If you’re in the market for a high efficiency washing machine in 2011, you will find a plethora of water and energy efficient washing machines available that are substantially better than their predecessors. If your current model is over ten years old, you should strongly consider an upgrade.

According to Energy Star, the average family washes almost 400 loads of laundry per year. If that describes you, then upgrading to a modern Energy Star-compliant high efficiency washer will save you enough energy to pay for a matching dryer.

You would save enough water to allow you til fill up three backyard swimming pools! That’s a lot of water that you’re literally throwing down the drain if you stick with an old washer.

If you’ve decided to take the plunge, how do you choose which model to purchase? A logical choice would be to look at the most efficient of the efficient. Efficiency is measured through the use of formulas, depending on whether you’re talking about energy or water.

The formulas for judging a high efficiency washing machine are a little complicated, but basically the energy efficiency formula (MEF) takes a washer’s capacity and divides it by the sum total of its energy usage in operating the machine, heating water for it, and extracting moisture from it. The higher the value, the better. Energy Star requires that MEF be greater than or equal to 2.

The formula for water (WF) takes total water consumption and divides it by capacity. Lower values are better, and so Energy Star requires a WF lower than or equal to 6.

Finding the most water and energy efficient washing machines, therefore, is simply a function of finding those with the highest MEF and the lowest WF. I sorted the Energy Star list first by WF, then by MEF. For MEF tie breakers I sorted thirdly by kWh/year from lower to higher. Note that I don’t think that simply sorting by kWh/year gives the best view of energy efficiency, since it’s not counting the heating of water which usually takes place outside the machine.

It’s worth noting as well that the result varies if you sort first by MEF and then WF, rather than the other way around as I did. I made the judgment call that people buying clothes washers are looking for water efficiency over energy efficiency, so I gave more weight to water consumption.

Here are the five top high efficiency washing machine choices based on the criteria above. (Asterisks denote model variants.) All five models had a WF of 2.7, making them the most water efficient among all models.

1) Whirlpool WFW97HEX*+. This excellent high efficiency washer takes the #1 slot with a MEF of 3.35. It sips 114 kWh/year–not the lowest amount, but among the lowest for its size class of 4.33 cubic feet. And once again, water usage is king here.

2) Whirlpool WFW95HEX*+. Coming in just a hair behind first place, all the stats are identical to the higher ranked Whirlpool model except for MEF, which is slightly lower at 3.30. It is unclear why the MEF is different when the other stats are the same–perhaps it uses just a bit more hot water.

3) Maytag MHW7000X*. In third place comes this model, which is just a tad smaller at 4.3 cubic feet but consumes the same 114 kWh/year, causing a slightly lower MEF of 3.30. Since it’s slightly smaller, I ranked it lower.

4) Maytag MHW6000X*. This Maytag model has the same stats as the previous one for volume, MEF and WF–but it uses a fair bit more energy, 127 kWh/year.

5) Whirlpool WFW94HEX*+. This cousin of the other models is the same size of 4.33 cubic feet, but comes in with a MEF of 3.21 and a higher 123 kWh/year.

Picking a high efficiency washing machine from one of these five models guarantees you will save a large amount of water compared to older washers. You will also save a chunk of change in energy costs.