What "measuring electric" means
Measuring electric means finding out how much power your devices and appliances are using, usually in units called watts or kilowatt-hours. Your electric meter does this automatically for your whole house, but you can also measure individual devices with a plug-in meter or a clamp meter to see which ones cost the most to run.
Most people measure electricity for one of three reasons: to understand why their bill is high, to decide whether an upgrade (like a heat pump or new refrigerator) will save money, or to make sure a circuit isn't overloaded before plugging in something heavy like a space heater or air conditioner.
Key Takeaways
- Your electric meter measures all power entering your home in kilowatt-hours, and your utility company reads it monthly to calculate your bill.
- A plug-in power meter (around $15 to $30) shows you how much power one device uses, letting you compare appliances or find energy hogs.
- Watts measure power being used right now; kilowatt-hours measure total energy used over time, and that is what you pay for.
- Multiplying a device's watts by the hours you run it per day, then dividing by 1,000, tells you its monthly kilowatt-hour cost.
- A clamp meter measures current flowing through a wire without disconnecting anything, useful for checking if a circuit is overloaded.
Reading your home's electric meter
Your electric meter sits outside your house or in a basement or garage and measures all the electricity flowing into your home. It displays a number in kilowatt-hours (kWh), which is the unit your utility company uses to bill you. The meter spins or counts up continuously whenever any device in your house is running.
To read your meter, write down the number shown on the display. If you check it at the same time each day for a week, you can see how much power your household used that day. Most modern meters have a digital screen that shows the number clearly; older meters have five dials that you read left to right, skipping every other dial if there are five total.
Your utility company reads this meter once a month and charges you based on how many kilowatt-hours you used. If your bill seems high, comparing your meter reading week to week can tell you whether you used more power than usual or whether the rate itself changed.
Using a plug-in power meter to measure individual devices
A plug-in power meter (also called a kill-a-watt meter or power usage monitor) is a small device you plug into an outlet, then plug your appliance into it. It shows you the watts that device is drawing right now and can calculate total kilowatt-hours if you leave it plugged in over time. These meters cost $15 to $30 and work on any device that plugs into a standard outlet.
To use one, plug the meter into the wall outlet, plug your device into the meter, turn the device on, and read the wattage on the meter's display. Leave it running for a few hours or a full day to see the kilowatt-hour total. This tells you exactly how much that refrigerator, space heater, or window air conditioner costs to run.
Plug-in meters are most useful for finding energy hogs — devices that run all day or all night and add up to a large share of your bill. A refrigerator, water heater, or air conditioner running constantly will show high numbers. A device you use for an hour a day will show lower totals even if its wattage is high.
Understanding watts versus kilowatt-hours
Watts measure power being used at that exact moment. A 100-watt light bulb uses 100 watts while it is on. A space heater might draw 1,500 watts. These numbers tell you how much power the device needs right now, but not how much you will pay for it.
Kilowatt-hours measure total energy used over time, and this is what you pay for on your electric bill. One kilowatt-hour means running a 1,000-watt device for one hour, or a 100-watt device for ten hours. To estimate a device's monthly cost, multiply its watts by the hours you run it per day, divide by 1,000 to get kilowatt-hours per day, then multiply by 30 for a month.
For example, a 1,500-watt space heater running 8 hours a day uses (1,500 × 8 ÷ 1,000) = 12 kilowatt-hours per day, or about 360 kilowatt-hours per month. At a typical rate of $0.12 per kilowatt-hour, that costs about $43 per month. A 60-watt lamp running 5 hours a day uses only 0.3 kilowatt-hours per day, or about $1.08 per month.
Checking if a circuit can handle a new device
Before plugging in a heavy-load device like a space heater, air conditioner, or electric oven, you need to know whether the circuit it will run on can handle it. Every circuit in your home has a maximum amperage (usually 15 or 20 amps for standard outlets), and exceeding that trips the breaker and cuts power to that circuit.
To check safely, add up the watts of everything already plugged into that circuit, then add the watts of the new device. If the total exceeds 1,440 watts on a 15-amp circuit or 1,920 watts on a 20-amp circuit, you should not plug in the new device on that circuit. Instead, use a different circuit or have an electrician install a dedicated circuit for the heavy device.
A clamp meter can measure the current (in amps) flowing through a wire without unplugging anything. You clamp it around the wire and read the amperage directly. This is more accurate than adding up wattages and is the method electricians use, but clamp meters cost $30 to $100 and require some comfort working near electrical wiring.
Comparing appliances before you buy
If you are deciding between two refrigerators, air conditioners, or other major appliances, you can use a plug-in meter to compare their power use. Plug each one in (at different times) and let it run for a full day or a full week to see the kilowatt-hour total. The one with the lower number will cost less to run over time.
Many appliances also have an EnergyGuide label on the box or the manufacturer's website that estimates yearly kilowatt-hour use and cost. This label is standardized, so you can compare two models directly without testing them yourself. Newer appliances are usually more efficient than older ones, so replacing an old refrigerator or air conditioner with a new one often pays for itself in lower bills within a few years.
Frequently Asked Questions
How do I know if my electric bill is accurate?
Read your meter on the same day each month and compare it to your bill. The kilowatt-hours on your bill should match the difference between this month's reading and last month's. If they do not match, contact your utility company to report the discrepancy.
Can I measure electricity use on hardwired appliances like a dishwasher or oven?
No, plug-in meters only work on devices with a plug. For hardwired appliances, you need a clamp meter or an electrician to measure the current. Some newer homes have submeters that measure specific circuits, which can tell you what a hardwired appliance uses.
What is a normal amount of electricity for a household to use per month?
This varies widely by climate, home size, and how many people live there. A typical U.S. household uses between 800 and 1,200 kilowatt-hours per month, but homes in hot climates with air conditioning can use much more, and smaller homes or those in mild climates use less.
Will measuring my electricity use lower my bill?
Measuring itself does not lower your bill, but the information you learn can help you decide what to change. If you find that a space heater or air conditioner is costing a lot, you might use it less or replace it with a more efficient model, which would lower your bill.
Do smart plugs measure electricity the same way as a plug-in power meter?
Yes, smart plugs have the same measurement capability as plug-in meters and can show you watts and kilowatt-hours on your phone. They cost more ($20 to $50) but let you monitor devices remotely and set schedules to turn devices on and off automatically.