Generator sizing comes down to two numbers: the running watts of everything you want on at once, and the extra surge the hungriest motor needs for a second or two when it starts. Tick what you want to power during an outage, adjust the figures to match your own appliances’ labels, and the calculator gives you both numbers and the size class that fits.
Figures are typical values from generator manufacturers’ sizing charts. The nameplate on your appliance (or its manual) is more accurate; edit any number to match. Enter total starting watts, not just the extra surge above running watts. Washers vary widely (models that heat their own water draw far more), and some shallow-well jet pumps run on 120V, so check those labels in particular.
| Appliance | Qty | Running W | Starting W |
|---|
- Largest single start-up surge—
- Generator class that fits—
- 240V appliances selected—
How the numbers work
This follows the standard method in generator makers’ own sizing guides. Running watts is the sum of everything you have ticked. Starting watts adds only the single biggest start-up surge on top. Motors like a well pump, sump pump, furnace blower or air conditioner briefly draw two to three times their running load when they kick on. In normal use those motors rarely start at the same instant, so you plan for the largest one, not all of them together. A generator has to cover both figures: its running (rated) watts must exceed your running total, and its starting (peak) watts must exceed your starting total.
The recommendation adds a safety margin on top of the running total. Running a generator near its maximum shortens its life, raises fuel burn, and leaves no room for an appliance you forgot. Twenty percent is a common rule of thumb.
Things that change the answer
- Central air conditioning has the largest surge in most homes. A soft-start kit installed on the compressor can cut it substantially, which is often what makes a mid-size generator workable. That is a job for an HVAC technician.
- Electric resistance loads such as water heaters, ranges, dryers and baseboard heat have little surge but very high running watts. They are usually the first thing to leave off a portable generator’s list.
- Gas furnaces still need electricity for the blower, igniter and controls. Many furnace makers specify a clean (low-distortion) power source for their electronics, which favors inverter generators or standby units.
- Well pumps are often 240V. A 120V-only generator cannot run them, whatever its wattage.
Connecting a generator safely
A generator that powers household circuits must connect through a transfer switch or a utility-approved interlock kit, installed by a licensed electrician and permitted where your jurisdiction requires it. Never plug a generator into a wall outlet (“backfeeding”). It can energize the utility lines and electrocute line workers or neighbors. Run portable generators outdoors only, well away from doors, windows and vents, and keep a working carbon monoxide alarm on every level of the home. Generator exhaust can kill within minutes.
Related guides
- Standby generator brands compared
- Portable generators for a winter outage
- Best backup power for a 3-day outage
- Home battery or generator? It depends on your climate
- The full Power & Backup guide
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For planning only, not electrical advice. Confirm your actual loads from appliance nameplates and have a licensed electrician size and install any generator connection.