What is the difference between watts and watt-hours?

Watts measure how fast energy is being used at any instant, like the speedometer on a car. Watt-hours measure total energy consumed over time, like the odometer. A 100-watt light bulb running for 10 hours uses 1,000 watt-hours, or 1 kilowatt-hour. When sizing backup power, you need both numbers: watts tell you how large the generator or inverter must be, and watt-hours tell you how much fuel or battery capacity you need to run it for a given duration.

Can I measure 240-volt appliances with a Kill A Watt meter?

No. Standard plug-in meters like the Kill A Watt work only with 120-volt outlets. Most large appliances that use 240 volts, including electric dryers, electric ranges, central air conditioners, electric water heaters, and well pumps, plug into dedicated 240-volt outlets or are hardwired and cannot be measured with a plug-in meter. For these loads, you can estimate consumption from the nameplate rating on the appliance and the duty cycle, or use a whole-house energy monitor that clamps onto the circuit breakers at your electrical panel.

What is startup surge and why does it matter?

Startup surge, also called inrush current, is the brief spike in power that occurs when a motor-driven appliance first turns on. A refrigerator that runs at 150 watts may draw 800 to 1,200 watts for the first fraction of a second when the compressor starts. A window air conditioner running at 1,200 watts may surge to 3,600 watts on startup. This matters for generator and inverter sizing because the power source must handle the surge without tripping a breaker or shutting down. If you size your backup power to running watts alone, it may fail the moment a motor starts.

How do I know if a plug-in meter reading is accurate?

A single watt reading at one moment is accurate but incomplete. Cycling appliances like refrigerators, dehumidifiers, and air conditioners turn on and off throughout the day. A spot reading catches the appliance either running or idle, which gives you one extreme or the other. The accurate measurement is the 24-hour cumulative kWh reading, which captures all the on-off cycles over a full day and gives you the true daily energy consumption. For appliances that run continuously, like a cable box, a spot reading is sufficient.

How does measuring power use help with generator sizing?

A generator must be large enough to handle the combined running watts of everything you want to power simultaneously, plus the startup surge of the largest motor load starting at the same time. If your essential loads total 2,500 running watts and your refrigerator has a 1,200-watt startup surge, you need a generator rated for at least 3,700 watts. Without measured data, you are guessing, and people consistently guess wrong in both directions. Measuring your actual loads before buying a generator prevents buying one that is too small to run what you need or too large for what you will actually use.

Home Self-Reliance Energy Measuring Power Use

Energy Preparedness

Measure it before you size it.

The number on the label is not the number your appliance actually draws. A $25 plug-in meter tells you the truth, and that truth is what sizes your generator, your battery, and your backup plan.

How to measure Appliance reference

The Language

Watts, watt-hours, and amps

Three units, three different questions. Confusing them is the most common mistake in load sizing, and it leads to buying the wrong generator or the wrong battery.

Watts

How fast energy is being used right now. Like the speedometer on a car. A 1,500-watt space heater draws 1,500 watts every moment it runs.

Answers: "How big must my generator or inverter be?"

Watt-hours

Total energy consumed over time. Like the odometer. That 1,500-watt heater running for 4 hours uses 6,000 Wh, or 6 kWh. Your utility bill is measured in kWh.

Answers: "How much fuel or battery capacity do I need?"

Amps

The volume of electrical current flowing. At 120 volts, amps = watts / 120. A 1,500-watt heater draws 12.5 amps. At 240 volts, amps = watts / 240.

Answers: "Will this trip my circuit breaker?"

The math that connects them

Watts = Volts x Amps. Watt-hours = Watts x Hours. A 100-watt bulb running for 10 hours = 1,000 Wh = 1 kWh. At 17 cents per kWh, that costs about 17 cents. These three formulas are the entire foundation of household energy measurement.

The Problem

Why the label is not enough

The wattage on an appliance nameplate is the maximum power the device can draw. The DOE is clear about this: the rated wattage is the maximum, not the average.^[1] Most appliances spend little or no time at their rated maximum.

A refrigerator nameplate might say 700 watts, but the compressor only runs about a third of the time. Its actual daily consumption is closer to 1,000 to 2,000 watt-hours, not 16,800 Wh (700W x 24h) as the label would suggest. If you sized a battery based on the label, you would overbuy by a factor of eight or more.

The reverse is also a problem. A microwave rated at 1,000 watts may draw 1,500 watts from the outlet because the rated wattage is cooking power, not electrical input. Using the label to size a generator would leave you 500 watts short.

Cycling appliances: the duty cycle

Refrigerators, air conditioners, dehumidifiers, freezers, and electric water heaters cycle on and off throughout the day. They are not running at full wattage continuously. The duty cycle, the fraction of time the appliance is actually drawing power, is typically 30 to 60 percent.

