Using a Wattage Meter to Size a Generator

What a watt meter shows

A watt meter is a small device that plugs into a wall outlet. An appliance plugs into the meter instead of the wall, and the display reads the watts the appliance draws at that moment. Watts are the same unit of power that generator listings use for their stated starting and running figures. The U.S. Energy Information Administration explains that electricity is measured in units of power called Watts, and one Watt is a very small amount of power; some devices need only a few Watts while others need far more.

For a Texas home preparing before winter storm season or summer heat, a watt meter turns a vague question into a concrete list. Measure a refrigerator, a window air conditioner, a well pump, or a space heater one at a time, and the meter shows the number each one draws. That list becomes the reference you carry to a generator listing.

The meter is not a battery gauge or a fuel gauge. It only reads the draw of the appliance plugged into it, and it does not read a generator's output. That distinction matters when you compare a measured appliance with a listed generator.

Starting watts versus running watts on a listing

Generator listings show two watt figures. Starting watts, sometimes called surge or peak watts, is the higher figure. Running watts is the figure the listing states the unit can deliver continuously. Many appliances with a motor or compressor draw more for an instant when they start than they do while they run. Read more about the two figures.

A watt meter shows both if you watch the display through the appliance's start-up. The peak you see at start-up lines up with what a listing calls starting watts; the figure the display holds while the appliance runs lines up with running watts. The listing's first watt figure is often the starting figure, so reading the second figure on the maker's page is part of the comparison.

Some meters show a peak-hold figure, meaning the display keeps the highest reading from the start-up moment. Check the meter's own instructions to know whether the display holds the peak or shows the current draw.

How to measure an appliance draw

The steps are the same for every appliance with a standard plug. A watt meter is read on the appliance side, not on the generator side.

The measured figures are your own reference list. They use the same unit as the generator listing. Take the list to the maker's page for a candidate generator and compare. This site does not publish appliance wattage figures, wattage worksheets, or run-time estimates, so the numbers you measure are the ones to use.

• Plug the watt meter into a standard wall outlet.
• Plug the appliance into the watt meter instead of the wall.
• Turn the appliance on and watch the display as it starts; note the highest figure you see.
• Let the appliance run through its normal cycle, such as a refrigerator's compressor cycle, and note the figure it holds while running.
• Write the two figures down for each appliance you plan to power.
• Repeat for each appliance, one at a time.

What a watt meter does not answer

A watt meter measures one appliance at one moment. It does not state how long a fuel tank lasts, how much fuel a generator uses per hour, or whether several appliances can run at the same time. Those questions are about the generator's stated figures, the maker's guidance, and a licensed electrician's review of your home.

The meter also does not show power quality, such as total harmonic distortion, which is a separate listing topic. See what THD means on a generator listing. For any connection to your home's wiring, a licensed electrician, the maker, and the local permit office are the ones who decide it.

Measured figures also change with conditions. A window air conditioner draws differently on a hot afternoon than on a mild morning, and a well pump draws differently at first start than on a later cycle. Your list is a snapshot of your home at the time you measured it.

Using your measured list with this site's guides

The buying guides group generators by their stated watts groups and by Texas home situations. For the essentials like a refrigerator and lights, the mid-size generator guide is a starting point. For a rural home on a well, the rural generator guide covers larger portables. For a unit an electrician installs for the whole house, the whole house generator guide is the match. If you are still deciding between a generator and a power station, the backup power overview compares the two.

Use your measured list against the stated figures on each listing. The listing states what the generator delivers; your measurements state what your appliances draw in your home. When the two are close, confirm the stated figures on the maker's page and ask a licensed electrician before you decide.

Preparedness and safety pointers

Measurement is part of preparing; the rest is planning. Before storm season, write down which appliances you will power, in what order, and how you expect to use them. TexasReady and the Texas Division of Emergency Management publish preparedness checklists for winter storms and extreme heat, and the sources below link to both.

Use a gasoline generator outdoors only, follow the maker's manual, and read the carbon monoxide guidance on the CPSC and CDC pages listed in the sources.

The EIA pages in the sources explain how electricity use in homes varies by region and type of home, and how household electricity is measured. Those pages give context for the figures you see on a meter and on a listing.

What to pick for your situation

Questions

What does a watt meter do?

A watt meter plugs between a wall outlet and an appliance and shows the watts the appliance draws at that moment. The U.S. Energy Information Administration explains that electricity is measured in Watts, and generator listings use the same unit for starting and running watts.

Does a watt meter tell me what size generator to buy?

It gives you the measured draw of each appliance, which you compare to the stated figures on a listing. It does not answer run-time or whole-home questions; the maker and a licensed electrician decide sizing.

Why do I need a meter if the listing already states watts?

The listing states what the generator is claimed to deliver. A watt meter shows what your appliance actually draws in your home, which can differ from a general figure. Your measured list is your own reference.

Can I measure a well pump or a window air conditioner the same way?

Yes. Any appliance with a standard plug can go through a watt meter. Watch the display as it starts to see the peak figure, then let it run to see the running figure.

What if my measured figure is close to the generator's stated figure?

That puts you at the edge of the listing's claim. Confirm the stated figures on the maker's page and ask a licensed electrician before deciding.