Portable Generator Wattage: How Many Watts You Need

Most homes need 3,000–5,000 running watts of portable generator wattage to cover outage essentials. For camping or a fridge-plus-lights setup, 1,000 to 2,500 running watts is enough; add up what runs at once, add the largest starting surge, and keep the running load near 80% of the generator’s rating.

Running watts vs starting watts

Running watts are what an appliance draws continuously. Starting watts (surge watts) are the brief spike that motors and compressors pull for a second or two when they start, often two to three times the running figure.

Generators list both numbers. Size for the running watts of everything on at once, then confirm the starting rating covers the biggest spike. Only the single largest extra surge counts, because you can avoid starting several motors in the same instant.

The gap between starting and running watts varies widely by model, so check the manual for the rated running figure instead of assuming a fixed percentage. Output also changes with fuel (dual-fuel units often list a lower rating on propane than on gasoline) and with altitude, so read the manual for your unit.

How to calculate portable generator wattage in 5 steps

1. List everything you want running at the same time. Leave out anything you can do without or run later.
2. Find each item’s running watts on its nameplate or in the manual. If you only see amps, use watts = volts × amps. A 120-volt, 5-amp device is 600 watts.
3. Add the running watts to get your running total.
4. Find the largest extra surge. For each motor, subtract running watts from starting watts. Take the biggest difference and add it to the running total. That is your peak need.
5. Add headroom. Divide the running total by 0.8 to get the minimum running rating. This is a rule of thumb to keep the engine from running flat out; some makers allow higher loads, so check your manual. Then confirm the peak from step 4 is at or below the generator’s starting rating.

Apply the headroom once, at the end. Padding every appliance and then dividing again overbuys by a full size class. The peak need is a separate check against the starting rating, so do not divide it by 0.8.

Typical appliance wattage chart

These are typical planning figures, not measurements of a specific model. Your appliance’s nameplate or manual is the number to trust, since efficiency and motor size vary.

Appliance	Running watts	Starting watts
Refrigerator (full size)	700	2,200
Sump pump, 1/3 hp	800	2,400
Furnace blower fan, 1/2 hp	600	1,800
LED lights (about 10 bulbs)	100	100
TV, Wi-Fi router, phone chargers	200	200
Microwave (1,000 W class)	1,000	1,000
Window AC, 10,000 BTU	1,200	3,600
Well pump, 1/2 hp	1,000	2,100
Central AC, 3-ton	3,500	6,000–10,000

Central AC varies the most. Its starting watts depend on the model, the compressor and whether it has a soft-start kit, so the 6,000–10,000 range is only a planning span.

Refrigerators are the most common first load. Our guide to the generator size needed to run a refrigerator covers surge and run-time details.

Worked example: sizing for outage essentials

Say you want the fridge, a sump pump, a furnace fan, lights and a TV with Wi-Fi during a winter outage. Using the chart above:

• Running total: 700 + 800 + 600 + 100 + 200 = 2,400 watts
• Extra surge for each motor: fridge 1,500, sump pump 1,600, furnace fan 1,200. The largest is 1,600.
• Peak need: 2,400 + 1,600 = 4,000 watts
• Minimum running rating with headroom: 2,400 ÷ 0.8 = 3,000 watts

You need at least 3,000 running watts and 4,000 starting watts. That points to a 3,500-watt-class unit, provided its starting rating is 4,000 or higher; see our guide to portable 3,500-watt generators and confirm the ratings in each manual.

Now add a 1,000-watt microwave. The running total becomes 3,400, the peak 5,000, and the headroom rating is 3,400 ÷ 0.8 = 4,250 watts. That moves you to a 5,000-watt class. A cheaper fix is to run the microwave only while the sump pump is idle, which keeps you in the smaller size.

Which portable generator wattage tier fits your use

Running watts	What it typically covers
1,000–2,500	Camping, small RV loads, a fridge with lights and electronics, small tools
3,000–5,000	Outage essentials like the example above; a window AC fits at the upper end
6,500–8,000+	Essentials plus a well pump, several window ACs, or larger tools

If your total lands in the middle tier, our guide to portable generators for home backup is the place to compare units in that 3,000–5,000 running-watt class.

Central AC and whole-house loads

A 5,000-watt portable does not run a whole house. It runs a chosen set of circuits. Central AC is the load that most often breaks the plan: a 3-ton unit can need about 3,500 running watts and a much larger start-up surge, so even a 7,000–8,000-watt portable may struggle, depending on the unit. A soft-start kit can reduce the surge, but results vary by model, so ask the installer or manufacturer. Electric ranges, dryers and electric water heaters usually call for a standby generator or for skipping them during an outage.

Inverter vs conventional

Inverter generators produce cleaner power for electronics and usually run quieter and burn less fuel at light loads, but many have lower surge ratings per dollar. Conventional open-frame units give more watts for the money. Run time varies by model and load; manufacturers list hours at a stated load, often 50%, so compare those figures rather than guessing.

Connecting your generator safely and avoiding overload

Each outlet has a limit. A 120-volt, 15-amp circuit tops out at 1,800 watts (120 × 15), and a 20-amp circuit at 2,400 watts. A 240-volt, 30-amp outlet maxes out at 7,200 watts. Spread loads across outlets so none trips, and use heavy-gauge extension cords rated for the load.

To power hardwired items like a furnace or well pump, have a licensed electrician install a transfer switch or interlock that meets the NEC and local code. Pump surge depends on horsepower and type, so see our guide to generator size for a well pump by horsepower. Never plug the generator into a wall outlet to feed the house. That backfeeds the lines and can kill utility workers.

Run the generator outdoors only, with the exhaust pointed away from windows and doors. Follow the manual or label for clearance from the house; many say 20 feet. Keep a battery-powered CO alarm inside. Let the engine cool before refueling, and keep the unit dry.

Our Top Picks

Ready to shop? These are the top-rated options from our hands-on roundup, Best 5000 Watt Portable Generators For Home of 2026.

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Frequently Asked Questions

Can a 5,000-watt generator run a whole house?

Not all at once. It can run chosen essentials such as a fridge, sump pump, furnace fan, lights and perhaps one window AC. Central AC, an electric range, a dryer and a water heater together would overload it. Add up your actual loads first.

What if my generator lists only one wattage?

Check the manual or spec sheet for the rated running watts. A single listed number may be the starting (peak) figure, and the gap to running watts varies by model, so do not size your loads against it without confirming.

Can I run a generator in the rain?

Not uncovered, because moisture can reach outlets and cause shock or damage. Many manuals allow use only under an open-sided, ventilated cover made for generators, with the exhaust pointed away from the home; follow yours. Never run one in a garage, even with the door open.

Can I use a 3,500-watt generator with a transfer switch?

Yes, if the circuits you connect stay within its running rating. A transfer switch moves selected circuits to the generator; it does not add capacity. Estimate: 3,500 W ÷ 120 V is about 29 amps, so check that the unit’s outlet and your inlet are rated for roughly 30 amps.

Is an inverter generator good enough for a refrigerator?

Yes, if its running rating covers the fridge and its starting rating covers the compressor start-up. The chart above lists about 700 running and 2,200 starting watts for a full-size fridge, but your nameplate is the number to use. Inverters also supply cleaner power for electronics.

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