Electric Energy Equation - Formulas and Calculations

Quick answer

The electric energy equation is E = P x t: energy equals power multiplied by time. Power is how fast a device uses energy, measured in watts. Time is how long the device runs. Multiply the two and you get the energy used, which is what your electricity bill charges for in kilowatt-hours.

Key facts:

One kilowatt-hour is 1,000 watts running for one hour, which equals 3.6 million joules.
The average US home pays about 17 cents per kilowatt-hour, according to the U.S. Energy Information Administration.
A 100-watt bulb left on for 10 hours uses exactly 1 kWh, about 17 cents of electricity.

Using this to understand a real bill? The home energy cost guides apply this equation to average bills, savings, and solar payback.

The Basic Equation

E = P x t

E is energy. P is power. t is time.

Power is the rate of energy use. Think of speed. Time is how long you run. Think of hours. Energy is the total. Think of distance.

If a 100 watt bulb runs for 10 hours, the energy is:

100 W x 10 h = 1000 Wh = 1 kWh

For scientific work, use seconds for time and joules for energy.

100 W x 36,000 s = 3,600,000 J

That is the same energy. Just different units.

For Younger Learners (Ages 7-10)

Think about two flashlights. One has a 60 watt bulb. The other has a 100 watt bulb. Which uses more energy? It depends.

If you run the 60 watt one for 5 hours, it uses 300 watt-hours. If you run the 100 watt one for 1 hour, it uses 100 watt-hours. The 60 watt one used more energy. It ran longer.

The equation is like a seesaw. More power OR more time means more energy. Less power OR less time means less energy.

Try this at home. Look at a light bulb package. It says something like “60W.” That is the power. Now think about how long you leave that light on. The longer it is on, the more energy it uses.

For Older Learners (Ages 11-14)

Let us explore all the forms of the equation.

From power and time: E = P x t

From voltage and current: P = V x I, so E = V x I x t

From current and resistance: P = I^2 x R, so E = I^2 x R x t

From voltage and resistance: P = V^2 / R, so E = V^2 x t / R

Each form is useful in different situations. If you know voltage and current, use the second form. If you know resistance, use the third or fourth.

Here is an example. A toaster has a resistance of 10 ohms. It runs on 120 V. How much energy does it use in 5 minutes?

First find power. P = V^2 / R = 120^2 / 10 = 1440 W

Then find energy. 5 minutes = 300 seconds.

E = 1440 x 300 = 432,000 J = 0.12 kWh

Cost Calculations

Your electric bill uses kilowatt-hours. Here is how to calculate cost.

Cost = Energy (kWh) x Rate ($ per kWh)

A 1500 W space heater runs for 8 hours. Rate is $0.12 per kWh.

Energy = 1.5 kW x 8 h = 12 kWh

Cost = 12 x $0.12 = $1.44 per day

For a month, that is $43.20. That is why space heaters can make your bill go up fast.

Common Mistakes

Wrong units. Using watts with hours and expecting joules does not work. Match your units. Use kW and hours for kWh. Use W and seconds for joules.

Forgetting to convert. A 100 W bulb is 0.1 kW. If you forget to divide by 1000, your answers will be off by a factor of 1000.

Confusing power and energy. “My microwave is 1200 W.” That is power. It uses 1200 joules every second. If you run it for 1 hour, the energy is 1.2 kWh.

Fun Facts

The formula E = P x t works for more than electricity. It works for all forms of energy. A 100 W light bulb and a 100 W human both use the same energy in the same time.
The first electric meters were mechanical. They had a spinning disk. The faster it spun, the more power you used. The total spins measured your energy.
A lightning bolt has about 5 billion joules of energy. Using E = P x t, that is enough to run a 1000 W microwave for 5 million seconds. That is about 58 days.
Your phone battery holds about 10 Wh of energy. That is 0.01 kWh. Charging it costs less than a penny.
A data center can use 100 million kWh per year. That is enough to power 10,000 homes.

References

Last updated: July 06, 2026

Quiz: Test What You Know

1. What does E stand for in E = P x t?

2. A 100 W bulb runs for 10 hours. How much energy in kWh?

3. What is the power equation using voltage and current?

4. A device uses 1.5 kW for 4 hours. What is the cost at $0.12/kWh?

5. How many joules are in 1 kWh?

Frequently Asked Questions

What is the basic electric energy equation?

The basic equation is E = P x t. Energy equals power multiplied by time. Power is how fast energy is used. Time is how long it runs. The total energy is what you pay for on your bill.

What units do you use in the electric energy equation?

For scientific work, use joules (energy), watts (power), and seconds (time). For home bills, use kilowatt-hours (energy), kilowatts (power), and hours (time). Always keep the units matched.

How do you calculate the cost of electric energy?

First find energy use in kWh. Multiply power in kW by time in hours. Then multiply by your rate per kWh. Cost = (kW x hours) x rate. If your rate is 12 cents, that is $0.12 per kWh.

What is the relationship between voltage, current, and power?

Power equals voltage times current. P = V x I. This is one of the most important equations in electricity. It connects the push (voltage) with the flow (current) to give the rate of energy use.

Can you use the equation for any electrical device?

Yes. The same equation works for everything. A light bulb, a motor, a phone charger. As long as you know power and time, you can find energy. Or if you know two of voltage, current, and resistance, you can find power first.