What size generator do you need?
Two numbers decide it: the running watts of everything you want on at once, and the extra surge the biggest motor needs to start. Tick what you'd run in an outage and the calculator does the sums, then shows the generators we compare that cover it.
Work out your watts
Tick everything you'd want running at the same time. It adds up the running watts, then adds the biggest start-up surge, because motors start one at a time. It uses the cautious figures from generator makers' and utilities' wattage charts, so check your own appliances' labels too.
You need about 1,646 running watts and 3,496 starting watts.
The gas furnace blower is wired in, not plugged in: running it needs a transfer switch fitted by a qualified electrician. Never plug a generator into a wall outlet.
What will each size run?
Everyday outage combinations checked against each size, with the generators we compare in it.
Generator wattage chart
Running watts, and the total each appliance needs for the moment it starts, from the charts behind the calculator.
| Appliance | Running W | To start |
|---|---|---|
| Refrigerator | 400–750 | 2,600 |
| Freezer (chest or upright) | 400–750 | 2,600 |
| Gas furnace blower (1/3 to 1/2 HP)Wired in: needs a transfer switch | 450–600 | 1,800 |
| Sump pump (1/2 HP) | 1,000 | 3,000 |
| Well pump (1/3 to 1/2 HP)Wired in: needs a transfer switch | 750–1,000 | 3,000 |
| Window air conditioner (10,000 BTU) | 625–1,200 | 3,200 |
| Microwave | 600–1,500 | 2,800 |
| Coffee maker | 750–1,200 | – |
| Electric space heater | 500–1,500 | – |
| TV (32 to 65-inch LED) | 30–128 | – |
| LED light bulb (60W equivalent, each) | 8–10 | – |
| Modem and Wi-Fi router | 10–18 | – |
| Phone or tablet charging | 5–45 | – |
| Laptop (plugged in, in use) | 30–70 | – |
| Desktop computer | 100–200 | – |
| CPAP with heated humidifier (and heated tube) | 21–67 | – |
| Home oxygen concentrator | 275–325 | – |
| Garage door opener (1/2 HP) | 1,050 | 3,150 |
| Electric water heater (50 gallon, 240V)Wired in: needs a transfer switch | 4,500–5,000 | – |
| Central air conditioner (3 ton)Wired in: needs a transfer switch | 6,000 | 18,000 |
Running watts from Champion Power Equipment's wattage chart and DTE Energy's appliance usage guide, Energy Star's product data and the device makers' manuals. "To start" is the cautious total for the moment a motor or compressor starts. Your appliance's nameplate beats any chart: amps × 120 = watts.
How to work it out yourself
- List what you want running at the same time. Most households start with the refrigerator, lights, Wi-Fi and phones, then add the furnace blower in winter, a sump or well pump, or a window air conditioner in summer.
- Add up their running watts. That's what the generator has to supply continuously, so its running (rated) watts must cover it.
- Find the one appliance with the biggest extra starting load (a compressor or pump motor) and add that extra to your running total. That's the starting (peak) watts you need. Champion's chart tells you to add the "highest single" starting watt, because motors rarely start at the same moment.
- Wired-in loads such as a furnace, well pump or water heater need a transfer switch fitted by a qualified electrician. Never plug a generator into a wall outlet.
Ready.gov says to use a generator ONLY outdoors, at least 20 feet away from windows, never in a home or garage. Fit carbon monoxide alarms with battery backup on every level of your home. Our generator safety guide covers fuel, cords and transfer switches.
Questions
What size generator do I need to run a refrigerator?
Generator makers' charts give a refrigerator 400 to 750 running watts and up to about 2,600 watts for the moment its compressor starts. A generator with at least 3,000 starting watts covers it with room for lights, Wi-Fi and phones; check the label inside your own refrigerator for its amps (amps × 120 = watts).
How do I work out what size generator I need?
Add up the running watts of everything you want on at the same time. Then add the extra starting watts of the one appliance with the biggest start-up surge, because motors start one at a time. Choose a generator whose running watts cover the first number and whose starting watts cover the total.
Can a portable generator run central air?
Usually not. DTE Energy's guide puts a 3-ton central air conditioner at 6,000 running watts and about 18,000 watts to start, more than the portable generators we compare can supply. Central air is a job for a standby generator sized by an electrician.
Compare the generators
- Generators compared: inverter models for the essentials and open-frame models for home backup.
- Want something silent you can run indoors? Portable power stations store less energy but need no fuel.
- Your utility's outage map, for every state.
Sources (16)
- Champion wattage chart
- DTE Energy appliance usage guide
- Energy Star certified residential refrigerators (full-size, 2,986 models, October 2026)
- Energy Star certified residential freezers (October 2026)
- a full freezer holds its temperature for about 48 hours (24 if half full) if the door stays closed
- Energy Star certified room air conditioners, 9,800–10,200 BTU (112 models)
- Energy Star certified televisions (October 2026)
- Energy Star connected light bulbs, 60W equivalent (219 models)
- Netgear Small Network Equipment Voluntary Agreement idle power (2025)
- Apple fast charge
- Apple
- AirSense 10 + humidifier (@4) 1.73–4.19 A; (@8) 3.44–4.59 A; + ClimateLineAir (@30°C) and humidifier (@8) 4.71–5.61 A, all @12V DC; original
- DreamStation 2 manual gives input rating and 80W supply only; no per-mode watts
- Inogen At Home
- Invacare Perfecto2 V
- Ready.gov: Power outages (generator safety)