About Size My Generator

A simple, practical tool from a licensed HVAC pro to help you think through backup power—without the fluff.

About SizeMyGenerator.com

Practical, no-nonsense generator sizing guidance—free to use and updated as assumptions improve.

Who’s behind this

I’m an HVAC professional with hands-on experience sizing, installing, and servicing residential systems. I built this calculator so families who don’t speak kilowatts all day can quickly get to a sensible starting point—before talking to a pro.

About this calculator

This site helps homeowners ballpark a generator size using a simplified interpretation of the National Electrical Code (NEC) dwelling load method, tailored for residential optional standby systems.

How the calculator works (high level)

General lighting load: 3 VA/ft² of floor area you enter.
Small appliance & laundry circuits: 1,500 VA each.
Fixed appliances: nameplate running watts at 100%.
Demand factor: first 10 kVA at 100%, remainder at 40% for non-A/C portion.
Cooling vs heating: larger of A/C or heat added at 100% (with special handling for heat pumps + strip heat).
EV charging: Level 2 loads added at 100% based on breaker size.
Headroom: default 20% safety factor before rounding to a common generator size.

What this site is (and isn’t)

This tool is oriented to residential optional standby systems (NEC Article 702). It is not intended for emergency systems (Article 700), legally required standby (Article 701), or critical operations power systems (Article 708), which have different requirements.

It’s planning guidance, not a site survey. Homes vary, nameplates can surprise, and local AHJs may amend the NEC. Before purchasing or installing a generator, consult a licensed electrician/contractor and your utility to confirm sizing, transfer equipment, interconnection rules, and code compliance.

Monetization & independence

SizeMyGenerator.com is free to use. We fund the site with display ads and affiliate links. If you buy through a link, we may earn a commission at no extra cost to you. Our recommendations are driven by practical assumptions and field experience—never pay-to-play.

See our Affiliate Disclosure and Privacy Policy for details.

Ownership & compliance

SizeMyGenerator.com is owned by Atacama Cooling & Heating, LLC (Pennsylvania).
PA HIC#: PA 073102
Contact: bill@atacamahvac.com

Corrections & updates

We periodically review load tables and assumptions. If you spot anything off—or have a suggestion—please email us.