Water heaters can be one of the largest loads in a home backup plan. The right generator size depends heavily on heater type, and the difference between gas and electric models is huge.
Build a complete load plan with the WattSizing Calculator.
Quick Answer
Most electric tank water heaters need a 6,000 to 10,000 watt generator. Gas water heaters often run on just 500 to 2,000 watts for controls and fans. Electric tankless units are usually too large for typical portable generators.
Water Heater Sizing Basics
- Identify heater type: gas, electric tank, or electric tankless.
- Use rated running watts from the nameplate.
- Add any startup/peak allowance (especially for fan-equipped models).
- Add simultaneous household loads and a 20% margin.
In extended outages, many homeowners use a smaller generator for essentials and postpone electric water heating to keep fuel use manageable.
Related planning guides:
Water Heater Generator Sizing Table
| Water Heater Type | Typical Running Watts | Typical Peak/Starting Watts | Recommended Generator Size |
|---|---|---|---|
| Gas tank heater (controls + blower) | 100 - 500 W | 300 - 900 W | 800 - 2,000 W |
| Hybrid heat pump water heater | 500 - 1,500 W | 1,000 - 2,500 W | 2,000 - 4,000 W |
| Electric tank heater (240V) | 3,000 - 5,500 W | 3,500 - 6,000 W | 6,000 - 10,000 W |
| Electric tankless (whole-home) | 12,000 - 28,000+ W | 12,000 - 30,000+ W | 15,000 - 35,000+ W |
Worked Sizing Example
You have a 240V electric tank water heater and want limited additional loads:
- Water heater element: 4,500 W
- Fridge + lights + router: 450 W
- Small allowance for switching transients: 300 W
Calculation:
- Peak combined load:
4,500 + 450 + 300 = 5,250 W - Add 20% margin:
5,250 x 1.2 = 6,300 W
Practical pick: a 6,500 to 7,500 W generator.
This is why electric hot-water backup often pushes people into larger and less fuel-efficient generator classes.
Practical Backup Strategy
- Prioritize shower timing instead of continuous water reheating.
- For gas units, verify ignition/control power requirements.
- Avoid running water heater alongside ovens, dryers, or large HVAC loads.
- If outage durations are long, consider alternative hot-water methods to save fuel.
FAQs
Can a 5,000W generator run an electric water heater?
Sometimes for smaller elements with no other loads, but it is usually tight. Most setups need more headroom.
Why can gas water heaters run on small generators?
Because the fuel does the heating. Electricity mainly powers controls, ignition, and fan components.
Can I run an electric tankless water heater on a portable generator?
Usually no. Whole-home electric tankless units often require far more power than typical portable generators provide.
Should I include other circuits when sizing for hot water?
Yes. Real operation includes refrigerators, lights, internet gear, and occasional motor starts.
CTA
Need to balance hot water comfort with outage fuel limits? Use the WattSizing Calculator to compare full-load and essentials-only generator sizing in minutes.
