Generator Size Calculator (Standby & Portable)

How to Use This Generator Size Calculator

Choose standby or portable, then tick every appliance you want running during an outage. Edit quantity, running watts and starting watts if your nameplates differ. The tool returns total running watts, a motor-surge allowance, the required running and surge ratings, the next standard generator size up, and the amperage figures an electrician will ask about for the transfer switch and conductors.

Why This Matters

An undersized generator trips its breaker the moment a well pump or air conditioner starts. It can also run overloaded for hours, which overheats the alternator and shortens engine life. An oversized one costs thousands more up front and burns more fuel at light load. Getting the number right matters in both directions.

Take a rural family with a well. They want the refrigerator, sump pump, furnace blower, a few lights and internet through a three-day winter storm. That is roughly 3,000 running watts, but a 1/2 HP well pump can pull 3,000 watts for a second or two when it starts. A 3,500-watt unit looks adequate on paper and fails in practice. Another household wants central air through a summer blackout. A 3-ton unit needs about 3,500 running watts and roughly 10,500 at startup unless it has a soft starter, which pushes a single-unit recommendation to about 8 kW standby or 9,500 watts portable before anything else is added. The same math applies to contractors sizing portable units for saws and compressors on a job site.

How It's Calculated

Every appliance has two ratings. Running watts is what it draws continuously. Starting watts is the brief surge, mostly for motors (refrigerators, pumps, compressors, air conditioners). Resistive loads such as heaters and water heaters have almost no surge. The calculator keeps the two ratings separate:

The surge factor is a planning assumption: portable generators can typically sustain about 125% of their running rating for a few seconds, and home standby units are assumed to start motors at about 150% of their kW rating with an acceptable voltage dip. Real models differ, and some inverter units have a smaller surge margin, so compare against the manufacturer's motor-starting data or the starting rating on the spec sheet. The result is rounded up to the next common size: portable sizes run from 1,000 to 12,000 running watts, standby sizes from 8 to 60 kW.

This is a conservative estimate. By default it assumes every ticked load can run at the same time, adds 20% headroom, and lets two large motors start together. Many homes run well on less, using load-shed modules, a smart panel, or simple habits such as no dryer while the AC runs. If you will manage loads, lower the diversity or choose the staggered motor option.

For the electrical side, the suggested breaker is the next standard size (NEC 240.6) at or above the generator's full-load current at 240 V single-phase. The conductor figure is the larger of 115% of that current (the generator-conductor rule in NEC 445.13) and the breaker rating, so the conductor is never protected by a device larger than its ampacity (NEC 240.4). It is a minimum ampacity, not a wire size: gauge depends on insulation, temperature, conduit fill, run length and voltage drop (NEC 310.16). Standby systems are also covered by NEC Article 702 or related articles where adopted, plus utility and manufacturer requirements. The adopted code edition and local amendments vary, and the generator maker's supplied breaker and instructions govern. This tool is a planning estimate, not engineering, permit or inspection approval.

Worked Example

A homeowner selects the Essentials preset, portable generator, 20% headroom, two-motor surge and 100% diversity:

• Refrigerator 700 W (starts at 2,200), sump pump 800 W (2,400), furnace blower 800 W (2,350), garage door opener 500 W (1,500), LED lights 100 W and electronics 150 W.
• Running total: 700 + 800 + 800 + 500 + 100 + 150 = 3,050 W.
• Running need: 3,050 × 1.20 = 3,660 W (610 W headroom).
• Surge: the two largest starting increments are the sump pump (2,400 − 800 = 1,600) and the furnace blower (2,350 − 800 = 1,550), so 3,150 W. Starting peak = 3,050 + 3,150 = 6,200 W.
• Portable rating needed: 6,200 ÷ 1.25 = 4,960 W, which is more than the 3,660 W running need, so motor starting controls. The next portable size up is 5,500 W running (about 6,875 W surge). The design load would use about 55.5% of that.
• Amperage: 5,500 ÷ 240 = 22.9 A, so the suggested breaker is 25 A and the conductors need at least 22.9 × 1.15 ≈ 26.4 A of ampacity.
• For a standby unit the same loads need 3,660 W running and 6,200 ÷ 1.5 ≈ 4,133 W for starting, so the smallest standard size, 8 kW, covers it.

Tips & Common Mistakes

• Use nameplate data. Typical wattages are averages. A well pump or AC compressor label gives locked rotor amps (LRA); starting watts ≈ LRA × voltage.
• Don't add every appliance in the house. Load management, such as a smart load-shed module for standby units, can cut the size needed. Reduce the diversity setting only if you will really prevent those loads from overlapping.
• Consider soft starters for air conditioners. They can cut starting surge by 50–70%, which can drop a standby unit one or two sizes. Re-enter the lower starting watts to see the effect.
• Never backfeed through a dryer outlet or “suicide cord.” Use a listed transfer switch or interlock installed by a licensed electrician, and obtain a permit where required.
• Keep portable generators outdoors, far from windows and doors. Carbon monoxide kills. Use CO alarms indoors and follow manufacturer clearances for standby units.
• Don't forget fuel. A bigger generator uses more fuel, and natural gas or propane standby units depend on supply pressure, pipe sizing and tank size.

Frequently Asked Questions