How do I calculate how much electricity things in my house use?

Question

Can anyone help? I know you have to know the volts, watts, and all that. I would like to figure out how much it costs to run certain appliances. Thanks.

Thanks for all the great answers, they definitly clear things up.

Answer 1

The two pieces of data you need are the watts and how long the device is operated.

You can generally find the wattage on the appliance nameplate, however some devices may only show volts and amps. A reasonable approximation is to multiply the volts x amps to get watts. (The concept of power factor is assumed to be beyond the scope of this question and answer)
Take the watts and divide by 1000 to convert the value to kilowatts (kW).

Once you know the device's wattage, you have to estimate how long the device will be operated. The electric utility generally measures consumption in kilowatt-hours (kWh). A 1 kW appliance operated for one hour consumes 1 kWh. If the device ran for an entire day it would consume 24 kWh. A one hundred watt light bulb (0.1 kW) operated for six hours would consume 0.1 kW x 6 hrs = 0.6 kWh.

For the purposes of this answer I will assume an energy cost of 12 cents per kWh. ($.12 / kWh)

There are many devices in your home that have a large power draw, but only run for a short period of time. For example, a microwave oven might draw 1000 watts (1 kW), but a bag of microwave popcorn only takes about 3 minutes. Three minutes is 3/60 = 1/20 of an hour.

1 kW * 3 minutes * 1 hour / 60 minutes = .05 kWh.

.05 kWh * $.12 / kWh = $.006 per bag of popcorn. (6/10th of one cent).

Other short duration appliances in a home include: hair dryers, garage door openers, garbage disposals, and toasters.

Other appliances might run continuously, or cycle on and off throughout the day. Your refrigerator's compressor will cycle on and off as called for by its thermostat. Your computer and monitor may power down as part of the energy saving features that began appearing several years ago.

The most expensive device to operate in my home is the central air conditioner. My 4-ton AC unit consumes about 4 kW while operating. On the hottest days in summer, the AC might run for a total of 8 hours. The cost to cool my house for that one day could be calculated as 4 kW * 8 hours * $.12 / kWh = $3.84.

Even when some devices are turned off they are consuming a small amount of electricity. Every device that uses a remote control must be constantly "watching" for the signal to turn on. Starting in the late 90's manufacturers began trimming down the phantom loads that were occurring when the device was turned off. Today many consumer appliances have this power draw down to less than 5 watts.

Perhaps the best "tool" to get a handle on energy usage around the house is the Kill-A-Watt device. It is easy to use, and provides volts, amps, watts, and if you leave it plugged in, will begin to rack up kWh. The LCD display allows you to easily monitor the usage. I have one and used it to check the actual load of various devices around the house. A friend used it to monitor the old refrigerator in his basement. He left it on for a week to see what that cold soda and beer was costing him.

You should be able to pick one up a Kill-A-Watt for less than $30. At one time I saw them at Radio Shack.

It is important to recognize that most all appliances have nameplates that overstate the actual power consumption. Devices with a motor generally rate the "locked rotor" current that is far higher than normal operation. Most motors also never reach their rated output. The work they do at the shaft is often far lower than their rating. Another example is desktop PC's that might have a 500-watt power supply, but if you perform measurements you'll find that it is drawing less than 150 watts.

Architects and electricians add up the nameplates of devices and size the electric service panel accordingly. They are not allowed to take into account the intermittent usage of the devices, nor the overstated nameplates. In reality the actual load seen at the electric meter rarely even reaches half the original predicted value.

Answer 2

The easiest way is to look at the nameplates on the appliances. They will tell the voltage and current (volts times amps = watts). The problem is to guess how long they are on for every day. The electric bill is based on (volts x amps x hours)/1000 since you are billed for kilowatt-hours.

There are three tough problems involving big consumers of electricity. Your stove doesn't use all of its rated power all the time. If the oven is on that adds a lot and if a top burner is at a middle setting it may be halfway between zero and its max. That makes the tracking of power and time harder. The refrigerator is about the biggest power user. It runs its max power all the time it is on but a lot of that on-time is at night when you are asleep not keeping track of it. The lights are also a factor to worry about. If you look at the lamps you can read their power ratings but again how many are on at a time?

Best way is to do some estimates for the unknown ones. Try to see how often the fridge runs in a typical hour and multiply that by 24 hrs/day. Look to see which burners are used most often and take a guess for their total time. For lamps see which are biggest and on the most and scale up from there. At the end of the month compare your calculations to your electric bill to see if you are close to the billed total.

Answer 3

Most of your house hold appliances have a wattage rating listed which you can use to calculate kilowatt-hour which is 1000 watts-hours. KW-H is the unit used by most power companies to determine your energy bill. The equation is simple, watt (wattage rating) X time (hours) /1000. This will give you KW-H or kilowatt-hours.

For example, if your 100 watt light bulb was on for 10 hours then it would have used 1000 watt-hours or 1 KW-H. If the power company charged $2 for KW-H then your bill for the light bulb would be $2 (1 KW-H X $2/ KW-H).

More complex information on kilowatts below:

When a constant power exists for a time, energy is transferred. Energy is the product of power and time, in the case of constant power; if power varies, the energy transferred is calculated from the integration of power with respect to time. When paired with a unit of time the term watt is used for expressing energy consumption. For example, a kilowatt hour, is the amount of energy expended by a one kilowatt device over the course of one hour; it equals 3.6 megajoules. A megawatt day (MWd or MW·d) is equal to 86.4 GJ. These units are often used in the context of power plants and home energy bills.

Answer 4

We were billed in our electricity consumption by the wattage we use. Kilowatts (1000 watts) per Hour.

A 1000watts rice cooker will consume 1kwh in 1 hr
A 500watts washing machine consumes 0.05Kwh in an hr.

Answer 5

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Starting with the UVB lamp, the consumption for 12 hours is 26W x 12h = 312 watt-hr = 0.312 kWh Cost then is: 0.312 kWh x $0.117 /kWh = $0.0365 You multiplied by 12 twice. If this is typical daily use then it would add $0.0365 /day x 30.5 days/mo = $1.113 per month I have actually measured current in quite a few desktop computers, and even in ones with large power supplies, I have found most drawing about 1.8 to 2.2 amps at 120 volts, or an average of 240 watts. But, assuming yours is unusually power hungry, if you assume 850 watts and it is on continuously, monthly bill would be 0.85 kW x 24 h/day x 30.5 day/mo x $0.117 / kWh = $72.80 / mo. If you had it on an average of 4 h per day as you have stated, monthly bill woud be 0.85 kW x 4 h/day x 30.5 day/mo x $0.117 / kWh = $12.13 / mo.

Answer 6

Rice Cooker Watts

Answer 7

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RE:
How do I calculate how much electricity things in my house use?
Can anyone help? I know you have to know the volts, watts, and all that. I would like to figure out how much it costs to run certain appliances. Thanks.

Answer 8

Somewhere on the appliance there is a "power rating." It is listed in WATTS. To determine how many amperes of electricity they use, divide the number of watts on the power rating by 120.

Answer 9

u can see a label on the appliance for example like :- 220V 300W...etc
300W is the power it uses. multiply it with the time(in hrs) u use it for and divide by 1000, it gives u units consumed. multiply this value by the price per unit.
eg:- xW * t(hrs) * p(price per unit) /1000