Best overall
Westinghouse iGen4500
4,500W start · 3,700W run
Gasoline · Inverter · 52 dBA · remote start · RV-ready TT-30R
Quiet enough for a campground and strong enough to start a 30-amp RV air conditioner.
Find on AmazonWhat size generator to run sump pump, furnace and fridge?
Short answer
To run sump pump, furnace and fridge at the same time you need a generator rated for at least 4,100 starting watts — about 2,550 running watts combined, plus the furnace blower fan's startup surge. A 5,000W (midsize portable) unit gives comfortable headroom.
Running watts
2,550W
Peak (surge)
4,100W
Minimum generator
4,100W
Recommended
5,000W
Wattage breakdown
| Appliance | Running watts | Starting watts |
|---|---|---|
| Sump Pump (1/2 HP) | 1,050W | 2,150W |
| Furnace Blower Fan (1/2 HP) | 800W | 2,350W |
| Refrigerator / Freezer | 700W | 2,200W |
Sizing a generator for sump pump, furnace and fridge
Together these draw about 2,550 running watts. You don't add every appliance's starting watts, though — only one motor surges at a time. So the generator only has to cover that running total plus the single largest startup surge (the furnace blower fan's), which brings the peak to 4,100 watts.
To power the whole set, choose a generator rated for at least 4,100 starting watts. A 5,000W unit leaves comfortable headroom to add more — the calculator below does that math for your exact list.
Size your full appliance list
This page sizes sump pump, furnace and fridge together. Open the calculator with them pre-selected, then add everything else you plan to power at the same time.
Size a generator for sump pump, furnace and fridgeRelated
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Find on AmazonFrequently asked questions
What size generator do I need to run sump pump, furnace and fridge?
To run sump pump, furnace and fridge together you need a generator rated for at least 4,100 starting watts. We recommend a 5,000W (midsize portable) unit so you have headroom to add other essentials without running the generator at its limit.
How many watts does it take to run sump pump, furnace and fridge at the same time?
About 2,550 running watts combined, but you must size for the peak: 4,100 watts, which adds the single largest startup surge in the group on top of the running total. Only one motor surges at a time, so you don't add every appliance's starting watts together.
Why isn't it just the two appliances' watts added together?
Because only one motor surges at the instant it starts. You add up everyone's running watts (2,550W here), then add just the single biggest startup surge — the furnace blower fan's — to get the 4,100W peak. Adding every appliance's starting watts would badly oversize the generator.