How to Estimate an Appliance's Operating Cost by Hand

Why this matters

"What does that thing cost to run?" is a question techs get on every kind of call, and the answer usually comes out of someone's head as a number that is off by a factor of two or three. The customer then checks it against the bill, sees it cannot be true, and stops trusting anything else the shop says about energy. The arithmetic is simple enough to do on the back of a work order. What takes care is the two inputs people guess at: the watts and the hours.

This is the hand method, from a rating label and a reasoned estimate of run time. It gives a range, not a number, and the range is the honest answer. When the range is too wide to be useful, the next step is measuring, which a sibling covers.

The one formula

Every electrical operating-cost estimate is the same line:

kWh = watts x run hours per day x days / 1000

cost = kWh x the rate per kWh from the bill

The 1000 converts watt-hours to kilowatt-hours (kWh), which is what the meter counts and the bill charges for. See related: kW Versus kWh: The Difference That Confuses Every Bill Call.

Two things make or break the answer: which watts, and which hours.

Step 1: Find the watts on the rating label

Look for the rating label (also called the nameplate or data plate): on the back or side of the appliance, inside a door, or on the compressor housing. It lists volts and either watts or amps, sometimes both.

• If it lists watts (or "rated input"), use that. It is the maximum draw at the test condition the manufacturer rated it at. Real running draw is often lower, rarely much higher.
• If it lists only amps, go to step 4. Amps x volts gives a ceiling, not an estimate.
• If it lists a heating element and a motor separately (a dryer, a dishwasher), note both. They run at different times.

Skip this and the common substitute is the breaker size. A 20-amp breaker tells you what the circuit can carry, not what the appliance draws, and it overstates almost every load.

Step 2: Work out run hours, not clock hours

This is where most estimates fail. Equipment that cycles (a compressor, a pump, a heater) runs until a control is satisfied, then waits. The fraction of time it actually runs is the duty cycle. A dehumidifier plugged in for 24 hours that runs its compressor 60 percent of the time has 14.4 run hours, not 24. See related: Duty Cycle: Why Run Hours, Not Clock Hours, Set the Cost.

Ways to estimate run hours, best first:

1. The equipment tells you. A thermostat with runtime history, a pool pump timer, an hour meter.
2. Watch it. Time one full on period and one full off period, then a second pair; divide on-time by total time. One pair is not enough, because the first cycle after you arrive is often not typical.
3. The customer tells you. "It runs all evening" is weak evidence, but "the bucket fills twice a day" on a dehumidifier is good evidence.
4. A reasoned range. When none of the above is available, give a low and a high duty cycle and say why.

Skip this and you get the 24-hour estimate, which reliably comes out two to three times the real figure on cycling equipment.

Step 3: Use the right rate

The rate is not the bill total divided by the kWh. That number includes the fixed monthly customer charge, which does not change when one appliance runs more or less. Add up every line on the bill that is charged per kWh (supply or generation, delivery or distribution, and any per-kWh riders), and use that sum. On a tiered or seasonal rate, use the rate of the block the extra kWh land in, which is usually the top block that month. See related: Tiered and Seasonal Rates: Why the Last Kilowatt-Hour Costs More.

Write it as R, the per-kWh total from this customer's bill. A rate from memory or from another customer's bill is wrong for this one.

Step 4: When only amps are known

Watts = volts x amps is exact for a resistance load, like a heating element or an incandescent lamp. For anything with a motor or a compressor, the real watts are lower than volts x amps because of power factor: the current and voltage are not perfectly in step, and some of the current does no work. Volts x amps is then a ceiling, called volt-amperes (VA). See related: How to Measure Real Watts Instead of Trusting the Nameplate, which covers why and how to get the true figure.

A ceiling is still useful, in one direction: it can rule a load OUT. If a load running nonstop at its label amps could not produce the excess on the bill, it is not the cause. It cannot rule a load IN.

For a 240-volt load (a well pump, a water heater, a dryer), use 240 V, not 120. Using 120 halves the answer.

Step 5: State the answer as a range

Carry the low and high duty cycles all the way through, and give the customer both ends. "Between about 250 and 400 kWh in July" is believable and checkable. "About 320 kWh" sounds better and is less honest, because nothing you have measured supports the third digit.

Worked example: a new basement dehumidifier

The July bill is up, and per day it runs 13 kWh above last July, both bills on actual reads, 30 days each. That is an excess of 13 x 30 = 390 kWh. The one change in the house is a dehumidifier the customer put in the basement in June. Its label reads 115 V and a rated input of 690 W. No runtime record. The customer says it runs "most of the time" and they empty the bucket every morning.