A plug-in meter solves this automatically. Leave it connected for 24 hours and it measures the cumulative kWh, which accounts for all the on-off cycles without you having to estimate the duty cycle yourself.

The Method

The 24-hour measurement method

A plug-in energy meter like the Kill A Watt (about $25) plugs between the wall outlet and your appliance. It displays live watts, cumulative kWh, volts, and amps. Here is how to get an accurate measurement for any 120-volt device.

Plug in the meter and reset it

Plug the meter into the wall outlet. Plug the appliance into the meter. Reset the cumulative kWh counter to zero. Note the time.

Read the live watts several times

Check the watt display at different times during the day. For cycling appliances, you will see two states: the high reading when the compressor or heating element is on, and the low reading (often near zero) when it is idle. Record both. The high reading is the running watts you need for generator sizing.

Note the peak or maximum reading

If your meter records a peak watt value, check it after the appliance has started at least once. This approximates the startup surge. For a refrigerator, the peak might be 800 to 1,200 watts even though running watts are 100 to 200. This number matters for inverter and generator sizing.

Read the 24-hour total

After a full 24 hours, read the cumulative kWh. This is the appliance's real daily energy consumption, accounting for all on-off cycles. Multiply by 30 for a monthly estimate. Multiply by your rate per kWh for the monthly cost of running that single appliance.

Record and move to the next device

Write down the appliance name, running watts, peak watts, and 24-hour kWh. Then move the meter to the next appliance on your list. Work through the house one device at a time. The full inventory takes a week or two at this pace.

The Spike

Startup surge: the spike that trips generators

Every motor-driven appliance draws a burst of extra power for a fraction of a second when it starts. This is called inrush current or startup surge. It can be two to six times the running wattage and lasts less than a second, but it is enough to trip a breaker or overload a generator that was sized only for running watts.

Appliance	Running watts	Startup surge	Surge multiplier
Refrigerator	100-200 W	800-1,200 W	4-6x
Window AC (10,000 BTU)	1,200 W	3,000-3,600 W	2.5-3x
Sump pump (1/3 HP)	800 W	1,300-2,400 W	2-3x
Furnace blower	500-800 W	1,000-2,400 W	2-3x
Dehumidifier	300-700 W	600-1,400 W	2x
Well pump (1/2 HP)	1,000 W	2,000-3,000 W	2-3x

Ranges reflect variation across makes, models, and ages. Your plug-in meter's peak reading gives you the actual number for your specific appliance. Generators list both "running watts" and "starting watts" or "surge watts" in their specifications for this reason.

Reference

Common appliance power draws

Use this table as a starting reference, not a substitute for measuring your own appliances. Real-world draws vary by age, model, and condition. The 24-hour kWh column reflects typical daily consumption including duty cycles.

Appliance	Running watts	Typical daily kWh	Voltage
Kitchen
Refrigerator (modern)	100-200 W	1.0-2.0	120V
Refrigerator (15+ years old)	200-400 W	2.5-5.0	120V
Chest freezer	50-100 W	0.5-1.5	120V
Microwave	1,000-1,800 W	0.2-0.5	120V
Dishwasher	1,200-1,800 W	1.0-2.0	120V
Electric range/oven	2,000-3,500 W	1.5-3.0	240V
Laundry
Washing machine	350-500 W	0.5-1.0	120V
Electric dryer	2,800-5,000 W	2.0-4.0 per load	240V
Gas dryer	300-600 W	0.3-0.5 per load	120V
HVAC and Water
Central AC (3 ton)	3,000-3,500 W	15-30	240V
Window AC (10,000 BTU)	1,000-1,200 W	5-10	120V
Furnace blower	500-800 W	4-8	120V
Space heater	750-1,500 W	6-12	120V
Electric water heater (tank)	3,000-4,500 W	8-14	240V
Dehumidifier	300-700 W	3-8	120V
Well pump (1/2 HP)	750-1,000 W	1-3	240V
Electronics and Lighting
LED bulb (60W equivalent)	8-10 W	0.04-0.08	120V
TV (55-inch LED)	50-100 W	0.2-0.6	120V
Cable box / DVR	20-35 W	0.5-0.8	120V
Laptop (charging)	30-65 W	0.1-0.3	120V
Phone charger	5-20 W	0.01-0.04	120V
Game console (active)	100-200 W	0.3-1.0	120V
Wi-Fi router + modem	10-20 W	0.2-0.5	120V
Medical
CPAP (without humidifier)	30-60 W	0.2-0.5	120V
Oxygen concentrator	300-600 W	5-10	120V

Daily kWh figures reflect typical residential use patterns and duty cycles. Your actual consumption may differ. Measure to be sure. 240V appliances cannot be measured with a standard plug-in meter.

