Generator Size Calculator

Multiply your running watts by a surge factor for your load type, and separately divide running watts by 0.8; the larger result, rounded up, is your generator size. A 4,000-watt mixed household load with a 1.5× surge factor peaks at 4,000 × 1.5 = 6,000 W, while the 80% derate gives 4,000 ÷ 0.8 = 5,000 W. The larger figure is 6,000 W, so you need a 6,000-watt generator rated for about 7.5 kVA.

The surge factor is a shortcut for the startup spike of your equipment. Pure resistive loads such as heaters, incandescent lights, and electric kettles have essentially no inrush (factor ≈ 1.2 for safety). A typical mix of household appliances lands near 1.5×. Homes dominated by motors — well pumps, compressors, shop tools — need 2× or even 3× to cover locked-rotor inrush. Choosing the right factor is the difference between a generator that starts your loads and one that stalls.

The surge factor multiplies steady running watts to approximate the momentary starting demand. It exists because electric motors draw a large inrush current — often called locked-rotor amperage — for the fraction of a second before the rotor spins up. The heavier and harder-starting the motor, the larger the multiplier:

• 1.2× — mostly resistive. Space heaters, electric ranges, toasters, incandescent bulbs, and heating elements. These have almost no surge; the 1.2 keeps a small margin.
• 1.5× — mixed household. A blend of electronics, a refrigerator or two, lighting, and a small pump. This is the safe default for typical backup planning.
• 2× — motor-heavy. Multiple refrigeration compressors, a well pump, power tools, or a single window/central A/C on a soft start.
• 3× — hard-starting motors. Capacitor-start pumps, older single-phase compressors, air compressors, and central air without a soft-start module.

If you know the exact starting watts of your largest appliance, the what size generator do I need calculator gives a more precise "running + largest surge" result. Use this surge-factor tool when you only know the total running load and its general character.

Even when the surge factor is small, a generator must not run permanently at its maximum rating. Manufacturers publish two numbers — a higher starting/peak watt figure the unit can hold for seconds and a lower running/continuous watt figure it can sustain for hours. Continuous operation should stay near 80% of the running rating so the engine runs cool, the alternator holds a clean 60 Hz, and there is room for cycling motors.

That is why the calculator also computes running ÷ 0.8. For a 5,000 W resistive load with only a 1.2× surge, the peak is 6,000 W but the derate need is 5,000 ÷ 0.8 = 6,250 W — actually the governing number. Whenever your load is mostly steady, the 80% derate, not the surge, sets the size. The calculator always reports the larger of the two so you never under-buy. The same 80% ceiling appears in inverter selection; compare with the what size inverter do I need calculator.

Generators are often specified in kVA (kilovolt-amperes) rather than kilowatts, especially standby and commercial units. The difference is power factor:

Real motors and reactive loads have a power factor below 1.0, meaning the generator's alternator must supply more apparent power (kVA) than the real power (kW) delivered to the load. The industry standard assumption for generator sizing is a 0.8 power factor. So a generator delivering 6,000 W of peak real power needs an alternator rated for 6,000 ÷ 0.8 = 7,500 VA = 7.5 kVA. This calculator reports that required kVA automatically.

When a spec sheet lists only kVA, multiply by 0.8 to estimate usable watts: a "10 kVA" standby generator supplies about 8,000 W of real power. For deeper background on the relationship, see kVA to watts and the power factor explainer.

Example 1 — Mixed household. 4,000 W running, 1.5× factor. Peak = 6,000 W; derate need = 5,000 W. Recommend 6,000 W (7.5 kVA). This is the default above.

Example 2 — Well-pump cabin. 3,000 W running, mostly motor loads, 2× factor. Peak = 6,000 W; derate need = 3,750 W. Surge governs → 6,000 W. A cabin whose biggest draw is a pump needs a generator sized well above its steady watts.

Example 3 — Electric-heat shop. 5,000 W running, resistive, 1.2× factor. Peak = 6,000 W; derate need = 5,000 ÷ 0.8 = 6,250 W. Here the 80% derate governs, and 6,250 W rounds up to the next standard rating → 7,500 W. Note how a nearly surge-free load still needs a generator well above its running watts because of the continuous derate plus rounding to a real size.