Run hours: in a damp summer basement, a dehumidifier sized for the space typically runs a large share of the day. Without a record, take 50 percent as the low end and 80 percent as the high end.

DEHUMIDIFIER, BASEMENT, JULY, 30 DAYS
WATTS FROM LABEL (RATED INPUT)        690 W
RUN HOURS PER DAY, LOW (50 PCT)       12.0 H
RUN HOURS PER DAY, HIGH (80 PCT)      19.2 H

LOW:  690 W x 12.0 H x 30 / 1000 = 248 KWH
HIGH: 690 W x 19.2 H x 30 / 1000 = 397 KWH

EXCESS ON BILL                        390 KWH
SHARE EXPLAINED, LOW: 248 / 390     = 64 PCT
SHARE EXPLAINED, HIGH: 397 / 390    = 102 PCT

COST, JULY: 248 x R TO 397 x R
(R = PER-KWH LINES SUMMED FROM THIS BILL)

Check it the other way: 397 kWh over 30 days is 13.2 kWh a day, and 13.2 kWh divided by 0.69 kW is 19.2 hours, the high run time. The low end: 248 / 30 = 8.27 kWh a day, divided by 0.69 kW is 12.0 hours. Both close.

What the range says: the dehumidifier alone explains between 64 and 102 percent of the July excess. It is very likely most of the answer, and it may be all of it. The tech tells the customer exactly that, and that the next July bill against this one, or a plug-in watt meter for a week, would narrow it.

The garage refrigerator. The customer also asks about an old refrigerator in the garage. The label shows 115 V, 6.5 A and no watts. Volts x amps = 747.5 VA. Running nonstop at that figure for 30 days would be 747.5 x 24 x 30 / 1000 = 538 kWh, a ceiling well above any real refrigerator, partly because label amps on a refrigerator often include the defrost heater, which runs only briefly. Here the ceiling is above the 390 kWh excess, so it does not rule the refrigerator out on its own. What rules it out is that it was in the garage last July too: a load that has not changed cannot explain a change, unless it is failing and running longer. If the customer reports it running constantly or the freezer icing up, measure it. Otherwise leave it.

A resistance load, where the label is the answer. The same customer runs a portable electric heater in a cold back bedroom from 9 p.m. to 7 a.m. through January, label 1,500 W. A heating element has no power factor problem, so 1,500 W is the real draw whenever the element is on. The heater's own thermostat cycles it; on a cold night in a poorly insulated room, say it is on 40 percent of the 10 hours, or 4 hours. That is 1,500 x 4 / 1000 = 6.0 kWh a night, and 6.0 x 31 = 186 kWh for January, 186 x R. If the room never warms and the element runs the full 10 hours, it is 15.0 kWh a night and 465 kWh for the month, 2.5 times as much, which is why "it runs all night" is worth asking about precisely.

What changes the method

• Variable-speed equipment. An inverter compressor or a variable-speed pump does not cycle on and off at one draw; it runs longer at lower watts. Duty cycle from step 2 means little. Use measured average watts over a day, or the manufacturer's rated annual energy where one is published.
• Loads with a standby draw. A device that draws a few watts continuously when "off" adds watts x 24 hours to the estimate. Small per device, and it is worth chasing only in total. See related: Standby and Phantom Loads: What Is Worth Chasing.
• Season. A duty cycle measured in mild weather understates the same equipment in the worst month, and the reverse. Estimate for the month on the bill, not the day of the visit.
• Gas, propane and oil appliances. Same idea, different units: input BTU per hour instead of watts. See related: How to Estimate a Gas Appliance's Use From Its Input Rating.

How you know you got it right

The estimate should be compared against something independent before it goes to the customer as a finding: the bill excess, the change against last year, or a measured week. If the range lands well below the excess, the appliance is part of the answer at most, and the note says so. If the range lands far above the whole bill, one of the inputs is wrong: usually 24 hours used as run hours, 240 V used on a 120 V load, or watts read off the wrong line of the label.

References

• See related: Duty Cycle: Why Run Hours, Not Clock Hours, Set the Cost
• See related: Reading Customer's Electric Bill Reference
• See related: Anatomy of a Residential Electric Bill, Line by Line
• See related: How to Diagnose a High-Bill Call, in Order