There are quite a few "studies" around that claim to show that electric dishwashers are green friendly and use less energy and water than handwashing. Although the studies have been done by various "researchers" worldwide, an interesting commonality is that all the studies were paid for by appliance manufacturers, and all of them compare the most efficient washers in the world under the most restricted circumstances against the most wasteful theoretical hand dishwashing practices that can be imagined. A typical result is to say that a dishwasher will use 4 gallons of water and handwashing uses 25-40 gallons.

Let's use some common sense here. 30 gallons is the capacity of your entire water heater, and is enough to fill a full sized bathtub. Have you ever run out of hot water while handwashing dishes? How many of you have even seen a house that had a sink that was the size of a bathtub? Do you believe that you use anywhere near 30 gallons while handwashing dishes? I didn't think so!

The studies will hide in their footnotes some assumptions and tricks. Let's look at some of them.

One trick is instead of measuring how much water is actually used, estimate the amount of time it takes to wash dishes instead. Then, you can multiply that time by some large flow rate numbers and get outlandish water usage numbers. In other words, assume that the kitchen plumbing is capable to deliver a lot of water quickly and is run at that maximum flow rate during the entire dishwashing session. So they will say, well a really good plumbing system can deliver 5 gallons a minute (GPM) through the kitchen sink, and people run the water for 6 minutes, so that is 30 gallons. This is already wrong for almost everyone and not a reasonable assumption that any honest researcher would make, or would find by actually measuring data. Now, 5GPM isn't an unreasonable rate for an outside hose spigot connected to 1/2 pipe. But kitchen faucets aren't outdoor faucets. If you want to fill a bucket of water to wash your car, you know it will fill faster on an outside spigot, or using the bathtub than using the kitchen faucet. Almost all kitchen faucets in the US nowadays have a flow regulator that limits them to 2.2GPM flow. In addition, few people run them at the full rate while washing dishes. As a calibration, I run my sink at the moderate rate I use while rinsing and filled a gallon jug. It took exactly 2 minutes, so the flow rate was 0.5GPM.

Another assumption is that no pots pans or very dirty items will be washed. This is true for people who just reheat frozen food, but if you actually cook your meals, you'll have to wash pots and pans as well. This will almost always involve soaking the pans a bit, and often these pans can't even be washed in a dishwasher anyway so you have to do the sink batch anyway whether you do the dishes themselves by hand or in the washer. And if you are using the sink for pans, why not share the water and do the dishes there as well? This is never addressed, pots and pans are assumed not to exist in these studies.

Another assumption is that no one stops the sink while washing. They just run that faucet to wash and to rinse and the water flows right down the drain. I suppose there are people that do this. It might even make sense if washing a few items. But a typical washing session here fills the entire sink with dishes at least twice, and the sink is certainly stopped up for this.

Let's compare their theoretical numbers with some real ones. Here's how I do it, and I assert that this is a pretty common method.

I stop up the sink. Then I start to fill it with warm water at a moderate rate. I do scrubbing at this time, while the sink fills. I might do some rinsing as well. Eventually it is half full. Our sink is average sized, each basin is 16 by 14 by 6 inches. It holds 5.8 gallons while full, so half full is about 3 gallons.

Washing the dishes, they come out and go on the counter. Then I stop up the second sink and start a 1/2 GPM trickle to rinse. I start with the large items. By the time there is about 1 inch of water in the sink (1 gallon), I am done with the large items. The plates and utensils then I can rinse in that 1 inch of rinse water.

This way I do two sink loads. I might stretch it to a second load over the day. If I need the use of a sink during the day, I'll drain the filthy water, but retain the rinse. Then, the next wash starts with the clean rinse water from the previous wash. So the total water per wash tends to average out to about 2-3 gallons, and that includes all the pots and pans that can't be done in the dishwasher anyway. This water goes into the septic system and ends up watering the lawn via the leach lines, so the water is entirely reclaimed.

I also have used a two-bucket method that uses about 1.5-2 gallons altogether, but doesn't handle very large pots and pans. The advantage is the water can be poured on the garden in some cases, however if there is much oil in it or any meat residue I don't really recommend that as it will attract varmits to your garden.

The studies claim that the most efficient washing machines will use 4-5 gallons. That's true, but it's not any washing machine that you are likely to own in your home so it's irrelevant for most people. A typical washing machine uses 6-10 gallons for a normal cycle, or 8-15 gallons if it's 10 or more years old. Plus it uses electric heaters to dry the dishes, which uses a lot of energy, plus dishwashing detergents have to be more harsh in order to wash things without scrubbing, and often contain phosphates. Plus, most people find with the efficient machines they have to prerinse their dishes anyway in the sink if they want dishwasher to have a chance to get the dishes clean.

Even if you have generously sized 6 gallon capacity sinks and fill both of your wash and rinse basins to the very brim, that's 12 gallons. A more reasonable "full sink" usage would be 10 gallons, which is the same as the washing machine, but includes the complete washing of the pots and pans and any presoaking or such that is not accounted for with machine usage.

The results of my research? I claim both that it's pretty difficult to use 20 gallons or more to hand wash dishes (unless doing them in the bathtub for some reason), and that typical dishwasher usage is not really 5 gallons per load when you factor in actual machines in homes and the need to clean pots and pans. I also claim less energy usage due to no heat drying, and less soap usage due to leveraging the ability to scrub.

All these other studies, yes all of them, done by guys with PhDs who have university positions, are a bunch of hogwash. For them to call themselves researchers or scientists is a bunch of bull as well. The typical university "scientist" is nothing more than a paid propaganda pundits for for-profit corporate interests. Alas, the brainwashing works well because the false claims are then uncritically regurgitated by naïve idealists who do not do a proper critical analysis. Because this second level of propaganda propagation consists of sincere, trusted regular people, the ideas spread like wildfire, gaining a thought momentum that makes their obviously false claims difficult to get people to challenge.