When you're comparing laptop batteries, you need a practical way to contrast them side by side. The amount of charge they can hold is one method, and its measurement is in WHr, or watt-hours. The more WHr, the longer the battery will last between recharges.

## Energy Measurements

Watts are a measurement of power. Recall that common light bulbs in your home use 60 watts. How long a battery can produce a watt's worth of power is a good way to measure the amount of charge it holds. If your battery can produce one watt of power for 12 hours, then its rating is 12 WHr. Recall also that your electric company bills you for the charge your consume in kilowatt-hours, or 1,000 watt-hour increments.

## Amp-Hours

Sometimes your battery's charge is also measured in amp-hours. Amps are a measure of current. The product of voltage and current is power. Therefore, since your laptop battery provides a fixed voltage to your computer, measuring charge in watt-hours or amp-hours are equally valid methods. For example, if your battery lasts for 20 watt-hours and always provides 10 volts, then it is valid to state your battery's energy storage capacity as 2 amp-hours.

## Cost Per WHr

Different batteries use different chemicals to store charges. Some chemicals are more efficient than others and some batteries cost more per WHr than others. Lead-acid batteries cost the least per Whr: just 17 cents. On the higher end are Lithium-ion batteries, which power many laptops; they cost £2.70 per Whr they give you.