What Size Generator Do I Need for a Power Outage?

Learn how to add up running and starting watts, size a generator for your essentials, and avoid buying too big or too small.

Picking a generator size feels like guesswork until you break it into two numbers: the watts your gear needs to keep running, and the extra burst some of it needs to start. Get those right and the rest is simple arithmetic.

This guide walks through the process with a pencil and a list of what you actually want powered. It applies to portable gas, propane and dual fuel units, and the same maths works for battery power stations.

Running watts versus starting watts

Running watts (also called rated or continuous watts) are what a device draws while it is operating normally. Starting watts (also called surge or peak watts) are the short burst some devices pull for a moment when they switch on. Anything with a motor or compressor, such as a fridge, freezer, sump pump, furnace blower or well pump, usually needs a noticeably higher burst to start than to run.

Lights, phone chargers, laptops, routers and TVs have almost no surge. Resistive heating devices such as space heaters, kettles and toasters draw a high steady load but do not surge. Those are better left off a small generator entirely.

How to add up your load

1. List only the essentials: fridge, freezer, a few lights, phone and device charging, a furnace fan or heating source, medical equipment, a sump pump or well pump if you have one.
2. Find the running watts on each label or in the manual. If you only see volts and amps, multiply them: watts equal volts times amps. A 120 volt device marked 5 amps draws about 600 watts.
3. Add the running watts of everything you want on at the same time.
4. Find the single device with the biggest starting surge and add the extra it needs above its running watts. You add only one surge, because you start devices one at a time.
5. Add a margin of roughly 20 percent so the generator is not working at its limit.

If a label does not list starting watts, check the manual or the manufacturer's website, or ask an electrician. Do not guess on motor loads, since they are the ones that trip a generator that is too small.

Staggering loads to keep the size down

You rarely need everything at once. Run the fridge and freezer in turn, charge devices during the day, and keep the microwave, kettle and other big draws off while a motor is starting. Staggering can let a modest unit cover a household that looked like it needed something far larger. For tips on keeping food safe while you rotate loads, see our fridge and freezer food safety guide.

A rough way to sort your needs is by tier. Tier one is food safety, communication and lighting. Tier two is heat or fan power and medical devices. Tier three is comfort, such as a TV or a coffee maker. Size for tier one and two, then add tier three only if the numbers allow.

Generator or power station?

If your essentials are mostly small, quiet and short duration, a battery power station may cover you with no fuel and no exhaust. The same running and starting watt logic applies, plus a second number: watt hours of storage. Our guide to what size power station you need covers that step, and you can compare the two approaches in solar generator versus gas generator. Shop the category through our power stations hub.

A generator makes sense when you need many hours or days of steady power, or a larger load such as a well pump. A power station suits lighter loads and indoor use, since it makes no exhaust.

Safe connection and use

Whatever size you choose, run it safely. Gas and propane generators produce carbon monoxide, which cannot be seen or smelled and can kill quickly.

• Run a generator outdoors only, well away from doors, windows and vents. Never run one in a garage, basement, shed or carport, even with the door open.
• Follow the minimum distance in your manual, and use battery powered or plug in carbon monoxide alarms with battery backup inside the home.
• Let the engine cool before refuelling, and store fuel in approved containers away from living space.
• Use heavy duty outdoor rated cords sized for the load, or have a transfer switch installed by a qualified electrician. Never plug a generator into a wall outlet.

For hands on setup advice, read how to use a portable generator safely and transfer switch versus extension cords.

A quick worked example

Say your list has a fridge, a freezer, six LED bulbs, a furnace fan, a router and phone chargers. You add the running watts from each label to get a total. The freezer has the largest starting surge, so you add its extra start watts on top. Then you add 20 percent as margin. The result is your minimum generator output rating, and the next size up that is commonly sold is a sensible choice. If the total lands near a size boundary, round up. A generator running far below its rating is usually more efficient and quieter than one that is maxed out.

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