What Size Generator Do I Need? Calculator

Add up the running watts of everything you want to power, divide that by 0.8 to leave 20% headroom, then separately add your single largest motor-starting surge to the running total — the bigger of those two numbers is the generator size you need. For a home running 5,000 watts of essentials with a 2,200-watt well-pump or A/C startup surge, the running requirement is 5,000 ÷ 0.8 = 6,250 W and the peak requirement is 5,000 + 2,200 = 7,200 W. The larger figure, 7,200 W, rounds up to a 7,500-watt generator.

Two different limits decide generator size and you must satisfy both. The first is the continuous running load: a generator should not run flat-out for hours, so we size it to carry your steady draw at about 80% of its rating. The second is the starting (surge) load: motors in refrigerators, air conditioners, well pumps, and power tools draw several times their running watts for a fraction of a second at startup, and the generator must survive that spike without stalling. This calculator solves both and reports the size that clears each one.

Every appliance with a motor or compressor has two wattage figures, and confusing them is the number-one cause of undersized generators.

• Running watts (also called rated or continuous watts) is the power an appliance draws while it operates normally. A refrigerator might run at 150 W, a window air conditioner at 1,200 W, a well pump at 1,000 W.
• Starting watts (also called surge, peak, or locked-rotor watts) is the brief spike an electric motor demands to overcome inertia the instant it turns on. This inrush lasts a fraction of a second but can be 2 to 7 times the running watts.

A generator is rated for both: a "7500 running / 9500 starting watt" generator can carry 7,500 W continuously and absorb a 9,500 W momentary surge. Because only one large motor usually starts at any given instant, the practical peak for a whole house is your total running watts plus the single largest surge — not the sum of every appliance's surge. That is exactly the peak figure this tool computes.

To break down individual appliance loads before you total them, use the power consumption calculator, and for a room-by-room or panel-level tally use the electrical load calculator for a house.

Generators — like circuit breakers and inverters — are sized to run at about 80% of their maximum rating for extended periods. The final 20% is reserved as headroom. There are solid engineering reasons for this rule:

• Heat. A generator running near 100% load produces maximum heat in its windings and engine. Sustained full-load operation shortens life and risks overheating, especially on hot days.
• Voltage and frequency stability. A lightly loaded generator holds a cleaner 120/240 V, 60 Hz output. Near full load, voltage sag and frequency droop worsen, which matters for electronics and motors.
• Surge room. Leaving 20% free means a refrigerator or pump can cycle on without the generator bogging down or tripping.

That is why this calculator divides your running watts by 0.8. A 5,000 W continuous load needs a generator rated for 5,000 ÷ 0.8 = 6,250 W of running capacity so it spends its life at a comfortable 80%. The National Electrical Code applies the same 80% (or 125% sizing) logic to continuous loads on branch circuits, so the number feels familiar to any electrician. The same 80% principle governs inverter sizing — see the what size inverter do I need calculator.

Example 1 — Storm-outage essentials. You want fridge (200 W run), furnace blower (800 W run / 2,350 W start), a sump pump (800 W run / 2,000 W start), lights and electronics (500 W), and a microwave (1,000 W). Total running = 200 + 800 + 800 + 500 + 1,000 = 3,300 W. Largest single surge is the furnace blower's extra 1,550 W over running, but the tool uses running + full largest starting: peak = 3,300 + 2,350 = 5,650 W. Running need = 3,300 ÷ 0.8 = 4,125 W. The larger is 5,650 W → a 6,000 W generator.

Example 2 — Well + A/C home. Running load 5,000 W with a well pump that surges to 2,200 W above running. Running need = 6,250 W; peak = 7,200 W; recommend a 7,500 W unit — the default in the calculator above.

Example 3 — Large all-electric home. Running 8,000 W with a central-A/C compressor surging 4,000 W. Running need = 10,000 W; peak = 12,000 W; recommend a 12,000 W (12 kW) standby generator. Homes with central air, electric range, and a well often land at 10–15 kW, which is why air-cooled standby units cluster around those ratings.