What a Plug-In Meter Cannot Reach

Measuring 240-volt and hardwired loads

Standard plug-in meters work only with 120-volt outlets. Your largest energy consumers, the electric dryer, electric range, central air conditioner, electric water heater, and well pump, all run on 240-volt circuits or are hardwired directly to the panel. You cannot plug a Kill A Watt into a dryer outlet.

Option 1: Estimate from the nameplate and duty cycle

Read the nameplate on the appliance for its wattage or amperage rating. Multiply amps by 240 to get watts. Then estimate the duty cycle: a water heater runs about 3 to 4 hours per day, a dryer runs 45 to 60 minutes per load, central AC runs 30 to 50 percent of the time on a hot day.

Example: Electric water heater rated at 4,500W, running 3 hours per day = 13.5 kWh/day.

Option 2: Whole-house energy monitor

Devices like the Emporia Vue, Sense, or IoTaWatt clamp onto individual circuit breakers in your electrical panel and monitor every circuit in real time. They can measure 240-volt circuits, identify individual appliance usage, and show you exactly what is drawing power at any moment.

These cost $100 to $300 and typically require installation at the breaker panel. Some are simple clamp-on installations; others may need an electrician.

Option 3: Read the circuit breaker label

For a rough upper bound, read the amperage on the circuit breaker serving the appliance. A 30-amp breaker on a 240V circuit means the maximum load is 30 x 240 = 7,200 watts. The actual draw will be lower than this, but it gives you a ceiling for generator sizing. Your electrical panel has a label on the inside of the door that maps each breaker to its circuit.

Hidden Draws

Phantom loads: always on, always drawing

Phantom loads are devices that draw power even when they appear to be off. A cable box in standby draws 20 to 35 watts, 24 hours a day, 365 days a year. That is 175 to 300 kWh per year from a device you think is off. The DOE estimates phantom loads account for 5 to 10 percent of household electricity use.^[2]

How to find them

Plug your meter into anything that stays plugged in when not in active use. Check the watt reading when the device is "off" or in standby. Anything above zero is a phantom load. Common offenders:

Cable/satellite box: 20-35W standby. Often the largest single phantom load in the house.

Game console: 10-15W standby, more if instant-on mode is enabled.

Printer: 5-15W standby, even when not printing.

Smart speaker: 2-4W continuously, always listening.

Phone/tablet charger (no device): 0.1-0.5W. Negligible individually, but many homes have 5-10 chargers plugged in.

Desktop computer (sleep): 2-10W depending on sleep mode depth.

How to reduce them

A smart power strip cuts power to connected devices when the primary device (the TV, the computer) turns off. The cable box is the one phantom load that is hardest to eliminate because many boxes take 5 to 10 minutes to reboot, which makes putting them on a power strip impractical. For everything else, a strip with a master switch solves the problem.

Preparedness Connection

Build your load priority list

The measurements you have now are the foundation of backup power planning. The next step is a priority list: what must stay on during an outage, what would be nice, and what can wait.

Tier 1: Must have

Medical equipment (CPAP, oxygen concentrator), refrigerator/freezer (food preservation), sump pump (flood prevention), well pump (water access), basic lighting, phone charging, and communications (router/modem). These loads define the minimum size of your backup power system.

Tier 2: Important

Heating system blower (winter), one window AC or fan (summer), cooking (microwave or electric kettle), hot water (if electric). These loads make an outage manageable rather than merely survivable. They roughly double the generator or battery size needed.

Tier 3: Comfort

TV, washer, full kitchen, multiple room lighting, central AC. These make an outage feel normal. Powering all of Tier 3 typically requires a whole-house generator or a very large battery system. Most households run Tier 1 and part of Tier 2 on portable generators or batteries.

From list to generator size

Add the running watts of everything in Tier 1. Add the startup surge of the largest motor in Tier 1. The total is the minimum generator size. Add Tier 2 loads if you want more capacity. Our generator guide and portable power station guide cover the sizing math in detail.

Without the measured data from this page, that sizing calculation is a guess. With it, you buy the right tool for your actual loads.

Sources

DOE. "Energy Saver: Estimating Appliance and Home Electronic Energy Use." energy.gov. Accessed August 2026.
DOE. "Energy Saver: Reduce Standby (Phantom) Loads." energy.gov. Accessed August 2026.
EIA. "Electricity Use in Homes." eia.gov. Accessed August 2026.
ENERGY STAR. "Heat & Cool Efficiently." energystar.gov. Accessed August 2026.

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