Electric Energy Formula - How to Calculate Electrical Energy

Quick answer

The two formulas you need are E = P x t (energy equals power times time) and P = V x I (power equals voltage times current). Chain them together and you can work out the energy use and running cost of any electrical device from its label.

Key facts:

Utilities bill in kilowatt-hours: power in kilowatts multiplied by hours of use.
One kilowatt-hour equals 3.6 million joules.
A device drawing 5 amps from a 230-volt outlet uses 1,150 watts, or 1.15 kWh every hour.

The Core Formula

E = P x t

Let us break it down.

E is energy. We measure it in joules (J) or kilowatt-hours (kWh).

P is power. We measure it in watts (W) or kilowatts (kW).

t is time. We measure it in seconds (s) or hours (h).

The units must match. Use W and s to get J. Use kW and h to get kWh.

Power comes in several forms. Each is useful in different situations.

P = V x I. Power equals voltage times current. This is the most common power formula.

P = I^2 x R. Power equals current squared times resistance. Use this when you know resistance.

P = V^2 / R. Power equals voltage squared divided by resistance. Use this when you know voltage and resistance.

Combine each with E = P x t to get the full energy formula.

E = V x I x t

E = I^2 x R x t

E = V^2 x t / R

For Younger Learners (Ages 7-10)

Here is the formula in plain words. A bigger number on the device plus more time equals more energy.

Look at a hair dryer. It might say 1500 W. That is a big number. It uses a lot of power. Now look at a night light. It might say 5 W. That is a small number. It uses very little power.

But time matters too. A 5 W night light left on all year uses more energy than a 1500 W hair dryer used for 5 minutes.

The formula shows you this. Power x time = energy. It is a balance.

For Older Learners (Ages 11-14)

Let us work through real examples using the formula.

Example 1: LED vs incandescent.

An LED bulb uses 9 W. An incandescent uses 60 W. Both give the same light. Run each for 6 hours per day for a year.

LED: 0.009 kW x 6 h x 365 = 19.7 kWh

Incandescent: 0.060 kW x 6 h x 365 = 131.4 kWh

LED saves 111.7 kWh per year. At $0.12 per kWh, that is $13.40 saved. For one bulb.

Example 2: Finding current.

A 1500 W space heater runs on 120 V. What current does it draw?

P = V x I

1500 = 120 x I

I = 1500 / 120 = 12.5 A

A standard circuit can handle 15 A. This heater uses most of that capacity.

Example 3: Phone charging.

A phone charger outputs 5 V at 2 A. What is the power?

P = 5 x 2 = 10 W

If you charge for 2 hours, what is the energy?

E = 0.01 kW x 2 h = 0.02 kWh

At $0.12 per kWh, that costs $0.0024. Less than a penny.

Common Mistakes and Tips

Tip 1: Always convert units. Divide watts by 1000 to get kilowatts. Divide seconds by 3600 to get hours.

Tip 2: Check your answer. A typical home uses about 900 kWh per month. If your calculation says your phone charger uses 500 kWh per month, something is wrong.

Tip 3: Remember the difference. Power is how fast. Energy is how much. Power x time = energy.

Fun Facts

The formula P = I^2 x R explains why power lines use high voltage. Higher voltage means lower current for the same power. Lower current means less loss from I^2 x R heating.
Thomas Edison used the first electric meter to measure energy. It was a chemical meter. The current caused a chemical reaction that showed how much energy was used.
The most efficient LED bulbs turn about 80 percent of input energy into light. The rest becomes heat. Applying the formula shows that a 9 W LED does the same work as a 60 W incandescent.
A microwave oven uses about 1000 W. Running it for 6 minutes uses 0.1 kWh. That costs about 1.2 cents.
If you left a 100 W bulb on for an entire year, it would use 876 kWh. That costs about $105. An LED giving the same light would use 131 kWh. That costs about $16.

References

Last updated: July 06, 2026

Quiz: Test What You Know

1. What is the formula for electric energy?

2. A 2 kW heater runs for 3 hours. What is the energy in kWh?

3. What is P = V x I used for?

4. What happens to power when you double the voltage across a resistor?

5. A 60 W bulb runs for 5 hours. Cost at $0.10/kWh?

Frequently Asked Questions

What is the main electric energy formula?

The main formula is E = P x t. Energy equals power multiplied by time. This works for any electrical device. Know the power and the running time, and you can find the total energy used.

How do you calculate power from voltage and current?

Power equals voltage times current. P = V x I. This is the basic power formula. If a device runs on 120 V and draws 2 A, its power is 120 x 2 = 240 W.

How do you figure out the cost to run a device?

Use the formula Cost = (P x t) x rate. First convert power to kilowatts. Use time in hours. Multiply to get kWh. Then multiply by your rate per kWh. A 1500 W heater for 3 hours at $0.12 per kWh costs 1.5 x 3 x 0.12 = $0.54.

What is the formula involving resistance?

There are two. P = I^2 x R uses current and resistance. P = V^2 / R uses voltage and resistance. These come from combining Ohm's Law (V = I x R) with the basic power formula (P = V x I).

Can you use the formula to compare appliances?

Yes. The formula shows which appliance costs more. A 2000 W oven uses 2 kWh per hour. A 100 W TV uses 0.1 kWh per hour. The oven costs 20 times more per hour of use.