What size generator do I need?
A refrigerator, furnace blower, lights, TV, and phone chargers add up to about 1,900 running watts, and the blower's start-up spikes it to roughly 3,450 for a second. That's a 3,500 watt generator, not the 7,500 the box store leads with. Flip the switches for your own list and read the panel.
How the math works
Every motor on your list has two numbers: what it draws while spinning, and the bigger gulp it takes for the first second or two while it gets the rotor moving. A 700W refrigerator wants about 2,200W at start. The generator has to carry your whole running total continuously, and it has to survive the biggest single spike on top of that total without stalling. So the sizing rule is: running watts of everything, plus the largest (start minus run) difference in the list.
Then we add headroom. Manufacturer manuals put the continuous-load ceiling at about 80% of the rating, and a set working at its limit runs hotter, drinks more fuel, and drifts in voltage. The recommendation row picks the smallest class whose rated watts cover your total with that 25% buffer and whose peak rating covers your spike. The bare-minimum row is the same math with no buffer, for reference, not for shopping.
One honest caveat: chart watts are planning figures. Your fridge's nameplate might say 4.5 amps (540W), your neighbor's ancient garage unit might say 9. We plan at the worksheet numbers, which lean high on purpose. Check the nameplate before you buy anything expensive.
Quick reference: common load lists
Computed with the same engine as the panel above. Each scenario assumes everything in it runs at the same time.
| Scenario | Running | Peak | Buy |
|---|---|---|---|
| Fridge + lights + phones + Wi-Fi | 900 W | 2,400 W | 3,400W inverter |
| The storm basics (adds TV + furnace blower) | 1,900 W | 3,450 W | 3,500W portable |
| Basics + sump pump (1/2 HP) | 2,950 W | 4,500 W | 5,000W portable |
| Basics + well pump (1/2 HP) | 2,900 W | 4,450 W | 5,000W portable |
| Basics + window AC (10,000 BTU) | 3,100 W | 4,650 W | 5,000W portable |
| Basics + central AC (3 ton) | 5,400 W | 7,900 W | 7,500W portable |
Questions people ask
What size generator do I need for my house?
For the storm basics (refrigerator, furnace blower, lights, TV, phones, Wi-Fi) the math lands at about 1,900 running watts and a 3,450W peak when the blower starts, so a 3,500W portable covers it with headroom. Add a 1/2 HP sump pump and you want the 5,000W class. Whole house with central air is standby territory, 10 to 26 kW. Flip the breakers above and the panel does your exact math.
Why does the recommendation look bigger than my total watts?
Two reasons. Motors pull two to three times their running watts for the first second or two, so the generator has to survive your biggest start-up spike on top of everything already running. And a generator should not sit at 100% of its rating for hours; manuals put the continuous ceiling around 80%, which is the 25% buffer we add. The bare-minimum row shows the math without that buffer, so you can see it is a real margin and not sales padding.
Can I just add up the starting watts of everything?
No, and doing so is the most common oversizing mistake. Start-up spikes last a second or two, and your appliances do not all start in the same second. The standard method (also what the Honda and Generac worksheets do) is total running watts plus the single largest extra surge. If two big motors genuinely can start together, like twin sump pumps in a wet spring, count both.
Are these wattages exact for my appliances?
They are planning numbers from manufacturer wattage charts, deliberately on the high side. Your appliance has a nameplate (a sticker near the plug or behind the door) with the real figure; amps times volts equals watts. A new fridge can run under 400W where our chart says 700. When the nameplate and our table disagree, believe the nameplate.
What about a generator for sensitive electronics?
Laptops, TVs, and CPAPs prefer the clean output of an inverter generator. Conventional open-frame portables are fine for pumps, heaters, and fridges but their voltage wobbles more. If the load list is mostly electronics and a fridge, buy the inverter; it is also half the noise.