Generator Sizing Calculator
Calculate the minimum generator wattage needed to power your home's essential loads, motors, and well pump during an outage. Use it before buying a standby or portable generator to avoid under-sizing.
Last updated: September 2026
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
Generator sizing requires accounting for both running watts and the starting surge of motors. A motor’s electrical draw is much more than its shaft rating of 746 W per horsepower, because of motor losses and power factor, so this calculator takes each motor’s running volt-amperes from the full-load currents in NEC Table 430.248 (single-phase, 230 V): ½ hp 4.9 A (about 1,130 VA), 1 hp 8 A (1,840 VA), 2 hp 12 A (2,760 VA), 3 hp 17 A (3,910 VA), 5 hp 28 A (6,440 VA), interpolating between table sizes. The formula is: Required Watts = (essential_watts + motorVA × startup_factor + wellPumpVA × 4) × safety_margin. The startup factor (1.5 for soft-start, 3 for standard and 5 for hard-start motors) multiplies running VA to cover the surge; well pumps use a fixed factor of 4 because submersible pump motors have high inrush current. The calculation assumes the motor and the well pump could start at the same time, which is conservative. Multiplying by a safety margin (e.g., 1.25) keeps the generator below its maximum rated output. Choose a generator whose rated wattage equals or exceeds this result, and check its surge rating against the motor starting load.
How to use
Suppose essential loads total 3,000 W, you have a 1 HP HVAC motor with a startup factor of 3, a 0.5 HP well pump, and a 1.25 safety margin. Motor running draw: 1 HP = 8 A × 230 V = 1,840 VA; surge 1,840 × 3 = 5,520 W. Well pump: 0.5 HP = 4.9 A × 230 V = 1,127 VA; surge 1,127 × 4 = 4,508 W. Total before margin: 3,000 + 5,520 + 4,508 = 13,028 W. Multiply by the safety margin: 13,028 × 1.25 = 16,285 W. You would need at least a 16.3 kW generator, so the next standard size, typically 17-18 kW. Using 746 W per HP instead would have suggested only about 8.4 kW, too small to start these motors.
Frequently asked questions
How do I calculate the startup surge watts for a motor when sizing a generator?
Start from the motor’s running draw, not its horsepower: a 2 HP single-phase motor draws about 12 A at 230 V (NEC Table 430.248), or 2,760 VA, roughly twice its 1,492 W shaft output. Multiply the running draw by the startup factor — about 1.5 for soft-start motors, 3 for standard induction motors and up to 5 or more for hard-starting compressors — so a 2 HP motor with a factor of 3 needs about 8,280 W of surge capacity. Appliance manuals and manufacturer data sheets often list the starting (locked-rotor) amps; use them when available.
Why does a well pump require a higher startup factor than other motors?
Submersible well pumps start against the static head pressure of the water column above them, meaning they must overcome both mechanical inertia and hydraulic resistance the instant power is applied. This combination produces inrush currents 4–6 times the running current, far higher than a motor starting under no load. Using a factor of 4 (as in the formula) is the industry standard conservative estimate. Undersizing the generator for a well pump is one of the most common mistakes homeowners make, resulting in generator shutdown or failed pump starts during an outage.
What safety margin should I use when sizing a home backup generator?
A 1.25 safety margin (25% overhead) is the standard recommendation for residential backup generators. Running a generator at 100% of its rated capacity continuously shortens engine life, increases fuel consumption, and leaves no room for temporary load spikes. The 25% buffer keeps the generator in its optimal operating range (75–80% of rated output), reduces wear, and accommodates small additional loads you may have overlooked. For critical applications like medical equipment or server rooms, consider a 1.5 margin or install a generator with automatic load shedding to prioritize the most essential circuits.