Generator sizing, worked out What size generator do I need? The size you need is the running watts of everything you power, added together , plus the single largest starting surge , plus 20% headroom. Check your items below — the arithmetic happens as you go.
A portable generator sized for a sump pump, refrigerator, LED lights and a Wi-Fi router
Diagram: a portable generator on the left connects to four appliances on the right -- sump pump 1,050W, refrigerator 1,000W, LED lights 13W, Wi-Fi router 20W -- totalling 2,083 running watts.
Generator
Sump pump
1,050 running watts
Refrigerator
1,000 running watts
LED lights
13 running watts
Wi-Fi router
20 running watts
Total: 2,083 running watts
Sourced wattages from this site's own dataset, the same figures the calculator below uses. For carbon-monoxide safety, the CPSC says to run portable generators outdoors, at least 20 feet from the home, with exhaust facing away. Generator safety guidance .
Start with a common setup Choose a realistic starting point, then adjust individual appliances below. Your selections stay in this browser.
Outage essentials Fridge + Wi-Fi + lights Sump-pump backup ½ HP pump + fridge RV weekend 13.5k BTU AC + fridge Job-site tools Saw + charger + lights
Check appliances below — running total, peak demand and a recommended rating appear here.
kitchen 1,000W / 2,200W start 720W / 1,890W start 500W / 2,000W start 625W 1,000W / 2,000W start 1,200W 900W 1,200W 2,100W 2,400W 400W 500W / 700W start 168W 1,500W 1,200W / 2,000W start 250W 1,500W 700W / 1,400W start 4,000W 1,600W 800W / 1,400W start 500W hvac 700W / 1,400W start 1,100W / 2,350W start 1,200W / 2,000W start 3,250W / 3,950W start 1,500W / 3,000W start 3,800W / 4,950W start 6,000W / 7,200W start 5,400W / 7,200W start 1,800W 175W 5,500W 800W / 2,100W start 1,050W / 2,200W start 1,250W / 3,750W start 1,000W / 2,100W start 5,000W / 19,000W start 7,200W 1,200W essentials 875W / 2,350W start 550W / 1,000W start 75W 190W 800W 250W 20W 25W 500W 250W 200W 450W 1,600W / 3,400W start 2,000W / 4,000W start 2,280W / 4,000W start 1,800W / 2,700W start 13W medical 90W 90W laundry 1,150W / 3,450W start 5,400W 700W / 1,800W start 1,200W 1,250W 1,500W rv 1,800W / 2,800W start 2,000W / 3,500W start 180W / 600W start 1,650W 1,300W 500W / 1,500W start 100W 1,500W 800W 60W workshop 1,500W / 4,200W start 2,000W / 4,000W start 1,800W 1,000W / 3,000W start 440W / 600W start 600W / 900W start 1,000W 1,200W / 3,300W start 800W / 2,400W start 975W / 1,600W start 1,600W / 4,800W start 600W / 1,800W start 1,800W 9,000W 1,000W 380W
As an Amazon Associate, needwatts.com earns from qualifying purchases. Links below are affiliate links — this never affects our wattage math or which units we list.
Sizing rule: running total + the single largest starting surge, + 20% headroom. Sources on the methodology page . Your nameplate is authoritative.