1. What do you need to run?
Battery power station instead of gasoline?
If your outage load is small - a fridge, a CPAP, phones and lights - a portable power station may be quieter and indoor-safe. See how long a given battery size will actually run your appliances:
Power Station Runtime CalculatorWhat size generator to run a fridge and AC?
A refrigerator plus a window air conditioner is the classic storm-outage pair. The fridge's start-up surge is the biggest extra load, and it sets the starting total.
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator | 150W | 1,200W |
| Window AC (10,000 BTU) | 1,000W | 1,500W |
Running total 1,150W + largest start-up extra 1,050W = starting total 2,200W. With 20% headroom the target is 2,200W, so the recommendation is a 3,500W class generator. Shop 3,500W class generators on Amazon
What size generator for a 3-ton AC?
A 3-ton central AC needs the furnace blower running too, and most people keep the fridge on. The AC's start-up surge (5,500W vs 3,500W running) pushes the recommendation up a class.
| Appliance | Running | Starting |
|---|---|---|
| Central AC (3-ton) | 3,500W | 5,500W |
| Furnace fan / blower | 800W | 1,200W |
| Refrigerator | 150W | 1,200W |
Running total 4,450W + largest start-up extra 2,000W = starting total 6,450W. With 20% headroom the target is 6,450W, so the recommendation is a 7,500W class generator. Shop 7,500W class generators on Amazon
What size generator to run a fridge, TV and lights?
The essentials-only setup. Running watts are tiny - the fridge's start-up surge is almost the whole requirement.
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator | 150W | 1,200W |
| TV | 100W | 100W |
| Lights (10 LED bulbs) | 100W | 100W |
Running total 350W + largest start-up extra 1,050W = starting total 1,400W. With 20% headroom the target is 1,400W, so the recommendation is a 3,500W class generator. Shop 3,500W class generators on Amazon
What size generator for a well pump?
Well pumps have one of the largest start-up surges on this list (2,500W starting vs 1,000W running), so the surge - not the running total - decides the class.
| Appliance | Running | Starting |
|---|---|---|
| Well pump | 1,000W | 2,500W |
| Refrigerator | 150W | 1,200W |
| Lights (10 LED bulbs) | 100W | 100W |
Running total 1,250W + largest start-up extra 1,500W = starting total 2,750W. With 20% headroom the target is 2,750W, so the recommendation is a 3,500W class generator. Shop 3,500W class generators on Amazon
What size generator for a sump pump and fridge during a storm?
The storm combo: sump pump, fridge, freezer and some lights. Both pumps surge, but only the single largest surge is added - motors rarely start in the same instant.
| Appliance | Running | Starting |
|---|---|---|
| Sump pump | 800W | 1,800W |
| Refrigerator | 150W | 1,200W |
| Freezer | 100W | 800W |
| Lights (10 LED bulbs) | 100W | 100W |
Running total 1,150W + largest start-up extra 1,050W = starting total 2,200W. With 20% headroom the target is 2,200W, so the recommendation is a 3,500W class generator. Shop 3,500W class generators on Amazon
What size generator for a CPAP and a fridge overnight?
A CPAP draws very little (about 40W). Even adding the fridge, this stays in the smallest class - though for overnight use many people pick a quiet battery power station instead (see the link below).
| Appliance | Running | Starting |
|---|---|---|
| CPAP machine | 40W | 40W |
| Refrigerator | 150W | 1,200W |
Running total 190W + largest start-up extra 1,050W = starting total 1,240W. With 20% headroom the target is 1,240W, so the recommendation is a 3,500W class generator. Shop 3,500W class generators on Amazon
What size generator to run a washing machine and electric dryer?
An electric dryer is a heavy continuous load (3,000W with no surge). Together with the washer and fridge it lands in the 5,000W class.
| Appliance | Running | Starting |
|---|---|---|
| Washing machine | 500W | 1,200W |
| Electric dryer | 3,000W | 3,000W |
| Refrigerator | 150W | 1,200W |
Running total 3,650W + largest start-up extra 1,050W = starting total 4,700W. With 20% headroom the target is 4,700W, so the recommendation is a 5,000W class generator. Shop 5,000W class generators on Amazon
What size generator to run a whole house?
A typical whole-house set - central AC, fridge, freezer, well pump, furnace fan, washer, microwave, lights, TV - fits a 10,000W portable if loads are staggered. Running literally everything at once, including an electric dryer, is what pushes people to a 22kW standby unit.
| Appliance | Running | Starting |
|---|---|---|
| Central AC (3-ton) | 3,500W | 5,500W |
| Refrigerator | 150W | 1,200W |
| Freezer | 100W | 800W |
| Well pump | 1,000W | 2,500W |
| Furnace fan / blower | 800W | 1,200W |
| Washing machine | 500W | 1,200W |
| Microwave | 1,000W | 1,000W |
| Lights (10 LED bulbs) | 100W | 100W |
| TV | 100W | 100W |
Running total 7,250W + largest start-up extra 2,000W = starting total 9,250W. With 20% headroom the target is 9,250W, so the recommendation is a 10,000W class generator. Shop 10,000W class generators on Amazon
How the sizing works
Running watts are what an appliance draws continuously. Starting watts are the brief surge a motor (fridge compressor, pump, AC) needs to start - often 2-3x its running watts. A generator has to survive the moment the biggest motor starts while everything else is already running, so the formula is:
Starting total = (sum of running watts) + (largest single start-up extra), where the start-up extra is that appliance's starting watts minus its running watts. The recommendation then takes the larger of the starting total and the running total plus 20% headroom, and rounds up to the next standard size class. That headroom keeps the generator from running at 100% load for hours, which is hard on the engine and shortens its life.
Watt values on this page are typical published figures for each appliance type. Your actual appliance may differ - the metal nameplate on the appliance (or its manual) lists its real watts or amps (amps x volts = watts; most US household circuits are 120V).
Shop generators by size
Common questions
Can a 3,500W generator run a refrigerator? Yes - a fridge runs on about 150W and surges to roughly 1,200W when the compressor starts, so a 3,500W-class generator handles a fridge plus lights, TV and a few small loads easily.
What size generator to run a 3-ton AC? With the furnace blower and a fridge, the worked example above lands in the 7,500W class, because the AC's start-up surge is about 5,500W. A soft-start kit on the AC can cut that surge substantially.
Why not just add up all the starting watts? Because motors almost never start in the same instant. Adding every surge would oversize the generator (and the price) for a coincidence that rarely happens. The standard method adds only the largest single surge to the running total.
Portable or standby? Portable generators (up to about 13,000W) are cheaper and movable but must be set up, fueled and connected each outage. A whole-house standby unit (about 22kW) starts automatically and is permanently installed - at a much higher installed cost.