Local water districts often offer rebates to customers who buy efficient washing machines. These washing machines use less water to clean clothing than older washing machine models. By using them, a customer helps the water district conserve its limited water supplies. Conservation is very important in arid regions such as Southern California and New Mexico. Water-conserving washing machines may also use less energy, especially if they’re Energy Star appliances, but water districts care more about how much water they use.

The water districts use a metric called the Integrated Water Factor to measure the efficiency of washing machines. This metric is necessary because just measuring the amount of water that a machine uses is not enough. A larger machine uses more water in each cycle but it also washes more clothing in each cycle. So the utilities have developed a metric that allows more accurate comparisons.

The integrated water factor is simply the amount of water that the washing machine uses in each cycle divided by its tank capacity. If the washing machine uses 37 gallons per cycle on average and its tank holds 10 gallons of water, then its integrated water factor is 3.7.

A washing machine with a 20-gallon tank would still have an integrated water factor of 3.7 if it used 74 gallons per cycle. It would be just as efficient as the smaller machine and it would use twice as much water to wash twice as much clothing.

A larger washing machine might even be more efficient than a smaller one. For example, a washing machine with a 20-gallon tank might use 70 gallons of water per cycle. That’s nearly twice as much water as the smaller machine. But the larger machine would have an integrated water factor of 3.5 and would thus be slightly better at conserving water.

On the other hand, a washing machine with a smaller tank could be less efficient. For example, if a washing machine had a 5-gallon tank and it used 25 gallons of water per cycle, it would have an integrated water factor of 5. That might make the washing machine too inefficient to qualify for rebates.

Utilities use the integrated water factor to determine whether a washing machine is eligible for a rebate. For example, the San Gabriel Valley Municipal Water District supplies water to several Los Angeles neighborhoods. It offers a $110 rebate for the purchase of a water-conserving washing machine. The washing machine has to have an integrated water factor of 3.7 or less to qualify for the rebate.

Water districts don’t all use the same efficiency standards. For example, the Albuquerque Bernallilo County Water Authority also has a rebate program. Albuquerque’s also a desert city where water is precious. But to qualify for the Albuquerque washing machine rebate, a washing machine needs to have an integrated water factor of 3.0 or less. A washing machine that satisfied the standard in LA would not meet the standard in New Mexico.

While this metric may confuse consumers who aren’t familiar with it, it’s a useful metric. It ensures that washing machines are compared on their efficiency instead of the total amount of water they use. It wouldn’t be reasonable to reward consumers for buying inefficient, but small, washing machines or to penalize them for buying large washing machines that do a good job at conserving water.