How-To Estimate Appliance and Home Energy Use

by andrewodom on August 30, 2013 · 2 comments


Part of moving off-grid is having a realistic understanding of your personal and your homes energy consumption. If – like most of American households – you use appliances or home electronics chances are you are not fully aware of your electricity load. If you’re trying to decide whether to invest in a more energy-efficient appliance or you’d like to just determine your electricity loads, you will probably want to estimate your energy consumption. This will certainly move you in the right direction of achieving an off-grid lifestyle.

Formula For Estimating Energy Consumption

To estimate a specific appliance’s energy use use the following formula:

(Wattage × Hours Used Per Day) ÷ 1000 = Daily Kilowatt-hour (kWh) consumption

1 kilowatt (kW) = 1,000 Watts

Editor’s Note: Multiply this by the number of days you use the appliance during the year for the annual consumption in kWh per year.

Estimating Annual Cost to Run An Appliance

Multiply the annual consumption in kWh per year (that you calculated above) by your local utility’s rate per kWh consumed to calculate the annual cost to run an appliance. To estimate the number of hours that a refrigerator actually operates at its maximum wattage, divide the total time the refrigerator is plugged in by three. Refrigerators, although turned “on” all the time, actually cycle on and off as needed to maintain interior temperatures.

EXAMPLES:

Oscillating Fan:
(200 Watts × 4 hours/day × 120 days/year)  ÷  1000
= 96 kWh × 11 cents/kWh
= $10.56/year

Desktop Monitor with Monitor:
[(120 Watts + 150 Watts) × 4 hours/day × 365 days/year] ÷ 1000
= 394 kWh × 11 cents/kWh
= $43.34/year

Wattage

Most new appliances (post-1993) list the wattage on the bottom or back of the appliance, or on its nameplate. The wattage listed is the maximum power drawn by the appliance when in use. Since a number of appliances have a range of settings (for example the volume on computer speakers) the actual amount of power consumed depends on the setting used at any one time.

120_240If the wattage is not listed on the appliance, you can still estimate it by finding the current draw (in amperes) and multiplying that by the voltage used by the appliance. Most appliances in the United States use 120 volts. This is the same for tiny houses as well if you are hard wired or plugged into the grid. Larger appliances, such as clothes dryers and electric cooktops, use 240 volts. Most people are familiar with the visual difference in 120 and 240 plugs. The amperes might be stamped on the unit in place of the wattage. If not, find a clamp-on ammeter — an electrician’s tool that clamps around one of the two wires on the appliance — to measure the current flowing through it. Take a reading while the device is running; this is the actual amount of current being used at that instant.

When measuring the current drawn by a motor, note that the meter will show about three times more current in the first second that the motor starts than when it is running smoothly. Think about the sound of an AC unit coming on. When the compressor fires up it expends more energy than when it is running consistently.

Many appliances continue to draw a small amount of stand-by power when they are switched “off.” This is called a phantom load and occurs in most appliances that use electricity, such as VCRs, televisions, stereos, computers, and kitchen appliances. Most phantom loads will increase the appliance’s energy consumption a few watt-hours. These loads can be avoided by unplugging the appliance or using a power strip and using the switch on the power strip to cut all power to the appliance.

Typical Wattages of Various Appliances

Here are some examples of the range of nameplate wattages for various household appliances:

  • iHome Clock radio = 10
  • K-Cup coffee maker = 900–1200
  • Clothes washer = 350–500
  • Clothes dryer = 1800–5000
  • Dehumidifier = 785
  • Fans
    Ceiling = 65–175
    Window = 55–250
    Furnace = 750
    Whole house = 240–750
  • Hair dryer = 1200–1875
  • Heater (portable) = 750–1500
  • Clothes iron = 1000–1800
  • Microwave oven = 750–1100
  • Laptop computer = 50
  • Refrigerator (frost-free, 10 cubic feet) = 525
  • Televisions (color)
    • 19″ = 65–110
    • 27″ = 113
    • 36″ = 133
    • 53″ – 61″ Projection = 170
    • Flat screen = 120
  • Toaster oven = 1225
  • DVD player = 17–21 / 20–25
  • Vacuum cleaner = 1000–1440
  • Water heater (40 gallon) = 4500–5500

A Practical Example

Here is what Tiny r(E)volution runs in terms of energy consumption. It is just an average and shows how to put the math into practice.

10 cubic ft. fridge – 525
Ceiling Fan (23″) – 75
Breville SmartOven – 1800
Overhead Light – 5
iPad Charger – 10
iPhone Charger – 5
SleepNumber bed – 60
————————-

Total Watts – 2480

(Wattage × Hours Used Per Day) ÷ 1000 = Daily Kilowatt-hour (kWh) consumption

1955 (total watts without fridge) x 1  ÷ 1000 = 1.955

525 (total watts of fridge which runs for 24 hours) x 1 ÷ 1000 = .525

Tiny r(E)volution uses 2.48 Daily Kilowatt-hour consumption. Multiply that by our current rate of $0.10836 per kwh and you get .269 cent per hour. Multiply that by 24 hours in a day and you get $6.54 per day. Multiply that by a 30-day utility month cycle and you get $196.20.

You have to remember that you don’t use an a SleepNumber pump for a full hour. You may not even use it for days. You don’t keep an overhead light on for 24 consecutive hours, etc. The list goes on. Remember, this is an average and is used to determine maximum energy consumption.

Our most recent bill can be seen here. Notice that it is vastly different from the maximum wattage our math indicated.

So what does your house require? Do you actually use that much power? How close are you to getting off-grid? Do you want to move off-grid? 

  • Pavel Ben

    Nifty little article, Andrew. Energy usage is a hot topic. Here in Australia it is not only the whinge – it’s far more than a talk – about town, but the basis for much political point-scoring. On one side of politics we have a party (who are presently in government) who brought in a carbon tax (a good thing, in my view) which is reputed to have impacted electricity prices and on the other we have a party that behaves as though electricity should be plentiful and cheap – for anything less would be some kind of violation.

    Personally, I look forward to the rawness of being off-grid. Having to be responsible for my own energy production. Having to take serious the responsibility of monitoring my usage. At present I live in an urban flat (apartment) which has really poor insulation – and it is facing east/west to boot. We are very particular about switching off, and using few, appliances. But the heater / cooler has gotten a workout this last winter. And the big bertha of a hot water service is electric.

    My tiny house will be positioned to face north, to benefit from the winter sun. It will have a pot-belly stove for additional warmth. In summer I see myself resorting to using an air conditioner – I hate the heat – but I am pretty sure I will be able to get enough energy of the solar panels to get by.

    I have done an inventory of what will be plugged in at the tiny house and it is minimal:

    A few 12V lights – maybe 3.
    An Apple MagSafe charger.
    A Billion modem.
    Occasionally – hopefully – an energy-efficient air conditioner.
    And not a lot else…

    • http://www.tinyrevolution.us/ anotherkindofdrew

      Awesome Pavel. It is amazing to me how some governments are truly trying to understand and accommodate energy usage (while cutting back on fossil fuel use) while others stand in ignorance. Thank you for sharing with us.

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