Power, energy and time calculators
This page contains a calculator for working out how many watts are used in a given time (i.e. power), as well as
calculators with the equation rearranged to make energy (joules) and time (seconds) the subject. The formulas are
shown together with other useful information and examples to try.
Both watts (W) and joules (J) are derived units in the SI system. You can see how they are derived on this page,
together with other derivations. The watt (W) is the unit of power.
Enter all figures without commas. For example, enter 2,400 as 2400.
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Calculate watts (W) from joules per second (J/s)
The equation for watts is given on the right.
Example: It takes a particular kettle 1 minute and 46 seconds to boil
a quantity of water. In doing so it uses 240,000 joules of energy. What
was the power used, i.e. how many watts (W) were used?
The time taken was 1 x 60s + 46s = 106 seconds. We can now simply
enter the number of joules (240,000) and the number of seconds
(106) into the calculator and click Calculate to show that the kettle
used just over 2264 watts.
Calculate energy in joules (J) using power (W) and time (t)
The equation for joules is given above right.
Example: A 10 kilowatt generator is run for 30 minutes. Assuming
100% running efficiency, how many joules does it use in that time?
10 kilowatts is 10,000 watts, and 30 minutes is 30 x 60s = 1,800
seconds. Putting these figures into the calculator and clicking Calculate
shows that the generator consumed 18,000,000 joules of energy.
Note: No machine can run at 100% efficiency, but such calculations
allow for good first estimates to be made.
The equation for energy is:
Where:
J = energy (joules)
W = power (watts)
t = time (s)
Quick Facts
1 watt (W) is 1 joule (J) per second (s)
1 kilowatt = 1,000 watts
It takes about 1 joule to lift a large
apple 1 metre
Calculate time in seconds (s) from joules and watts
The equation for time in this page’s context is given above right.
Example: An LED light bulb is rated at 12 watts, and over a period of
time is measured to have used 4,200 joules. How long was the light on
for?
In this case we simply enter the energy used (4,200 J) and power
rating (12 W) into the calculator and click Calculate. The answer is 350
seconds exactly. To convert this into minutes and seconds, start by
dividing by 60, i.e. 350 / 60 = 5.83 minutes. The .83 is a decimal and
because there are 60 seconds in a minute the final task is to multiply
0.83 by 60s = 49.8s. So the total time is close to 5 minutes and 50
seconds.
The equation for time is:
Where:
t = time (s)
J = energy (joules)
W = power (W)
The equation for watts is:
Where:
W = power (watts)
J = energy (joules)
t = time (s)
Power, energy and time
calculators
Power, energy and time calculators
This page contains a calculator for working out how many watts
are used in a given time (i.e. power), as well as calculators with
the equation rearranged to make energy (joules) and time
(seconds) the subject. The formulas are shown together with
other useful information and examples to try.
Both watts (W) and joules (J) are derived units in the SI system.
You can see how they are derived on this page, together with
other derivations. The watt (W) is the unit of power.
Enter all figures without commas. For example, enter 2,400 as
2400.
Calculate watts (W) from joules per second (J/s)
The equation for watts is given below.
Example: It takes a particular kettle 1 minute and 46 seconds to
boil a quantity of water. In doing so it uses 240,000 joules of
energy. What was the power used, i.e. how many watts (W)
were used?
The time taken was 1 x 60s + 46s = 106 seconds. We can now
simply enter the number of joules (240,000) and the number of
seconds (106) into the calculator and click Calculate to show
that the kettle used just over 2264 watts.
Calculate energy in joules (J) using power (W) and time (t)
The equation for joules is given below.
Example: A 10 kilowatt generator is run for 30 minutes. Assuming
100% running efficiency, how many joules does it use in that
time?
10 kilowatts is 10,000 watts, and 30 minutes is 30 x 60s = 1,800
seconds. Putting these figures into the calculator and clicking
Calculate shows that the generator consumed 18,000,000 joules
of energy.
Note: No machine can run at 100% efficiency, but such
calculations allow for good first estimates to be made.
The equation for energy is:
Where:
J = energy (joules)
W = power (watts)
t = time (s)
Calculate time in seconds (s) from joules and watts
The equation for time in this page’s context is given above right.
Example: An LED light bulb is rated at 12 watts, and over a
period of time is measured to have used 4,200 joules. How long
was the light on for?
In this case we simply enter the energy used (4,200 J) and power
rating (12 W) into the calculator and click Calculate. The answer
is 350 seconds exactly. To convert this into minutes and seconds,
start by dividing by 60, i.e. 350 / 60 = 5.83 minutes. The .83 is a
decimal and because there are 60 seconds in a minute the final
task is to multiply 0.83 by 60s = 49.8s. So the total time is close
to 5 minutes and 50 seconds.
The equation for time is:
Where:
t = time (s)
J = energy (joules)
W = power (W)
The equation for watts is:
Where:
W = power (watts)
J = energy (joules)
t = time (s)
Power, energy and time
calculators
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