Why this is a backup power product
Almost every page on this site eventually says the same thing: read the plate, measure it, do not estimate. That advice is only useful if measuring is easy, and a meter is what makes it easy.
The failure it prevents is specific and expensive. People size backup power from nameplate ratings and worst-case assumptions, and nameplate ratings are ceilings rather than draws. A computer with an 850W power supply may idle at 60W. A refrigerator rated at 800W starting averages 150W. A furnace blower rated 1,600W may draw 700W in practice.
Add those ceilings together and you buy a 7,500W generator for a house that needs 3,000. Measure them and the answer is smaller, cheaper and quieter.
Three tiers of measurement
| Tool | Measures | Right for |
|---|
| Basic plug-in watt meter | One 120V appliance, live reading only | A quick check with no app |
|---|
| Smart plug with energy monitoring | One 120V appliance, with logged history | Working out averages over days — the useful case |
|---|
| Panel-level monitor | Whole service plus individual circuits, including 240V | Sizing a transfer switch or a standby generator |
|---|
| Clamp meter | A live circuit, instantaneously | An electrician diagnosing something specific |
|---|
The middle two are what this page is about. The tier you need follows directly from what you are buying: a battery for some appliances, or a generator and a switch for circuits.
Peak, average and the difference between them
A meter gives you two numbers and they size two different things.
Peak draw sizes the inverter or the generator. It is the largest instantaneous demand, including the surge when a motor starts. Note that many smart plugs sample too slowly to catch a genuine millisecond surge — they will show the running draw well and understate the start.
Average draw sizes the battery. It is what the appliance uses over hours, and for anything that cycles — a refrigerator, a furnace, a well pump — it is dramatically lower than the running figure.
The starting watts page covers why the surge matters and why a meter is not the right tool to measure it. For surge, use the manufacturer's figure or the plate.
The week-long exercise worth running
This is the single most useful thing on the page, and it costs one evening plus a week of waiting.
1. Pick your critical list. Typically the refrigerator, the freezer, the network and lighting. Write it down.
2. Meter the two biggest items for a full week. Refrigerator and freezer are the usual pair, and a week captures the variation between a hot day and a cool one.
3. Read the daily kilowatt-hours, not the watts. That is the number that maps directly onto battery capacity.
4. Add the small stuff from the chart. Lighting and network gear are small enough that the appliance wattage chart is accurate enough.
5. Add 20% and buy against that. The sizing page walks through the arithmetic.
The second job: load shedding during an outage
A smart plug earns its keep twice, and the second use is the one people do not anticipate.
On a generator or a battery you are managing a budget, and the biggest risk is something switching itself on while you are not watching — a second fridge, a dehumidifier, a well pump.
Smart plugs with schedules and auto-off timers let you sequence loads deliberately: the freezer runs for two hours, then the fridge, rather than both compressors starting together and tripping the inverter.
The caveat is significant and worth stating plainly: a Wi-Fi smart plug needs the network, and the network is usually down. This only works if the router is on backup power too — the router page covers that, and the smart home page covers the failure order.
What a meter will not tell you
- Motor starting surge. Most sample too slowly. Use the plate or the manufacturer's figure.
- Anything on 240V, unless it is a panel-level monitor. Never put a 120V smart plug near a dryer or water heater circuit.
- Anything hardwired. The furnace, the well pump and the water heater need a panel monitor or an electrician with a clamp meter.
- Power factor subtleties. Cheap meters report apparent power rather than real power on some loads, which overstates motors.
The short version
- Measure before you size. Nameplates are ceilings, not draws.
- A smart plug for appliances, a panel monitor for circuits.
- Read daily kilowatt-hours, not instantaneous watts.
- Use the plate for surge. Meters are too slow to catch it.
