How Do Watts and Watt-Hours Differ?

What you will learn

• State which questions W and Wh answer.
• Follow power multiplied by duration.
• Distinguish stored battery energy from power capability.

Before you start

Read the linked prerequisite first. This article explains principles and does not require assembling or opening equipment.

Watts describe a rate

An operating device demands power. Watts measure that rate at a particular moment. A watt figure alone does not state how long the device has operated.

Ask “how quickly is energy being used now?” when reading power. That differs from asking how much energy was used over an entire session.

• Shorter operating interval
• Longer operating interval

Watt-hours include duration

For constant power, multiply watts by hours to obtain watt-hours. Running the same lamp for longer uses more energy.

When demand varies, divide the session into intervals with roughly steady demand, calculate each interval’s energy and add them. This relates a rate to an accumulated quantity, rather than renaming the same quantity by appending “hours”.

Read storage and output without mixing them

A battery energy figure concerns an amount available within its stated scope. A power figure concerns the rate of delivery or consumption. Keep those questions separate when examining storage equipment.

In an ideal model, usable energy divided by constant load power gives duration. This follows from the energy-power relationship; it does not predict real runtime without operating conditions.

A worked example

A hypothetical example, not a measurement of any real device. The first lamp uses 10 W for 3 h: multiplying power by time gives 30 Wh. The second lamp uses 40 W but is on for only 1 h, which gives 40 Wh. The second lamp has four times the power, yet it uses only about a third more energy because it runs for less time.

If power changes during a session, add the intervals: 10 W for 2 h gives 20 Wh, then 40 W for 1 h gives 40 Wh, for a total of 60 Wh. Run the other way, 30 Wh supplied to a 10 W load lasts 3 h (energy divided by power). Power alone does not tell you which lamp used more energy; you need the duration as well.

Frequently asked questions

Does Wh mean watts per hour?

No. Here energy is power multiplied by time, not divided by it.

Does higher power always mean more energy?

A fair comparison must also include duration.

Summary

• Watts measure power.
• Watt-hours measure accumulated energy.
• Compare energy using both power and duration.