave homeowners with an undersized generator that trips out the moment the well pump cycles.
Generator wattage basics
A generator nameplate quotes two figures: running watts (also called rated or continuous watts) and starting watts (also called surge or peak watts). The U.S. Department of Energy defines running watts as the load a generator can sustain indefinitely; starting watts is a short surge it can deliver for a few seconds, typically 10–20% above the rated figure on a conventional unit and as much as 25–30% above on an inverter model.
Generators are sold by their running figure. A "5,500 W generator" can run 5,500 W of steady load and surge to roughly 6,500–7,000 W briefly. Match it to your largest single starting load — not your total starting load if you stagger motor starts — and you have headroom.
Standard portable generator sizes in the U.S. cluster at 2,500, 3,500, 5,500, 7,500, 10,000, and 12,500 running watts. Above 12,500 W most units are standby (permanently installed) rather than portable, because moving them safely starts requiring two or three people.
Running watts vs starting watts
The gap between running and starting watts is almost entirely about induction motors. When an AC motor first energises, the rotor is stationary and the inrush current spikes to 3–7× the running current for a few cycles (50–100 milliseconds). Once the rotor accelerates to synchronous speed, current drops back to nameplate. The National Electrical Manufacturers Association (NEMA) standard MG 1 publishes the locked-rotor codes that quantify this surge.
Resistive loads behave completely differently. A 1,500 W space heater, a 100 W LED string, and a 1,000 W coffee maker all draw their rated wattage from the first millisecond — no surge, starting watts equals running watts. The only appliances with meaningful surge are those with motors (refrigerators, pumps, fans, compressors, power tools) and a few specialised loads like fluorescent ballasts and microwaves.
Manufacturers measure starting watts at room temperature with a perfectly seasoned motor. A 10-year-old refrigerator compressor on a cold morning can pull 30–50% more than its nameplate surge. Build that buffer into the safety margin or pick a generator one size up.
Sizing a generator for home backup
The Department of Energy and Ready.gov both recommend listing critical loads first and discretionary loads second. Critical for most U.S. households means refrigerator, sump pump (basements), well pump (rural), at least one furnace blower or window AC depending on season, and basic lighting plus a Wi-Fi router. The total running figure for that bundle lands around 2,000–3,000 W; the worst-case starting load is usually 3,500–5,000 W, set by the well pump or AC compressor.
Adding the discretionary loads — microwave, coffee maker, TV, a laptop or two — brings running totals to 4,000–5,500 W but barely moves the starting figure, because none of those are motor loads. A 7,500 W generator covers a typical four-bedroom U.S. home for most of an outage, provided the homeowner staggers starts: well pump first, then fridge, then everything else.
- Studio apartment — 2,000–3,000 W generator, no central AC
- 2-bedroom house — 3,500–5,500 W with well pump or window AC
- 4-bedroom house — 7,500–10,000 W with one AC zone
- Whole-home with central AC — 14,000–20,000 W (standby unit)
- Rural with well + septic + electric heat — 22,000–30,000 W standby
Generator fuel use and runtime
U.S. Energy Information Administration figures put gasoline portable generators at roughly 0.75 gallons per hour at 50% load for a 5,000 W unit. That scales close to linear: a 7,500 W unit runs ~1.1 gal/hr at half load, a 10,000 W unit ~1.5 gal/hr. Full-load consumption is roughly double the half-load figure, so the same 5,000 W generator burns 1.5 gallons per hour at full throttle.
Tank sizes vary — most portables hold 4–8 gallons — giving 6–12 hours of continuous runtime at half load. Diesel generators are roughly 25–30% more fuel-efficient by BTU content and also store fuel longer (diesel keeps for 6–12 months with stabiliser, gasoline only 3–6 months even treated). Propane runs cleaner and stores indefinitely but yields about 10% less power per gallon than gasoline.
The U.S. EPA limits portable generator emissions under 40 CFR Part 1054. Modern compliant units burn roughly 30% cleaner than 2010-era models. Honda, Yamaha, and Generac inverter generators meeting CARB (California Air Resources Board) standards are the strictest tier.
Generator safety and code rules
Carbon monoxide poisoning kills around 70–80 Americans per year from portable generator misuse, per the Consumer Product Safety Commission. The single rule that prevents almost all of those deaths: never run a generator indoors, in a garage, in a basement, or within 20 feet of any window, door, or vent. Outside, downwind, on level ground.
The National Fire Protection Association's NFPA 110 covers permanently installed standby generators. Key requirements include a transfer switch (manual or automatic) that physically separates the generator from utility power — back-feeding a panel without a transfer switch can electrocute utility workers restoring service — and a fuel system inspection schedule.
Plugging a generator into a wall outlet to "feed the panel" sends current backwards through the meter and into the utility lines. It violates NEC Article 702 and has killed utility workers restoring service after storms. Always use a properly installed transfer switch or interlock kit.
Modern generators (post-2020 PGMA-G300 compliant) auto-shut off when CO levels rise, but the standard does not yet cover all units sold. Treat every generator as a potential CO source: minimum 20 feet from the house, exhaust pointed away.
Generator wattage sizing mistakes
A 5,000 W generator looks fine for a 4,000 W running load — until a 1,200 W refrigerator surges to 2,400 W on start while a 1,500 W well pump is already running at full draw, blowing past the surge limit. Always size by worst-case starting, not running.
Running a generator at 100% of nameplate continuously cuts engine life roughly in half and shortens fuel filter intervals. The 25% margin built into a sizing calculation keeps the unit running at 70–80% load — the efficiency sweet spot for both gasoline and diesel.
The worst-case formula assumes one motor at a time. In practice a refrigerator can cycle on the same second a sump pump kicks. If two motor loads share the panel, add the second-largest starting watts to the safety margin or jump up a generator size.