GENERATOR SIZE · REVIEWED SEPTEMBER 2026 · BY BRENT

GENERATOR SIZE

kW = Σ run watts + largest motor surge

Refrigerator (running W, 0 if not on backup)?

Chest/upright freezer (W)

Well pump (½ HP = 1000 W, ¾ HP = 1500 W, 1 HP = 2000 W)?

Sump pump (W)

Furnace blower / boiler (W)

Lights + outlets (W)?

Microwave (W, 0 if not on backup)

Window AC or central AC (running W, 0 if not backed up)?

Other (W)

RESULT

—

FILL IN ABOVE

Running watts + 2.5× surge on largest motor + 20% headroom. Estimate only — verify with a licensed electrician and local code/inspector before purchase or installation. Not a substitute for engineered drawings.

ICC EXAM PREP

Studying for the ICC E2 exam? Practice tests written to the same NEC, 2020 and 2023 editions

→

RECOMMENDED TOOLS

Electrical Tools we recommend for projects like this

→

About this calculator

This generator size calculator adds up running wattage for the loads you want to back up, applies a startup surge to the largest motor (refrigerator, well pump, AC compressor — typically 2.5× running), and recommends a portable or standby generator size. Inductive loads — anything with a motor — are the trap that undersized generators fall into: nameplate running watts is fine, but the locked-rotor inrush at startup is 2-4× higher and stalls the genset. The calculator picks a generator with at least 20% headroom on running watts and headroom for the surge. Use the "essentials" preset for a portable, or "whole home" for a standby. ESTIMATE ONLY — interlock kits, transfer switches, and fuel sizing must be verified by a licensed electrician.

How to use this calculator

Enter the running watts for each load you want backed up. Use 0 for anything you're not powering on backup. The defaults reflect a typical "essentials" backup: fridge, furnace blower, lights, microwave. Add a well pump if you have one (the well pump is usually the biggest single load).

The calculator adds running watts for everything, then adds a startup surge to the largest motor (typically 2.5× for fridges/AC, 3× for well pumps and sumps). Result is the recommended generator size with 20% headroom — portable up to 12 kW, standby (whole-home) above. Standby generators require a transfer switch or interlock kit per NEC 702.

Worked example

For an essentials backup: fridge 700W, furnace blower 600W, microwave 1500W, lights/outlets 600W, well pump 1500W (¾ HP):

Running total: 4,900 W. Largest motor surge: well pump at 3× = 1,500 × 2 = 3,000 W extra. Peak demand: 4,900 + 3,000 = 7,900 W. With 20% headroom: 9,480 W.

Recommended generator: 9,000 W (closest standard size) → portable class.

For whole-home backup adding central AC at 3,500 W and electric water heater at 4,500 W:

Running total: 12,900 W. Largest motor surge: AC at 2.5× = 3,500 × 1.5 = 5,250 W extra. Peak: 18,150 W. With 20% headroom: 21,780 W.

Recommended: 22,000 W → standby (whole-home) class. Requires permanent install with transfer switch and a natural gas or propane fuel line.

Common mistakes & waste factors

Forgetting motor surge. A 700W fridge has a 1,800W startup inrush. Sizing for 700W means the genset trips when the compressor kicks on. The 2.5× surge factor handles this.

Ignoring the well pump. ¾ HP well pumps are the highest surge of common home loads (3× running watts). Adding a well pump to backup often jumps the recommended generator one or two sizes.

Buying portable for whole-home backup. Above ~12 kW, portable units become impractical (heavy, loud, single-fuel). Standby generators (Generac, Kohler, Cummins) start at 14 kW and run on natural gas/propane.

Forgetting the transfer switch / interlock. Connecting a portable generator to your panel without a transfer switch or interlock kit (NEC 702) backfeeds the utility line — kills line workers. Required by code; budget $300-$1,500 for the install.

Rules of thumb

Running watts + largest motor surge + 20% headroom = required generator capacity.

Motor surge: 2.5× for fridges/freezers/AC compressors, 3× for well/sump pumps, 2× for furnace blowers.

Portable class: up to 12 kW. Above that, standby is the practical option.

Well pumps: ½ HP ≈ 1,000 W running, ¾ HP ≈ 1,500 W, 1 HP ≈ 2,000 W. Add 3× surge.

Whole-home backup typically needs 18-26 kW depending on load mix.

Standby install requires NEC 702 transfer switch or interlock kit — $300-$1,500 plus electrician labor.

Common questions

Tool and material links below are affiliate links — we may earn a small commission if you buy, at no extra cost to you.

How many watts to back up the essentials?

A typical "essentials" load — fridge, freezer, furnace blower, lights, phone chargers, modem/router, microwave — totals 3,000–4,500 running watts. Add a well pump and you are at 5,000–6,000 W running with 9,000+ W surge. A 7,500 W portable will not start that well pump — a 9,000 W unit or a 10 kW standby is the honest answer once a well is in the load list.

What is starting (surge) wattage?

Motors draw 2-4× their running wattage for the half-second they are spinning up. A 700 W fridge surges to ~1,800 W; a ½ HP well pump runs at 1,000 W and surges to 3,000-4,000 W. The generator must have enough surge headroom (sometimes called "peak watts") to swallow the largest single motor without stalling.

Do I need a transfer switch?

Anything wired into the home panel needs a manual transfer switch or a UL-listed interlock kit per NEC 702. Backfeeding through a dryer outlet without an interlock is illegal and can kill a lineman working on the de-energized service. Portable generators powering extension cords directly to appliances are exempt.