How to Size a Backup Generator for Your Home

Written By Robert Frary

Choosing the right generator size is critical. Too small, and it won’t power essentials; too large, and you overpay.

Step 1: List Essential Loads

Identify appliances you must run during an outage:

  • HVAC system

  • Refrigerator/freezer

  • Well pump or sump pump

  • Lights and critical outlets

  • Medical equipment

Step 2: Find Starting & Running Watts

Check each appliance’s nameplate or manual. Add up the starting watts for equipment with motors (A/C, fridge) and running watts for everything else.

Step 3: Add a Safety Margin

A good rule is 25% headroom for unexpected loads or future upgrades.

Example:

  • HVAC: 4,000 start / 2,000 run

  • Fridge: 1,200 start / 200 run

  • Lights/outlets: 800 run

  • Total running: ~3,000 watts → add 25% → 7 kW generator recommended.

Professional sizing ensures accuracy, especially for whole-home systems.

Robert Frary
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