Generator sizing
What size generator do I need?
2-step calculator Based on NEC 702 (NFPA 70) & manufacturer specs Reviewed by the WhatSize editorial team
Last reviewed June 2026 • Sources: NEC 702, DOE Energy Use Chart, manufacturer spec sheets
Quick answer
Generator size is your appliances' total running watts plus the largest single starting surge. A typical 2,000 sq ft home essentials (fridge, freezer, sump pump, furnace blower, lights) needs 5,000-7,500 watts. The 2-step wizard below applies NEC 702 to your specific appliance list.
What do you need to power?
Check everything you want to run at the same time. We'll calculate the total wattage needed.
Running watts of selected devices. Starting watts are factored in next.
Based on NEC 702 + manufacturer spec sheets (Generac, Honda, Champion)
Starting wattage needed: 0 W
How we picked these products
Each product below is verified to meet or exceed your calculated spec. We sort by verified specifications first, then by availability and customer reviews. We never recommend a higher-priced product for a higher commission. Read our product recommendation policy.
Disclosure: WhatSize Appliance earns a commission when you buy through Amazon links. This doesn't affect which products we recommend. Read our full disclosure.
Got a result that doesn't match your situation?
Adjust your inputs above and re-run the calculator. Most users find their spec falls within 1-2 sizes of the recommendation.
Formula
Total = (∑ Running Watts × 1.25 buffer) + Largest Single Starting Surge
→ NEC 210.20(A) continuous-load margin + NEC 702 starting-surge rule
About this calculation
This is a simplified sizing tool based on published industry standards, not a full professional audit. Critical applications (electrical, HVAC, flooding prevention) require site evaluation by a licensed contractor.
Accuracy depends on your input — subjective factors like dampness level or insulation quality are estimates. Our formulas include a safety buffer that may result in oversizing for some situations (slightly higher cost, but safer than undersizing).
Local variables (climate zone, altitude, insulation levels, building codes) can affect real-world sizing needs. Standards are re-checked quarterly — see our Editorial Policy.
Common scenarios for generator sizing
If your situation matches one of these, jump straight to a detailed guide:
Frequently asked questions
What size generator do I need to run my house?
For essential circuits only (fridge, furnace fan, lights, phone charger): 5,000-7,500 running watts. For most of a 2,000 sq ft home including well pump or central AC: 10,000-15,000 running watts. Whole-house with every circuit: 20,000-25,000 watts.
How do I calculate generator size?
Add the running watts of everything you want to power simultaneously, then add a 25% safety buffer for startup surges. Motors (fridge, AC, well pump) draw 3-7× their running wattage for 2-3 seconds at startup. The wizard above does this automatically.
What size generator for a 200-amp service?
To cover a 200-amp residential service panel, a 15,000-25,000 watt standby generator is needed. For essential-circuit coverage of a 200-amp home, a 10,000-12,000 watt portable or mid-size standby unit is enough.
What's the difference between running watts and starting watts?
Running watts (rated watts) is the continuous power the generator delivers. Starting watts (surge watts) is the brief burst for motor startup, typically 2-3× running watts for 2-3 seconds. Always size by running watts plus the largest motor surge.