Appliance Electricity Cost Math: Watts, kWh, Duty Cycles, and Your Monthly Bill

The advice most people get about appliance energy use is vague: "your old fridge costs you money" or "unplug your chargers." That is not enough to make a decision. A kettle may draw 2,000 watts - more than a refrigerator - yet cost far less per month, because the kettle runs for minutes and the refrigerator runs all day.

To estimate what any appliance actually costs you, you need four numbers:

1. Its power draw, in watts.
2. How many hours it runs per month.
3. Its duty cycle - what fraction of that time it is actually drawing power.
4. Your electricity rate, in dollars (or your local currency) per kilowatt-hour (kWh).

This guide shows how to find each number, work the math, and build a monthly cost worksheet for your whole home. Every rate and price in the examples below is hypothetical and labeled as such - swap in your own numbers from your bill and your appliance labels.

The Core Formula

Electricity bills charge for energy, not power. Power (watts) is the rate at which an appliance uses energy at any moment. Energy (watt-hours or kilowatt-hours) is power sustained over time.

Monthly cost = (watts / 1,000) x hours used per month x your rate per kWh

Dividing watts by 1,000 converts to kilowatts. Multiplying by hours gives kilowatt-hours. Multiplying by the rate gives cost.

A quick example. Suppose you have a 1,500-watt space heater that runs 4 hours per day, and your rate is $0.17/kWh (hypothetical):

The heater feels powerful, but whether $30.60 a month matters depends on how it compares with everything else in the house - which is what the worksheet at the end is for.

Finding Your Real Rate: Read the Bill, Not the Headline

Many people grab the "energy charge" printed on their bill - say $0.09/kWh - and plug it into the formula. That often understates the true cost of running one more appliance, because bills also carry delivery charges, per-kWh riders, and taxes that scale with usage.

For appliance decisions, use an approximate marginal rate: the extra cost of one more kWh.

Marginal rate = (variable charges on your bill) / (kWh used)

Look at a recent bill and:

• Add up the charges that change with usage: energy charges, per-kWh delivery charges, per-kWh riders, and taxes on those amounts.
• Leave out fixed monthly fees (the "customer charge" or "service charge"), because you pay those whether the appliance runs or not.
• Divide the variable total by the kWh shown on the bill.

Example with a hypothetical bill: variable charges total $132 for 800 kWh. Marginal rate = $132 / 800 = $0.165/kWh, roughly $0.17/kWh.

If you are on time-of-use pricing, do this separately for the rate periods when the appliance actually runs, or use a weighted average. A dryer run at off-peak rates costs less per kWh than the same dryer run at on-peak rates, and the formula should reflect that.

Why Duty Cycle Matters

Many appliances do not draw their rated power continuously. A refrigerator compressor, dehumidifier, freezer, or electric water heater cycles on and off to hold a temperature. A gaming PC's power supply rating is a maximum, not what the machine draws while you browse the web.

The duty cycle is the fraction of time the device actually draws its rated power:

Average watts = rated watts x duty cycle

Then use average watts - not nameplate watts - in the cost formula.

Example: a dehumidifier draws 520 watts while running, with an estimated 60% duty cycle in a damp basement.

Average watts = 520 x 0.60 = 312 watts = 0.312 kW.

Using Energy-Guide Labels

If an appliance carries an energy-guide label with an estimated annual kWh figure, start there - the estimate already bakes in assumptions about typical use, so you can skip the duty-cycle guessing.

Annual cost = annual kWh x your rate

Monthly average = annual cost / 12

Example with hypothetical numbers:

Your actual result will differ with temperature setpoints, household size, weather, and maintenance - the label assumes a standardized test, not your kitchen. Treat it as a starting point for comparisons between models, which is what the label is designed for.

Phantom Loads: The Quiet 24/7 Draw

Phantom load (also called standby power) is the electricity a device uses while "off" or idle: a TV waiting for the remote, a cable box downloading guide data, a game console in instant-on mode, a phone charger left plugged in.

One device drawing 5 watts sounds trivial. But it runs 24 hours a day, 365 days a year:

5 watts x 24 hours x 365 days = 43,800 watt-hours = 43.8 kWh per year. At a hypothetical $0.17/kWh, that is about $7.45 per year - for one small device doing nothing.

To estimate any phantom load's monthly cost:

Monthly cost = (standby watts / 1,000) x 720 hours x rate

(720 = 24 hours x 30 days.)