A generator's nameplate rating assumes standard conditions — roughly sea level and 25°C (77°F). Thinner air at elevation and hotter intake air both reduce the engine's power because a naturally aspirated engine breathes less oxygen. The widely used rule of thumb:

• Altitude: derate about 3–3.5% of rated power for every 1,000 feet (305 m) above the first 500–1,000 feet.
• Temperature: derate roughly 1% for every 10°F (about 6°C) above 77°F.

Example: a 7,500 W generator used at 5,000 ft loses about 5,000 ÷ 1,000 × 3.5% ≈ 17.5%, leaving roughly 6,190 W usable. On a 97°F day add another ~2%. If you live at altitude, size up one notch — a mountain cabin needing 6,000 W at sea level should shop for a 7,500 W unit. Some manufacturers sell high-altitude carburetor jet kits that partially restore output; check the owner's manual for your engine's derating chart.

1. List every appliance you want to run at the same time. Backup planning is about essentials — don't size for the whole house unless you truly need it all running together.
2. Add up their running watts. Enter the total in the first field. Nameplate labels, spec sheets, or the power consumption calculator give you these numbers.
3. Find your single largest starting surge. Identify the appliance with the biggest motor — usually the A/C compressor or well pump — and enter its starting (surge) watts in the second field.
4. Read the recommendation. The tool shows your running need (with the 80% derate), your peak need, and the standard generator size that satisfies both.
5. Adjust for altitude. If you live above ~2,000 ft or in extreme heat, step up one size using the derating table above.

Sizing is only half the decision. A few practical factors shape which generator actually fits your home:

Transfer switch. To power hard-wired circuits (furnace, well pump, whole panels) safely and legally, you need a manual or automatic transfer switch — never backfeed through a dryer outlet. A standby generator pairs with an automatic transfer switch that starts it within seconds of an outage.

Fuel type. Gasoline portables are cheapest but store poorly; propane runs cleaner and stores indefinitely; natural-gas standby units never need refueling but deliver slightly less power than the same engine on gasoline. Factor a 10% derating when running many generators on propane or natural gas.

Runtime and tank size. A 7,500 W generator at 50% load typically burns 0.5–0.75 gallons of gasoline per hour, so a 7-gallon tank lasts roughly 8–10 hours. Larger loads shorten runtime. Inverter generators throttle the engine to load, dramatically improving fuel economy and noise at part load.

Once you know the wattage you need, the class of generator determines how you live with it. Three families dominate the residential market, and the right one depends as much on runtime and noise as on raw output.

Conventional portables (3,000–12,000 W). These open-frame gasoline units deliver the most watts per dollar and cover the widest sizing range, making them the default for storm backup. They run the engine at a fixed 3,600 RPM to hold 60 Hz, so they are louder (65–75 dBA) and less fuel-efficient at part load, and their power has more total harmonic distortion (THD) — usually fine for motors and heaters but occasionally rough on sensitive electronics.

Inverter generators (1,000–7,500 W). An inverter unit generates AC, rectifies it to DC, then electronically rebuilds a clean sine wave, which lets the engine throttle down to match load. The payoff is low noise (48–60 dBA), excellent part-load fuel economy, and under 3% THD that is safe for laptops and medical devices. They cost more per watt and top out lower, so they suit RVs, quiet backup, and sensitive loads more than a whole all-electric home.

Standby generators (8,000–26,000 W+). Permanently installed and wired through an automatic transfer switch, standby units run on natural gas or propane and start themselves within seconds of an outage. They are the choice for hands-off whole-home backup, but remember the ~10% fuel derating versus gasoline when you compare their nameplate kW to a portable's. Size a standby from your full connected load, not just the essentials, if you want the outage to feel invisible.

• Summing every appliance's surge. Only one large motor starts at a time; adding all surges wildly oversizes the generator.
• Ignoring surge entirely. Sizing only on running watts leaves nothing for startup, and the generator stalls when the A/C kicks on.
• Running at 100% load. No headroom means overheating, dirty power, and a short engine life. Keep continuous load near 80%.
• Forgetting altitude. A unit that's perfect at sea level is undersized by 15–25% at 5,000+ ft.
• Assuming propane equals gasoline output. Alternative fuels typically cut output ~10%.

Pair this tool with the generator wattage calculator to double-check by adding running plus starting watts, and the generator size calculator to size by load-type surge factor.