How to Size a Portable Generator for Home Backup

Size a portable generator for the loads that must run together and the largest plausible startup event—not the square footage of the house. Record each appliance’s running demand, starting demand, voltage, and connection method. Use manufacturer information for your exact equipment; a generic appliance chart cannot settle a motor’s starting requirement.

A worked example with deliberately assumed numbers

The following values are invented for calculation practice. They are not typical appliance ratings and should not be copied into a purchasing decision. “Total during start” includes the starting appliance’s own running component; it is not additional watts to add again.

If all four are running, the total is 600 + 800 + 100 + 50 = 1,550 watts.

Calculate what happens when a motor starts

If the pump starts while the refrigerator, lights, and router are already running, the example demand becomes 2,000 + 600 + 100 + 50 = 2,750 watts. Do not also add the pump’s 800 running watts; those are already included in its assumed 2,000-watt total startup demand.

If the refrigerator starts while the pump is running, the result is 1,800 + 800 + 100 + 50 = 2,750 watts. If both start together, the example is 1,800 + 2,000 + 100 + 50 = 3,950 watts.

The last case matters because automatic appliances can restart without someone deliberately turning them on. Staggering initial connection does not prove their later cycles can never overlap. Generac’s GP owner’s manual, “Know Generator Limits”, explains accounting for connected loads and motor starting.

Turn the arithmetic into equipment requirements

The illustrative plan calls for at least 1,550 watts of running capacity and the ability to handle the relevant startup event, with suitable allowance for the real equipment and operating conditions. A marketed “3,500-watt” number means little until you know whether it is running or starting output and how the manufacturer defines that rating.

For perspective, Generac’s iQ3500 7723 series is listed at 3,000 running watts and 3,500 starting watts, at 120 volts. The iQ3500 specification sheet therefore does not establish capacity for the example’s 3,950-watt simultaneous start. Passing the 1,550-watt running calculation would not resolve that shortfall.

Four checks that a watt total cannot replace

• Voltage: identify every 240-volt load; a 120-volt-only generator cannot supply it as specified.
• Fuel: use the generator’s running and starting ratings on the actual fuel.
• Connections: confirm individual outlet, cord, inlet, and transfer-equipment limits.
• Real starting performance: have large motors and uncertain appliance specifications reviewed before buying.

Use the fuel-output comparison and house-connection guide to complete those checks.

Bring the load list to Brite Generators at 270-223-6302. Include nameplate photos and say which appliances may be left off; that gives a sizing discussion something concrete to work from.