Example 4 — Small standby for essentials. 6,500 W running, 1.5× factor. Peak = 9,750 W; derate need = 8,125 W. Recommend 10,000 W (12.5 kVA) — a common small whole-home standby size.

A generator's kW/kVA rating is measured at sea level and moderate temperature. At elevation the engine breathes thinner air and loses power, and hot intake air compounds the loss. Standard derating guidance:

• ~3–3.5% less power per 1,000 ft above about 1,000 ft of elevation.
• ~1% less power per 10°F above 77°F.

A 6,000 W generator at 6,000 ft delivers only about 6,000 × (1 − 0.035 × 5) ≈ 4,950 W. If you operate above 2,000 ft, choose the next standard size up, or verify the manufacturer's high-altitude kit and derating chart. Standby generators on natural gas or propane already run about 10% below their gasoline rating, so altitude derating stacks on top of the fuel derating.

1. Total your running watts. Add the running (rated) watts of every load that will operate at once. The power consumption calculator and house electrical load calculator make this fast.
2. Pick the surge factor that matches your loads. Resistive 1.2×, mixed 1.5×, motor-heavy 2×, hard-starting 3×. When unsure, choose the higher factor.
3. Read the recommended size and kVA. The tool shows peak watts, the derated running need, and the standard generator rating that clears both.
4. Derate for altitude. Above ~2,000 ft or in extreme heat, size up one notch.

The recommended wattage points you toward a class of generator, and each class carries its own sizing habits. Matching the two avoids buying a unit that is technically large enough but wrong for the job.

Small portables (3,000–5,000 W). Sized from a short essentials list — fridge, lights, a furnace blower, and charging. Their surge headroom is modest, so pick loads with care and avoid starting two motors at once. A 1.5× mixed factor is usually right here.

Large portables (6,000–9,000 W). The workhorse range for gas-heated homes that also want a well pump or a window air conditioner. At this size the 80% derate and a 2× motor factor typically govern, so verify both the running and starting ratings on the label.

Standby generators (10,000 W and up). Sized from the whole connected load rather than a hand-picked subset, because an automatic transfer switch may energize entire panels. Two extra factors apply: natural gas and propane cut output about 10% below the gasoline rating, and the National Electrical Code requires the generator to safely serve the calculated load. This is where a formal house electrical load calculation — general lighting at 3 VA per square foot, appliance and motor circuits, and the largest-motor 125% rule — replaces back-of-envelope math.

Three-phase and commercial. Larger buildings use three-phase generators where kVA, not kW, is the headline number and power factor matters even more. The 0.8 power-factor assumption still holds, but balancing load across phases and accounting for the largest motor's inrush on a single phase become part of the sizing. For most homes, single-phase 120/240 V sizing as shown above is all you need.

One more sizing nuance separates the generator classes: step load, or how much load a generator can accept in a single instant without its voltage and frequency dipping out of tolerance. A large standby unit can swallow a big motor start in one step, while a small portable running near its limit may sag when the compressor hits. This is why sizing with genuine surge headroom — the 20% the 80% derate leaves free, plus a correct surge factor — matters even when the average load looks comfortable. If your loads are dominated by a single hard-starting motor, either size the generator so that motor's inrush is a small fraction of capacity, or fit the appliance with a soft-start device to flatten the step. The calculator's surge-factor and derate outputs together tell you whether your biggest motor will start cleanly or bog the engine down.

• Using a 1.2× factor on a motor load. A well pump or compressor can surge 2–3×; a low factor leaves the generator unable to start it.
• Ignoring the 80% derate on resistive loads. Even surge-free loads need a generator larger than their running watts.
• Confusing kVA with kW. A "10 kVA" generator delivers only ~8 kW of real power at 0.8 power factor.
• Forgetting fuel and altitude derating. Propane/natural gas and high elevation each cut usable output.

Cross-check your result with the generator wattage calculator and see real appliance fit in what will a 5,000-watt generator run.

Finally, remember that a surge factor is a planning estimate, not a guarantee. If you can measure the actual starting watts of your largest motor — from its locked-rotor amperage on the nameplate, multiplied by voltage — do so and confirm the recommended size still clears it with margin. The surge factor keeps you safe when exact figures are unknown, but real measurements always trump a multiplier when the biggest motor is close to the generator's ceiling.