Hand washing vs. Dishwasher redux

Ok… so I understand that the dishwasher is generally reputed to be more efficient than hand-washing under normal circumstances.

But here in Texas, we’re not experiencing normal circumstances. There are severe electricity issues, and calls to conserve as much as possible. And the same sort of thing but less so for natural gas, in that they’re asking for conservation, but I haven’t heard of natural gas outages yet. But gas scarcity is a factor in the electricity issues though.

So with that in mind, which is better under the current conditions (I have power and gas) for washing dishes for a family of four? I’m struggling- part of me thinks that the dishwasher might still be the most efficient, in a gas/electricity per dish washed idea, but I’m not sure if it’s better to spend the gas for all the hot water right now, because electricity is so scarce.

Anyone have any ideas?

Typical modern dishwashers seem to average at about 6 gallons of water per load, and energy star are less than 4 gallons. So it depends if you think handwashing can beat those values or not, for the same amount of dishes washed.

Dishwashers are enormously more efficient than hand washing, IF your hand washing method comprises washing things under a continuous stream of running water.

If you run a bowl of water and wash the dishes in it, then rinse in another bowl of water, it’s a very much closer game.

I’m ignoring the comparative efficiencies of heating water, because in general, turning mains power into heat is quite evenly efficient (it’s difficult to use electricity efficiently for other purposes without accidentally making heat, but it’s easy to use electricity efficiently for making heat)

I have a gas water heater, for what that’s worth.

My method is kind of a hybrid; soapy water in one side of the sink, and rinse in the other in a stream of cool/lukewarm water.

I would expect that whichever one uses less water would also use less energy, because most of the energy goes into heating up that amount of water.

Except that a dishwasher generally uses hotter water than humans will use for hand-washing.

That’s why I’m kind of stumped; it’s not an obvious or easy call. I lean toward the dishwasher, but with electrical power being at a premium, maybe the handwashing is a better choice right now.

The other factor is that a dishwasher typically uses nothing but hot water, wheras handwashing can use as much cold water as desired for rinsing.

Test it out then. Run the hot water tap until you get 4 gallons of hot water out into the sink. After that, run as much cold water as you want for washing/rinsing. If you can handwash the same amount of dishes under those constraints, you’re probably better off handwashing.

Handwashing also gives you a potential collateral water saving in either sourcing water that would otherwise be wasted [bucket under the shower until the hot starts to run] or using it afterwards on plants or the garden.

I’ve lived in two houses with dishwashers I’ve never used, although one was happily exchanged for a little beer fridge, which again saves soooo much water. If I got my act together I could save water just by organising the sink so really grimy dishes soaked, cutlery in the same water, vegetables were rinsed from a bowl rather than under a tap etc etc.

Is that correct? My guess would have been that the dishwasher uses a fair amount of energy not to heat water, but to move it around. So if your priority is conserving energy rather than conserving water, the dishwasher may not be the most efficient solution.

That was basically why I asked. On one hand, the dishwasher has been shown to be the most efficient in terms of overall energy and water usage. But it’s electric.

On the other hand, hand-washing isn’t nearly so efficient, but it only uses hot water, which in my house is natural gas heated.

The real quandary came in when they started asking for efforts to conserve gas as well as electricity. Before that, it was pretty clear that hand washing was the way to go. But if they’re asking for gas conservation, and millions of people are out of electricity due to electrical capacity problems caused in some part by gas supply issues, it suddenly became a lot more of a thorny problem.

Luckily, it’s kind of academic now, as ERCOT has announced that the rolling outages are essentially over, and that transmission companies can bring back any load they were required to shed.

At the beginning of the pandemic and stay at home orders, I didn’t have a home office set up yet so I spent a few weeks working at the kitchen table. Which is how I learned that many “energy star” dishwashers, including my own run for 3 freaking hours. And they only use a few gallons of water by recycling it through the machine for hours at a time. I don’t think it’s possible that they’re using a low amount of power too.

