Out of sheer boredom, I am trying to quantify the cost of washing dishes by hand V using our new dishwasher. I suspect the dishwasher is the winner, but I want to tell people about this (this is the exciting conversation we have around the braai) and they are going to try to sling mud at my calculations.

My calculation has to be for my house, with a heat pump being used for heating. Once I figure how much electricity is consumed in heating a liter of water, the rest is down to how much the city charges me per liter or per wH.

So, roughly, using the data that the Goodwe portal gives me.

My heatpump runs twice a day. It consumes about 1.2kwh. I have chosen a day when the housekeeper is here because she uses water for cleaning, thus the geyser has to work a little harder in it's mid day time slot as more hot water has been taken out.

So run time comes to about 70 minutes in total (heating to 55C) and the geyser is 200L. So my (admittedly rough) calculation is 

A = 1200 / 60 (to get watts per minute)
B = A * 70 (to get total watts per hour)
C = B/200 and thus the electricity to heat a liter of water.

This comes to 7wh per liter.

This seems a bit high compared to the theoretical cost of heating a liter of water by 1 degree, but that's in lab conditions with no losses through pipes, heat exchange or whatever.
, 
OK... I am not taking degrees in temperature change into account. This is just a crude calculation of what it costs to get the water from whatever temperature it is to 55C.

This raises another interesting point. The dishwasher's spec sheet says that in eco mode it uses 900wh of electricity. It goes on to say that the water is heated to 45C for the wash cycle and 70C for the rinse cycle (I presume this higher temperature is for sterilisation). But this would mean that in the winter it will use more because the incoming water is colder. So I guess this figure is an average over the year for X cycles per day, using the temperature range in some part of the EU. IE some days you will use less, and some days you will lose more.

7Wh to heat a liter of water to 55°C from whatever it was at the start is not bad at all. Your disregard for proper units is concerning, though. You also assume the temperature of the water is homogeneous throughout the geyser (which might be true, considering it is connected to a heatpump)...

Edited November 20, 20223 yr by P1000

1 hour ago, P1000 said:

7Wh to heat a liter of water to 55°C from whatever it was at the start is not bad at all. Your disregard for proper units is concerning, though. You also assume the temperature of the water is homogeneous throughout the geyser (which might be true, considering it is connected to a heatpump)...

Thanks.

It kind of doesn't matter what the starting temperature is. The point is that that's how much energy it costs me to heat up the water that would end up in the kitchen sink. But I'm open to hear more about my disregard for proper units. 

I am really trying to figure out if there is an overall rands and cents saving.

It's looking to me like the dishwasher is running at about 1/2 the price of using a sink - depending on mode and assuming I were paying for electricity at the pre-paid tariff in Jhb. Electricity use is more, but the savings on water is huge and more than offsets the increase in electricity. So the pain inflicted on one's back pocket is reduced.

Factor in solar power and the saving is greater.

Edited November 20, 20223 yr by Bobster.

2 hours ago, Bobster. said:

70C for the rinse cycle (I presume this higher temperature is for sterilisation)

This is then also the drying cycle, presumably it doesn't do any heating after getting the water to 70°C and once the rise side is done, it probably doesn't even drain the water immediately just stops the circulation pump to allow the internals to dry at that high-ish temperature...

12 minutes ago, Kalahari Meerkat said:

This is then also the drying cycle, presumably it doesn't do any heating after getting the water to 70°C and once the rise side is done, it probably doesn't even drain the water immediately just stops the circulation pump to allow the internals to dry at that high-ish temperature...

This makes sense. It pops the door open a couple of minutes before the cycle actually ends. If you wait until the end is actually signalled, then the dishes are pretty much dry. All there will be is some water that has gathered in tea cups etc.

Water has a specific heat capacity of 4184 Joules of heat (1 calorie) for the temperature of one kilogram (call it a litre) of water to increase 1°C.  1 Wh = 1 J/s x 3600s = 3600 Joules. 

21 minutes ago, Scubadude said:

Water has a specific heat capacity of 4184 Joules of heat (1 calorie) for the temperature of one kilogram (call it a litre) of water to increase 1°C.  1 Wh = 1 J/s x 3600s = 3600 Joules. 

I remembered from high school science that there was a specific heat, but I couldn't remember the number. However that's heat that actually gets to the water, it doesn't include inefficiencies and indirect use of energy in the system. EG in my case there's the "cost" of running the compressor, and there will be losses in the heat exchanger. Now I type all that, it seems amazing that the heat pump is more efficient than an element.

Edited November 21, 20223 yr by Bobster.
correct mistype "high school figures" should have been "high school science"

A heat pump has COP of around 3, so the apparent efficieincy is around 300%.  If you have an actual efficiency curve of your heat pump you can model this quite accurately. 

1 minute ago, Scubadude said:

A heat pump has COP of around 3, so the apparent efficieincy is around 300%.  If you have an actual efficiency curve of your heat pump you can model this quite accurately. 

All I can see, and it may be enough, is the load recorded by the inverter, and how that changes when the heat pump is running. Currently it runs at 6am when there's not a lot else going on in the house, so I get a fairly good idea of how much power it consumes whilst running. The power to run the heat pump, it seems to me, is the total power expended on heating water.