WireWorks

Generator Sizing Calculator

Total running load plus a safety margin, since generators need headroom for the startup surge of motor-driven appliances.

Total connected load (W)

Safety margin (%)

Recommended generator size

6,000 W

Add up the running watts of everything you'd power at once, and remember that motors (fridges, AC units, pumps) have a higher startup surge than their running wattage.

What you entered

Total connected load (W): 5000Safety margin (%): 20

1
Add a 20% safety margin for startup surges
5000 × 1.20= 6000 W

Result

Recommended generator size: 6,000 W

A 5000 W total load with a 20% safety margin calls for a generator rated at least 6000 W.

Generator size by connected load and safety margin

Add up the running watts of everything you need, then add headroom. The margin covers motor starting surges and leaves the generator off its limit.

Running load	Safety margin	Generator size
3,000 W	25%	3,750 W
5,000 W	20%	6,000 W
5,000 W	25%	6,250 W
5,000 W	50%	7,500 W
7,500 W	25%	9,375 W
10,000 W	25%	12,500 W

Motor-driven appliances draw two to three times their running watts for a moment at startup, so a fridge rated at 700 W can pull 2,000 W briefly. If several motors could start together, use the higher margin. Never run a generator indoors, in a garage, or near an open window: carbon monoxide from a portable generator can be lethal within minutes.

Starting watts vs. running watts

Motors (refrigerators, well pumps, AC units) draw a large surge of power for a fraction of a second at startup — often 2-3× their running wattage. A generator sized only for running watts can stall or shut down when a motor-driven appliance kicks on.

Don't run everything at once

For a portable backup generator, it's often cheaper and just as practical to plan on staggering appliance use (running the fridge and a few lights, but not the AC and the dryer simultaneously) rather than sizing a generator for every possible load running at the same time.

Frequently asked questions

My fridge (700W), sump pump (1,500W), and some lights (300W) total 2,500W — what generator do I need?

At least a 4,000W generator. The sump pump's startup surge can hit 3,000-4,500W momentarily, so you need headroom well above the 2,500W running total. A 4,000-5,000W generator handles the running load plus startup surges.

What's the difference between running watts and starting watts?

Running watts is the continuous power a device draws during normal operation. Starting watts is the brief surge (2-3x running watts) when a motor-driven device kicks on. Your generator must handle the highest possible starting surge on top of everything else that's already running.

Can a portable generator power my whole house?

Most portable generators (3,000-7,500W) can power essential circuits only — fridge, lights, sump pump, a few outlets. Powering a whole house including AC and electric stove typically requires a standby generator (10,000-22,000W) with a transfer switch.

How does this relate to the wattage calculator?

The wattage calculator finds the power draw of individual devices from their voltage and amperage. Add up all those wattages and enter the total here to size an appropriate generator for the combined load.