Okay, I’ll bite: how did getting the little beer fridge save you water? I can see how a little beer fridge would save energy because you’re not running the whole big fridge to cool down a six-pack or opening up the big fridge door whenever you want a beer.

But how does the little fridge save water, unless you mean that you gave up drinking water in favor of beer? Which if so, whoa dude. :crazy_face:

Thank you for asking. Since it was a small fridge that tended to ice up every time you opened it, it probably mostly saved water in the same sense that glaciers save and retain water that would normally be sloshing around the oceans. Much human energy was used in constant de-icing and attempting to hammer out cans from the depths.

I think in a situation like yours, the goal is to use less energy overall, rather than the same amount of energy more efficiently. You could vary how you hand wash or use the dishwasher to tweak either to be slightly more efficient, but I don’t think you could match the energy saving by simply washing less (re-use dishes, plan meals so dishes don’t get very dirty through the day, even use paper plates for the short term). That’s not a direct answer to your question but another way to look at the solution to your current problem.

Well sure, but you can only do so much of that kind of thing before you have to do dishes over the course of nearly a week.

Sure, a dishwasher is going to spend some energy to move water around, but it’s negligible compared to the energy for heating. The low end of dishwasher temperatures is 45ºC, or about 25ºC above room temperature. Water has a specific heat of 4 kJ/kgºC, so heating up water that much will take around 100 kJ/kg of energy. That amount of energy, if put into moving water, would have it moving at over 400 m/s (supersonic speeds).

You also have to keep in mind how many dishes are done.

If you are just doing a couple plates, then washing them by hand is probably better. If you have enough to fill your dishwasher, then using it is probably better.

Thanks. I confess the physics of this is beyond me. But isn’t there a time factor at work here? You only have to heat up your kilo of water once, and therafter you have to maintain its temperature; the energy required to do this will of course depend on how well the dishwasher is insulated. But you’re moving it around repeatedly, over two to three hours. Every time it hits a surface and drains to the bottom of the dishwasher it loses all momentum and you have to constantly expend more energy to get it moving again. Doesn’t this have to be accounted for?

Longer cycle times are more because of less powerful motors and also changes in detergent formulation. When phosphates were removed from detergents they mostly switched to enzymes. Those need more time to activate and do their job, so in my experience the longer cycles also spend some of that time just sitting idle. Letting the dishes sit in a hot steamy environment also helps, kind of like pre-soaking, irrespective of the detergent or spray strength. Using a less forceful pump also means it takes longer to blast away grime from plates, but that works with the enzymes and is also quieter and doesn’t bang around the glasses as much. New detergents may not have adequate time for enzymes to be as effective in older dishwashers, but they make up for it with a stronger spray and more water.

Is it? Never underestimate the amount of energy used by pumps, fans, or compressors, especially ones that run for a while. Many Energy Star labels for instance only report the primary fuel usage of the appliance. So in a gas furnace or boiler, they only report the gas usage. However, they don’t report the electricity usage for the draft fan, blower motor, or circulator pump.* A furnace blower can easily use 1,000 watts, but that’s not factored into the equation. I’m not sure how labels on dishwashers are reported. It could be their own electricity use, their own use plus a range of estimates for hot water usage, or just the hot water usage.

That said, I don’t think the sort of pump used in a dishwasher is as high-power as you’d find in a hot water heating system, because it’s not pushing water through long lengths of pipe. I just don’t know what it is.

*Yes in a hot water heating system the circulator pump is usually separate so that’s not fair, but when converting an old steam system to hot water, the greater gas-burning efficiency of the hot water boiler is somewhat canceled out by the additional electricity used for the draft blower and especially the circulator pump. A steam system likely has neither, or only a very tiny condensate pump, so to do a proper apples-to-apples you need to consider both gas and electric usage, which isn’t on the energy labels.