What size generator do I need?

1. Running watts vs. starting watts — the key distinction

Every generator has two wattage ratings: a running (rated) wattage and a peak (surge) wattage. The running wattage is what the generator can deliver continuously. The peak wattage is the short burst it can supply — typically for a few seconds.

Your appliances work the same way in reverse. Most draw a steady running wattage, but motor-driven appliances — refrigerators, air conditioners, pumps, furnace fans — draw a much higher starting wattage for a fraction of a second when the motor first spins up. This starting surge is caused by the locked-rotor current of an induction motor: the initial current to get the rotor moving from standstill.

Resistive loads (LED lights, electric heaters, microwaves, phone chargers) have no starting surge. Their starting and running watts are the same.

Typical starting surges for common motor appliances:

Figures are typical published values from generator manufacturer sizing guides (Generac, Honda, Briggs & Stratton). Your appliances may differ — check the nameplate or the manufacturer's datasheet for your specific model.

2. The generator sizing formula

Generator sizing uses three numbers:

1. Running total — add the running watts of every appliance you want to power simultaneously.
2. Starting surplus — find the appliance with the largest difference between starting watts and running watts. Only one appliance starts at a time, so only the single biggest surplus adds to the peak.
3. Peak demand = running total + starting surplus.

Peak demand = running total + (startingW − runningW) of the largest motor appliance
Recommended generator ≥ peak demand × 1.1

The 1.1 multiplier adds 10 % headroom — running a generator at 90 % of its rated capacity rather than 100 % reduces wear, improves fuel efficiency, and gives a margin for appliance-to-appliance variation. Most generator manufacturers recommend sizing for 70–90 % of rated output.

3. Worked example: basic emergency backup

Scenario: you want to keep the refrigerator running, power three rooms of LED lights, watch TV and charge phones during a power outage. No air conditioning, no sump pump.

• Refrigerator: 150 W running / 600 W starting
• 3 rooms LED lights: 150 W running / 150 W starting (no motor)
• TV / entertainment: 150 W running / 150 W starting (no motor)
• Phone / laptop charging: 100 W running / 100 W starting (no motor)

• Running total: 150 + 150 + 150 + 100 = 550 W
• Largest starting surplus: refrigerator = 600 − 150 = 450 W
• Peak demand: 550 + 450 = 1,000 W
• With 10 % headroom: 1,000 × 1.1 = 1,100 W → 2,000 W generator

4. Worked example: adding a sump pump

Same essential loads plus a ½ hp sump pump — a common combination for homes at risk of basement flooding during a storm.

• All essential loads above: 550 W running
• Sump pump ½ hp: 1,050 W running / 3,150 W starting

• Running total: 550 + 1,050 = 1,600 W
• Largest starting surplus: sump pump = 3,150 − 1,050 = 2,100 W (beats refrigerator's 450 W)
• Peak demand: 1,600 + 2,100 = 3,700 W
• With 10 % headroom: 3,700 × 1.1 = 4,070 W → 5,000 W generator

Note that even though the sump pump's running watts (1,050 W) dominate the running total, the peak is driven by its 3,150 W starting surge. This is why starting watts — not just running watts — determine the minimum generator size.

5. Worked example: adding air conditioning

A central AC unit is usually the biggest load on a residential generator. A 2-ton (24,000 BTU) unit draws about 2,500 running watts and surges to 7,500 starting watts.

• Essential loads (fridge, lights, TV, laptop): 550 W running
• Central AC — 2-ton: 2,500 W running / 7,500 W starting

• Running total: 550 + 2,500 = 3,050 W
• Largest starting surplus: AC = 7,500 − 2,500 = 5,000 W
• Peak demand: 3,050 + 5,000 = 8,050 W
• With 10 % headroom: 8,050 × 1.1 = 8,855 W → 10,000 W generator

This is why a 7,500 W generator cannot reliably start a standard 2-ton central AC compressor: the starting demand alone (8,050 W) already exceeds 7,500 W. A 10,000 W generator covers the starting surge and still has headroom for other loads.

6. Generator size quick reference

All running and starting watt figures are typical published values. Actual appliances vary — check your nameplates and use the generator size calculator with your own values for an exact result.

