Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases.
A battery label cannot tell you how long your household will run until you know what each essential device actually uses. The useful number is usually energy over time, measured in watt-hours (Wh), rather than the highest wattage printed on the appliance.
This guide gives you a repeatable way to measure plug-in loads, turn the readings into a daily energy budget, and carry that budget into our household power-outage planner. It is a planning method, not an electrical-safety inspection or a promise that a battery, inverter, or generator can start every appliance.
Start with the question you need to answer
Make three lists before buying anything:
- Essential: lighting, communications, refrigeration, and other loads your household has decided must continue.
- Useful: loads that improve comfort or reduce disruption but can be deferred.
- Usually off: high-energy or high-power appliances you can replace with a lower-energy method during the outage.
Medical equipment needs its own plan based on the manufacturer and care team. Do not use a household calculator as evidence that a medical device has safe backup duration.
Know watts, watt-hours, and startup power
Watts (W) describe the rate of power use at a moment. Watt-hours (Wh) describe energy used over time. A steady 20 W device running for 8 hours uses 160 Wh: 20 W × 8 h. A cycling appliance is different. A refrigerator may draw power when its compressor runs and little power between cycles, so multiplying its running watts by 24 hours can badly overstate energy.
Startup or surge power is a separate constraint. A motor can require more power as it starts than it draws while running. An energy total may fit inside a battery’s capacity while the appliance still exceeds the inverter’s output or surge rating. Check the appliance and power-source instructions; do not infer safe compatibility from Wh alone.
Choose the best evidence available
Use these methods in descending order of usefulness for an ordinary plug-in appliance:
- Measure energy over a representative period. A plug-in electricity-usage meter that reports kWh or Wh captures cycling and changing loads. Choose a meter whose voltage, current, and plug type match the appliance and receptacle, and follow its instructions.
- Use a stable watt reading. For a load that stays steady, record watts after it settles and multiply by expected daily hours. The Department of Energy’s standby-power measurement guidance says a stable reading can be read directly; when consumption fluctuates, measure energy over time and divide by the measurement period to find average power.
- Use the nameplate only as a conservative placeholder. A label’s volts and amps can estimate an upper input of volts × amps, but it may not represent ordinary consumption. Replace that placeholder when you have a measurement.
- Use an EnergyGuide or manufacturer energy figure carefully. Convert annual kWh to daily Wh with annual kWh × 1,000 ÷ 365. The result is an average under the stated test assumptions, not a guarantee for your room temperature, settings, appliance age, or outage conditions.
Do not open a panel, expose conductors, defeat grounding, or improvise adapters to take a reading. Plug-in meters are for compatible plug loads. Hardwired circuits, large 240 V appliances, and generator transfer equipment call for qualified help.
A seven-step load-measurement worksheet
- Write the appliance, model, location, and why it is essential.
- Record the measurement method: watt-hours over time, steady watts, label, or manufacturer figure.
- For cycling loads, measure long enough to include several normal cycles. A 24-hour reading is more informative than a brief compressor-on snapshot for refrigeration.
- Record the test duration and total Wh or kWh. Convert kWh to Wh by multiplying by 1,000.
- Normalize the reading: measured Wh ÷ measured hours × 24 = estimated Wh/day.
- Note conditions that may change the result, including room temperature, device settings, door openings, charging state, and seasonal use.
- Date the reading. Recheck after replacing equipment or changing how it is used.
If you want to compare currently available plug-in meters, this Amazon electricity-usage-monitor search is an affiliate link. We have not physically tested those listings. Check electrical ratings, measurement range, display resolution, safety certification, return terms, and recent buyer documentation rather than choosing by the first result.
Worked 72-hour household example
The following figures are fictional arithmetic inputs, not typical-device claims:
| Load | Evidence entered | Daily device energy |
|---|---|---|
| Refrigerator | 1.20 kWh measured over 48 hours | 600 Wh/day |
| Two LED lights | 9 W each × 5 h | 90 Wh/day |
| Communications | 18 W average × 6 h | 108 Wh/day |
| Phone charging | Measured 15 Wh per charge × 2 | 30 Wh/day |
| Total delivered to devices | 828 Wh/day |
For three days, the devices need 828 × 3 = 2,484 Wh. Battery-side demand will be higher because conversion and the power source’s own operation consume energy. Using the planner’s illustrative 85% conversion efficiency and 5 W continuous idle draw:
(828 Wh ÷ 0.85) + (5 W × 24 h) = about 1,094 Wh drawn from the battery per day. Three days would therefore draw about 3,282 Wh. If the plan also holds 20% in reserve, the modeled rated capacity is 3,282 ÷ 0.80 = about 4,103 Wh. Different efficiency, reserve, standby draw, temperature, battery limits, and actual use change the answer.
This example reveals a useful decision: refrigeration accounts for most of the device energy. Measuring it well matters more than polishing a tiny phone-charging estimate. It also shows why adding nameplate watts is not a runtime calculation.
Refrigeration is an energy problem and a food-safety problem
The FDA advises keeping refrigerator and freezer doors closed during an outage. It says an unopened refrigerator keeps food cold for about four hours, while a full freezer holds temperature for about 48 hours, or about 24 hours when half full. Those are food-safety planning intervals, not promises about a particular appliance’s electrical consumption. Use appliance thermometers and follow the FDA’s temperature-based discard guidance rather than assuming that intermittent power automatically made food safe.
A refrigerator measurement taken during normal grid service helps size an energy plan, but outage conditions can differ. Hotter rooms and frequent door openings may increase runtime. Starting the compressor also requires a compatible power source. Keep the food-safety fallback—closed doors, ice/coolers, thermometers, and a discard decision—alongside the electrical plan.
Turn the worksheet into a power-source decision
Enter each measured load in the planner, including hours and duty cycle only when you have not already entered measured average power. If a meter gives total daily Wh, divide by 24 to get average watts and enter 24 hours at 100% duty. Do not apply cycling twice.
Then check three separate limits:
- Energy: enough usable Wh for the planned duration, losses, and reserve.
- Running power: enough continuous watts for loads that may operate together.
- Startup power and compatibility: enough surge capability, correct output, and manufacturer-supported connections.
You may discover that no purchase is needed: a smaller essential list, charged power banks, cold-storage precautions, and a shorter plan may cover the likely interruption. If the measured need remains larger, use our research-based battery, solar, and generator guide to compare the roles of those systems. Fuel-burning equipment adds carbon-monoxide and connection hazards and should never be treated as a plug-in substitute without the required safe location and electrical arrangement.
Sources and update record
Checked September 15, 2026:
- U.S. Department of Energy: Measuring Standby Power — stable versus fluctuating measurement method and meter-resolution context.
- FDA: Food and Water Safety During Power Outages and Floods — refrigeration temperatures, closed-door intervals, thermometers, and discard guidance.
- Ready.gov: Power Outage Hazard Information Sheet — generator, carbon-monoxide, medical-device, food, and outage-planning precautions.
Our Top Posts About Prepping
The first 72 hours after a disaster are the most critical for survival. Enter your email below to get a checklist of essential items you need in your 72 hour emergency get home bag…
The first 72 hours after a disaster are the most critical for survival. Enter your email below to get a checklist of essential items you need in your 72 hour emergency get home bag…