A phone charger left plugged in might draw 0.5 to 1 watt doing nothing - roughly $0.12 per month, not worth chasing. A home-theater setup with several devices idling at 5 to 10 watts each is where phantom loads add up. The honest way to find standby draw is a plug-in power meter (a one-time purchase, usually inexpensive), because standby watts are rarely printed on the box.

Worked Examples: Four Common Appliances

All examples below use a hypothetical rate of $0.17/kWh and hypothetical device specs. Replace them with your own.

1. Refrigerator

Suppose the compressor is rated 150 watts and runs about 35% of the time.

About $6.43 per month, or roughly $77 per year. Note how the duty cycle cut the naive estimate (150 W x 720 h = 108 kWh, or $18.36) by almost two-thirds. This is the single most common error in home energy math.

2. Electric Clothes Dryer

Suppose a 3,000-watt dryer runs about 35 hours per month (roughly 8 loads a week at an hour each).

Dryers have no meaningful duty cycle - the heating element is on nearly the whole cycle - so high wattage times real hours gives a real number. This is why dryers surprise people: nothing else in a typical home combines this much power with this many hours except heating equipment.

3. Space Heater

Suppose a 1,500-watt heater runs 6 hours per day during cold weather.

At $45.90 per month, a single space heater can cost more than every other appliance in this list combined. A thermostat helps, but during genuinely cold weather the heater may run near full power most of the time it is switched on.

4. TV and Streaming Box (Including Standby)

Suppose a TV draws 80 watts while watched 4 hours per day, and the TV plus streaming box draw 6 watts combined in standby the rest of the time.

Active use: 0.08 kW x (4 x 30 = 120 h) = 9.6 kWh, costing 9.6 x $0.17 = $1.63. Standby: 0.006 kW x 720 h = 4.32 kWh, costing 4.32 x $0.17 = $0.73. Total: about $2.36 per month.

The standby portion is nearly a third of this setup's cost - a case where a switched power strip pays for itself in a few months and then keeps saving.

What to Check First: The Priority Table

Rank appliances by watts x hours, not by watts alone:

Short-runtime devices (kettle, microwave, hair dryer) usually matter less than people expect. A 1,800-watt hair dryer used 10 minutes a day uses 0.3 kWh per day - about 9 kWh per month, or roughly $1.53 at $0.17/kWh.

Your Monthly Cost Worksheet

Copy this table and fill in one row per appliance. Take rated watts from the label (usually on the back or bottom of the device), your best estimate of hours, and the marginal rate from your bill. Two example rows are filled in with the hypothetical numbers used above.

Formulas for each row:

• Avg watts = rated watts x duty cycle
• kWh/month = (avg watts / 1,000) x hours/day x 30
• Cost/month = kWh/month x your marginal rate

Add up the Cost/month column. Then compare the total with the variable portion of your bill. If the two are wildly different, your hours or duty-cycle estimates need adjusting - or something big (usually water heating) is missing from the table.

FAQ

Is watts the same as watt-hours?

No. Watts measure power - the rate of energy use at a moment in time. Watt-hours measure energy - power sustained over time. A 100-watt bulb running for 10 hours uses 1,000 watt-hours, or 1 kWh.

Where exactly do I find my rate on the bill?

Look for the total of the charges that scale with kWh (energy, per-kWh delivery, riders, taxes on those) and divide by the kWh used. Ignore the fixed monthly customer charge. If your bill shows time-of-use periods, compute the rate for the period when the appliance actually runs.

Should I buy a plug-in power meter?

If you are auditing a whole home or deciding whether to replace an appliance, yes - a plug-in meter measures real watts and real kWh over days or weeks, which beats guessing duty cycles. For one-off estimates, the formulas in this guide are enough.

Do switched power strips actually save money?

They eliminate standby draw for everything plugged into them. Whether that matters depends on the standby watts: 6 watts of media standby costs about $0.73/month at $0.17/kWh - a switched strip pays for itself in a few months and then keeps saving. A single phone charger idling at 1 watt is about $0.12/month; a strip is overkill there.

Why is my bill still high after I replaced one appliance?

Because the bill is the sum of everything: heating, cooling, water heating, cooking, lighting, plug loads, fixed charges, and rate structure. Replacing one refrigerator typically moves the total by a few dollars a month. The worksheet above shows where the big numbers actually live.

Does unplugging small chargers matter?

Barely, in dollar terms. A charger idling at 1 watt costs roughly $0.12 per month at $0.17/kWh. Unplug them if you like the habit, but put your effort into the top rows of the priority table.

What to Calculate Next

For household planning, pair this method with the utility offset math guide and the heat pump ROI calculator guide.