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.
About this calculator
Generator sizing requires accounting for both running watts and startup surge. Electric motors draw 3–7× their running current for a fraction of a second at startup, which can trip a generator's overload protection if not planned for. The formula is: Required Watts = (essential_watts + (motor_hp × 746 × startup_factor) + (well_pump_hp × 746 × 4)) × safety_margin. The constant 746 converts horsepower to watts. The startup_factor (typically 2–3) multiplies motor running watts to cover surge. Well pumps use a fixed factor of 4 because submersible pump motors are notorious for high inrush current. Summing these loads gives total peak demand; multiplying by a safety_margin (e.g., 1.25) ensures the generator runs below its maximum rated output for longevity and reliability. Choose a generator whose rated wattage equals or exceeds this result.
How to use
Suppose essential loads total 3,000 W, you have a 1 HP HVAC motor with a startup factor of 3, and a 0.5 HP well pump. No safety margin is entered, so use 1.25. Motor surge: 1 × 746 × 3 = 2,238 W. Well pump surge: 0.5 × 746 × 4 = 1,492 W. Total before margin: 3,000 + 2,238 + 1,492 = 6,730 W. Multiply by safety margin: 6,730 × 1.25 = 8,412.5 W. You would need at least an 8,500-watt (8.5 kW) generator — so choose the next standard size, a 9,000 W or 10,000 W unit.
Frequently asked questions
How do I calculate the startup surge watts for a motor when sizing a generator?
Multiply the motor's horsepower by 746 to get running watts, then multiply again by the startup factor — typically 2 for soft-start motors and up to 3–7 for standard induction motors. For example, a 2 HP air compressor motor has 2 × 746 = 1,492 running watts; with a startup factor of 3, its surge demand is 4,476 watts. Many appliance manuals and manufacturer websites list both running and starting watts explicitly. If only running watts are listed, applying a factor of 3 is a safe conservative estimate for standard single-phase induction motors.
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.