Generator size by appliance / water heater
What size generator runs an electric water heater
Electric water heater: 4,500 W running with no starting surge in the source table, so a generator needs 4,500 W of running capacity for it. The smallest of the six portables this site covers that can do it is the Honda EU7000iS (5,500 W running, 7,000 W max).
EWEB lists an electric water heater at 4,500 W running with no starting surge. Generac's brochure lists 2,000 W running and 1,000 W additional starting, less than half of EWEB's figure.
The element's rating is printed on the heater's label; that one number replaces both table rows. Electric water heaters are 240-volt appliances on their own breaker.
One way to fit a water heater onto a smaller generator is to switch its breaker off while other loads run and back on once they are done, so its 4,500 W never stacks on top of them.
The figures and where they come from
| Device | Source | Running W | Additional starting W | Starting need alone |
|---|---|---|---|---|
| Electric water heater | EWEB (2019) | 4,500 | — | 4,500 |
| Electric water heater | Generac GP brochure (2023) | 2,000 | 1,000 | 3,000 |
Highlighted rows are the figures the calculator uses. Every table here prints additional starting watts: the surge on top of running watts.
Which portables can run it
| Generator | Running / peak W | Alone | With fridge, 6 LED bulbs, 2 chargers |
|---|---|---|---|
| Honda EU2200i | 1,800 / 2,200 | No | No |
| Honda EU3000iS | 2,800 / 3,000 | No | No |
| Westinghouse iGen5000 | 3,900 / 5,000 | No | No |
| Honda EU7000iS | 5,500 / 7,000 | Yes | No |
| Champion 201040 | 7,500 / 9,375 | Yes | Yes |
| Westinghouse WGen9500DF | 9,500 / 12,500 | Yes | Yes |
A generator fits when its running rating covers every running watt and its peak (or "max") rating covers the running total plus the single biggest starting surge. Adding the basics — a refrigerator (700 W running, 2,200 W to start), six 15 W LED bulbs and two phone chargers — raises the running load to 5,330 W and the starting need to 7,530 W, because the refrigerator's surge is now the biggest one. The smallest portable here that covers that is the Champion 201040.
On a standby generator
A home standby unit is rated in kilowatts and has no overload capacity: Generac's rating definition says "No overload capability is available for this rating." Using Generac's own formula — sum of running watts plus the highest single starting burst — the water heater with the basics needs 7.5 kW. The smallest Guardian model that covers it on natural gas is the 10 kW, rated 9 kW on natural gas and 10 kW on propane, which leaves 1.5 kW of starting room for everything else. Whole-home lists are on the standby generator pages.
Before running anything, read the carbon monoxide distance rules and how a transfer switch keeps power off the utility lines. Other appliances: refrigerator · sump pump · well pump · furnace · window AC · central air · heat pump · space heater · washing machine · garage door opener · CPAP · oxygen concentrator.