Electricity & Energy - Units of Measure in the Home

When talking about electricity usage in a house, it's important to understand several key electricity units of measurement. These units help describe how much electricity you’re using, how fast you’re using it, and how that translates into your electric bill.

Here are the most important ones:

Watt (W)

• What it measures: Power — the rate at which electricity is used.
• Think of it as: The speed of electricity usage.
• Example: A 60-watt light bulb uses 60 watts of power when it's turned on.

Kilowatt (kW)

• What it is: 1 kilowatt = 1,000 watts.
• Why it's used: Appliances and home systems often use more than just a few watts; kilowatts make the numbers easier to understand.
• Example: A small window air conditioner might use about 1 kW (1,000 watts) of power.

Kilowatt-hour (kWh)

• What it measures: Energy — the total amount of electricity used over time.
• Think of it as: The amount of electricity used (like gallons of gas for your car).
• Why it matters: This is what your utility company charges you for.
• Example: If you run a 1,000-watt (1 kW) appliance for 1 hour, you use 1 kWh of electricity.

Volt (V)

• What it measures: Voltage — the pressure that pushes electric current through wires.
• Home standard: In the U.S., standard voltage for most household outlets is 120 volts, while large appliances like ovens or dryers use 240 volts.

Ampere or Amp (A)

• What it measures: Current — the flow of electricity.
• Think of it as: The volume of electricity moving through a wire.
• Example: A microwave might draw 10 amps at 120 volts.

A Note on “Power” vs “Energy”

It can be confusing sometimes between what is “power” and what is “energy” in electricity, because both have “kilowatt” in their measure. A simple analogy to Aesop’s fable of “The Tortoise and the Hare” can illustrate the differences in concept:

In the parable, the hare runs very fast (he has high power). He sprints ahead quickly — burning a lot of energy in a short time. But he takes breaks and naps, so doesn't keep moving the whole time. The tortoise moves slowly but steadily (he has low power). He never stops, using a little energy constantly. Over time, he ends up covering more ground.

Power = speed of energy use (how fast you're going)
The hare has more power, but doesn't use it continuously over time. The tortoise has less power, but uses it consistently over time.

Energy = total work done (how far you've gone)
The tortoise uses more total energy over the time to reach the finish line to win the race.

🔌 How It All Connects in Your Home:

Power (Watts) = Voltage (Volts) × Current (Amps)
Energy Used (kWh) = Power (kW) × Time (hours)

For a 1500-watt space heater plugged into a wall outlet:

• In the US the voltage of the wall outlet is 120V. So the amount of current drawn by the heater is:
  1500 W ÷ 120 Volts = 12.5 Amps
• If on for 2 hours, the amount of energy used by the heater is:
  1500 W ÷ 1000 = 1.5 kW
  1.5 kW × 2 hours = 3 kWh
• If your electricity rate is $0.15 per kWh, this costs:
  3 × $0.15 = $0.45

Extending the tortoise and hare analogy to common home appliances –

• An electric clothes dryer might have a power rating of 4,500 watts.
• A computer router might have a power rating of 45 watts.

The dryer requires 100X the power to run than the router!

But, the router is likely to run all the time – using 45w X 24 hours X 7 days / 1,000 watts/kWh = 7.6kWh in a week

Where the dryer might dry 2 loads of clothes in a week – using 4,500w X .75 hours per load X 2 loads / 1,000 watts/kWh = 6.8kWh

If your electricity rate is $0.15 per kWh, then:

• The router costs 7.6 x $0.15 x 4 weeks/mo = $4.56 per month
• The dryer costs 6.8 x $0.15 x 4 weeks/mo = $4.08 per month

The router (the tortoise) costs more to operate than the dryer (the hare) because it uses more energy, even at 1/100th of the power!

TL;DR — Household Electricity Units Cheat Sheet:

There are other types of energy used in the house, that we can compare with electricity, natural gas (methane) and gasoline being the most common.

Natural Gas units of measure – here are the most important ones:

British Thermal Unit (BTU)

• What it is: Energy — the total amount of gas energy used over time.
• Think of it as: The amount of energy used (heat created) burning gas.
• Example: A single burner on a gas stove might use 8,000 BTU’s per hour of energy.

Therm

• What it measures: Energy – 1 Therm = 100,000 BTU’s of energy.
• Why it's used: Appliances and home systems often use 10’s of thousands of BTUs; Therms make the numbers easier to understand.
• Why it matters: This is what your utility company charges you for.
• Example: An entire home might use 3,500,000 BTU’s of gas energy in a month, which would be expressed as 35 Therms of usage on the bill.

Cubic Foot (ft³)

• What it measures: Volume — the total amount of natural gas used over time. Many gas meters measure in increments of 100 Cubic Feet (CCF).
• Think of it as: The amount of natural (methane) gas used (like gallons of gasoline for your car).
• Why it matters: This is how your utility company measures how much natural gas you use.

Therm Conversion Factor (Therms per 100 Cubic Feet)

• What it measures: Energy per Volume — the amount of energy in 100 cubic feet of gas. Also referred to as ‘Multiplier’, ‘Heating Value’, ‘Gas Energy Conversion Rate’ or similar. This value usually varies from 0.95 to 1.03 depending on the energy content of the gas the utility is providing.
• Why it matters: Your utility company charges you for the energy in the gas, not just the gas itself.
• Example: If your gas meter measures 1900 cubic feet of gas consumed, and your utility has a Therm Conversion Factor of 1.02, then the number of Therms you would be charged for is 1900 ft³ X 1.02 / 100 = 19.4 Therms.

Gasoline Units of measure – here are the most important ones:

Is gasoline really an important measure for your home? Yes, if you are considering getting or have an electric car and want to know how to compare costs and energy consumption to a gasoline one.

Gallon

• What it is: Volume — the total amount of gasoline, used over time.
• Why it's used: While a gallon of gasoline measures volume, it is used interchangeably to represent a standard unit of energy.
• Think of it as: The amount of gasoline usage to create energy; can be compared to kWh of electricity or cubic feet of natural (methane) gas.

Miles per Gallon

• What it measures: Efficiency – the distance a car can go on one gallon of gasoline.
• Why it's used: It’s a simple metric to compare the cost and energy efficiency of different vehicles.

We’ve discussed different forms of energy in the home. Can all these different energy measures and units be compared and converted between them? You bet! Here’s a comparison table with conversion factors for all of the above metrics:

Converting Electric Energy to other fuels

Converting Natural Gas Energy to other fuels

Converting Gasoline Energy to other fuels