A plug-in watt meter costs around $25 and answers the one question every generator sales page guesses at: what your house actually pulls.
Size your generator →A home energy audit for backup power is not the utility company version, where somebody hangs a fan in your front door and then sells you attic insulation. It is a list. Every load in the house, what it pulls in watts while it runs, and what it eats in watt-hours over a full day. Get that list and every other decision on this site becomes arithmetic instead of opinion.
It takes one weekend, one cheap meter, and a notebook. Most people who skip it end up either with a generator too small to run the well or a battery that cost three times what their actual loads justified.
Watts tell you if it starts, watt-hours tell you how long
Watts are a rate. A 150 W refrigerator compressor is pulling 150 joules every second while it runs. Watt-hours are a quantity: that same fridge running 150 W for 8 hours out of 24 has consumed roughly 1,200 watt-hours, or 1.2 kWh.
The distinction matters because the two numbers size two different things. Watts decide whether your generator or inverter can turn the thing on at all. Watt-hours decide how many hours of runtime a battery or a tank of fuel buys you. A power station rated 1,800 W output and 1,000 Wh capacity will happily start your fridge and then die before morning. Both numbers, every time.
The meter that ends the argument
A plug-in energy monitor sits between the outlet and the appliance and reads instantaneous watts plus accumulated kilowatt-hours. Leave it on the fridge for 48 hours and it will tell you the real daily consumption, duty cycle included, which no spec sheet will.
Two limits worth knowing before you buy one. These meters only read 120 V plug loads on a standard 15 A outlet, so your well pump, electric range, dryer and electric water heater are out of reach. And most of them sample too slowly to catch the true inrush spike when a motor starts. For hardwired 240 V loads you either read the nameplate, look up the motor’s locked rotor amps, or have an electrician clamp it.
Typical loads in a house with a well
Use these as a starting point and replace each line with your own measurement as you get it. Ranges are wide on purpose: a 2008 side-by-side and a 2023 inverter fridge are not the same animal.
| Load | Running watts | Startup surge | Typical per day |
|---|---|---|---|
| Refrigerator, 18-22 cu ft | 100-200 W | 600-1,200 W | 1-2 kWh |
| Chest freezer, full | 80-150 W | 500-1,000 W | 0.8-1.5 kWh |
| Gas furnace, older PSC blower | 400-800 W | 1,500-2,500 W | 2-6 kWh in winter |
| Gas furnace, ECM blower | 100-300 W | 500-800 W | 0.8-2.5 kWh in winter |
| Submersible well pump, 1/2 HP | 800-1,000 W | 2,000-3,000 W | 0.3-0.8 kWh |
| Sump pump, 1/3 HP | 600-900 W | 1,300-2,000 W | Depends on the storm |
| Boiler circulator pump | 50-100 W | Minimal | 0.5-2 kWh in winter |
| CPAP, humidifier off | 30-60 W | None | 0.2-0.4 kWh per night |
| Router and modem | 10-20 W | None | 0.25-0.5 kWh |
| LED lighting, one floor | 40-100 W | None | 0.2-0.6 kWh |
| Microwave, 1,000 W rated | 1,400-1,700 W draw | Brief | Minutes at a time |
| Electric water heater element | 4,500 W at 240 V | None | 8-15 kWh |
Add up the right-hand column for the things you truly need and you have your daily energy budget. For most houses running on backup, that number lands somewhere between 3 and 8 kWh a day in mild weather, and considerably more in a winter storm when the furnace blower runs half the night.
Surge is what actually breaks things
Anything with an induction motor, meaning pumps, compressors, blowers and the well, draws several times its running current for a fraction of a second while the rotor gets moving. A half-horsepower well pump that settles at 900 W can ask for 2,000 to 3,000 W on start. The inverter or generator has to survive that spike or it trips out and you are standing in the dark wondering why a 1,000 W load defeated a 1,500 W machine.
Two things soften it. A soft starter on a pump or air conditioner cuts inrush substantially. And staggering your loads by hand, so the fridge and the furnace never start in the same second, buys you headroom for free.
How to run the audit in a weekend
- Walk the house with a notepad and list every load, then mark each one critical, comfort, or optional. Be strict. The espresso machine is not critical.
- Put the meter on the refrigerator for 48 hours. Record kWh, then divide by two for the daily figure. Repeat on the freezer.
- Measure the small stuff in one evening: router, laptop, lights, medical devices, the well pressure tank controller.
- For 240 V hardwired loads, read the nameplate on the equipment and the breaker size on the panel. Write both down.
- Build two totals: peak watts if your critical loads all ran at once, and watt-hours per day for the critical list alone.
- Redo the watt-hour total for a January day with the furnace running. That is your real worst case, not August.
Turning the numbers into a plan
Peak watts point you at a generator size or an inverter rating. Watt-hours per day point you at fuel or battery capacity. If your critical list comes to 4 kWh a day, a 2,000 Wh power station is a half-day tool no matter how impressive the marketing photo looks, while a 7,500 W portable generator will cover it easily but burn fuel doing it. The audit is what lets you see that before the money leaves your account. From here, the two next steps are sizing a generator against your peak or checking whether a battery covers your daily total.
Two things the audit will not tell you
It will not capture a bad start capacitor or a tired compressor that draws more than it should. If a measured number comes in wildly above the range above, that is a maintenance finding, not a sizing input. And it says nothing about the order in which things fail. A house can have a perfectly sized generator and still lose water because nobody thought about the pressure tank, which is the sort of thing covered in what actually breaks first when the grid goes down.
What I would do
Buy the meter before you buy anything else. Spend the $25, spend the weekend, and write the results on a card taped inside the panel door where your future self will find it during an outage at 2 a.m. Nine times out of ten the audit shrinks the purchase you were about to make, because the critical list turns out to be a fridge, a freezer, a furnace blower, a well pump and some lights, and that is a much smaller problem than the one the sales page described. The power and backup section assumes you have these numbers, so this is the article to do first.
Frequently Asked Questions
How many watt-hours does an average house use in a day?
A typical American house runs somewhere around 20 to 30 kWh a day on grid power. That is not the number you size backup for. The critical-loads-only figure is usually 3 to 8 kWh, which is why backup is affordable at all.
Can a plug-in watt meter measure my well pump?
No. Submersible well pumps are hardwired and usually 240 V, well outside what a 120 V plug meter handles. Read the pump nameplate for horsepower and service factor amps, or have an electrician put a clamp meter on it.
Why is my fridge listed at 700 watts but measures 150?
Nameplate ratings on appliances often reflect maximum draw with defrost heaters and everything else running, not the steady compressor load. The measured number is the one to plan with; keep the nameplate number in mind only for surge headroom.
Do I need to audit in both summer and winter?
Ideally yes, but if you only do it once, do it in the cold. Winter is when the furnace blower, the well and the lights all pile onto the same day, and it is also when an outage is most likely to last.