7. Portable vs. standby generators

Portable generators (typically 1,000–12,500 W) run on petrol (gasoline) or propane and are manually started and connected, usually through a transfer switch or extension cords. They are good for temporary power during outages and for job sites. They must always be operated outdoors — carbon monoxide from the exhaust is lethal indoors. Portable generators in the 3,500–6,500 W range cover most emergency home backup needs.

Standby generators (typically 7,500–22,000 W) are permanently installed, connected to natural gas or propane, and start automatically when grid power fails — usually within seconds. They are sized to cover all critical circuits or the whole house. Standby generators require a licensed electrician for installation and a permit in most jurisdictions.

Inverter generators are a type of portable generator that produces cleaner power (lower total harmonic distortion), making them safer for sensitive electronics. They are typically quieter and more fuel-efficient at lower loads. Sizes range from about 1,000 W to 7,000 W, so they are suited to camping and small backup loads rather than whole-house coverage.

The sizing formula — running watts + starting surplus × 1.1 — applies equally to all three types. What changes is fuel type, noise level, starting mechanism, and installation requirements.

8. Safety and installation notes

Generator safety is a separate topic from sizing, but a few rules are worth stating plainly:

• Never run a generator indoors or in a garage, even with the door open. Carbon monoxide is colourless, odourless and builds up rapidly in enclosed spaces. Keep any generator at least 6 metres (20 ft) from any door, window, or vent.
• Use a transfer switch.Connecting a generator directly to your home's wiring without a transfer switch (backfeed) can electrocute utility workers and damage equipment. A licensed electrician should install the transfer switch.
• Do not overload the generator. Operating at 100 % of rated wattage for extended periods shortens generator life. The 10 % headroom in the sizing formula is there for a reason.
• Check the run time at expected load. A generator's advertised run time is usually at 50 % load. At higher loads, fuel consumption rises significantly.

These are general safety principles. Your local authority having jurisdiction (AHJ) may have additional requirements for permanent standby generator installations — check before permitting.

Common questions

What size generator do I need to run my house?

For essential loads only (refrigerator, lights, TV, device charging), a 2,000–3,500 W generator is typically enough. Add a sump pump or well pump and you need 5,000 W. Add a window air conditioner and you need 5,000–6,500 W. Add central AC (2-ton) and the requirement rises to 10,000–12,500 W. A whole-house standby generator covering everything at once typically runs 14,000–20,000 W. Use the generator size calculator to add your specific appliances.

What is the difference between running watts and starting watts?

Running watts (rated or continuous watts) is the steady power an appliance draws in normal operation. Starting watts is the brief surge — typically 2–3 times the running watts — that induction motors (compressors, pumps, fans) draw for a fraction of a second when they first start. Resistive loads such as LED lights, electric heaters and microwaves have no starting surge. A generator must handle the starting watts without stalling, so starting demand — not just running load — determines the minimum generator size.

What size generator do I need for a sump pump?

A ½ hp sump pump draws about 1,050 running watts and up to 3,150 starting watts. You also need the generator to run other essentials at the same time: add a refrigerator (150 W running, 600 W starting), lights (150 W), TV (150 W) and a phone charger (100 W) and the running total is 1,600 W. The peak demand when the sump pump starts is 1,600 + (3,150 − 1,050) = 3,700 W. With 10% headroom, a 5,000 W generator is the recommended minimum.

How many watts does a generator need for a refrigerator?

A typical household refrigerator draws 150–200 running watts, but its compressor motor surges to 600–700 starting watts when it cycles on. If the refrigerator is the only load, a 1,000–2,000 W generator is sufficient. If you want to run other appliances at the same time, add their running watts to the total and still account for the refrigerator's starting surge in the peak calculation.

Can a 3,500 W generator run a house?

A 3,500 W generator can run essential loads: refrigerator, LED lights, TV and device charging — typically around 550–700 running watts with a peak around 1,000–1,200 W. It can handle a sump pump (starting demand ~3,700 W from the peak side) as long as you are not also running an air conditioner, electric stove or electric water heater at the same time. A 3,500 W generator cannot run central AC (which needs 7,500–10,500 starting watts for a 2–3 ton unit).