Estimate of cost to heat the water for a bowl of washing up.

tomstickland

I've just bought an energy meter from Amazon and plugged it into my kettle.
A brimmed kettle of water (about 8 cups) uses 0.16 units (Kilowatt hours, aka Kwh).

I estimate that it'd take two of these mixed with cold water to create a typical bowl of washing up liquid. Maybe slightly less. Call it 0.32 units.

I also did a theoretical study.
Dimensions of bowl 330x280x100mm, giving a volume of 9,240,000 mm3
which is 9.24l (a litre is 1/1000th of a cubic metre).
Density of water is 1000kg/m3, so bowl has 9.2Kg of water in it.
Sounds about right.
Say a typical bowl of washing up water is 50deg C, that's a 30 deg rise over room temperature.
The specific heat capacity of water is 4.2 KJ/(KG deg C).
Hence it takes 1160KJ to heat the water up.
A KwH is 3600 seconds of 1 KW, hence 3,600KJ.
One bowl of washing up is 1160/3600 units, which is 0.32.

Genius!

At a daytime rate of 12p per unit that's just under 4p for a bowl of washing up.

Say I use 4 bowls per day, then that's 1.3 units per day, which is 16p per day. Over 365 days it's £56.

Now here's the interesting thing for me.
At the moment I use economy 7 to heat a tank overnight. From numbers I've seen on the web, a tank will lose around 3 units per day.

So the daily cost of my washing up water based on a night time rate of 4p per unit is:

(1.3 + 3)* 4 = 5.2p + 12p = 17p.
ie: 17 p per day, of which 5.2p is for the water used and 12p is for heat lost from the tank during the day.

Compare that with heating the water when I needed it:
1.3*12 = 16p.

It's very close. If more hot water is used then night time heating will become the cheaper option.

Cardew

tomstickland wrote: »

Now here's the interesting thing for me.
At the moment I use economy 7 to heat a tank overnight. From numbers I've seen on the web, a tank will lose around 3 units per day.

Tom,
Have you got a link for that info please.

tomstickland

Heat loss data at bottom of page
http://www.elsonhotwater.co.uk/elson/elson.nsf/03798e9169de46dd80256a2a003c660b/5ecade7d9751175e80256ac300527099?OpenDocument

Heat loss table at lower right
http://www.copperform.co.uk/electrical_products/maxistore/index.htm

Heat loss data:
http://www.albionwaterheaters.com/Maxistore.html

Typical heat loss number:
http://www.tradingdepot.co.uk/DEF/product/!!CYLLE7BS15045!!/D003006/Plumbing/Central%20Heating/Hot%20water%20cylinders/Copperform%201500%20x%20450%20BS%20Economy%207%20Direct

This is interesting:
http://www.toolbase.org/PDF/DesignGuides/domestichotwater.pdf

tomstickland

http://www.leaningpinesoftware.com/hot_water_heater_tank_insul.shtml

tomstickland

One thing I haven't considered is heat loss through the pipework. When I used the daytime boost to heat my hot water the other day I noticed that the bathroom became very warm. This was because the thermostat on the upper heater was set high and was making the pipework hot.

mute_posting

I've not done the maths (but it looks okay from what bits I remember from my thermo lessons at uni!)

However, (I could have missed something important here!) are you really only heating your hot water from 20 degrees to 50? I thought it would be more like 10 degrees to 60. (the reccomended temp for a hot tank to prevent legionaires bugs is 60 degrees minimum IIRC)

I MIGHT HAVE MISSED SOMETHING - IT HAS BEEN A WHILE SINCE I DID THERMODYNAMICS! LOL

MP

tomstickland

The hot water might be heated in the tank to 60 degs, but the water is mixed with cold to create a bowl of approximately 50 deg and it's only the energy to create that that matters.

UtilityMan_3

Put your washing up in the washing machine with your clothes. Or better still fill your pockets with crockery and jump in the bath fully clothed.

redmatchstickman

UtilityMan wrote: »

Put your washing up in the washing machine with your clothes. Or better still fill your pockets with crockery and jump in the bath fully clothed.

that's what i do. or use my dishwasher :rolleyes:

tomstickland

I've got some more data.
With my hot water cylinder out of action I've been using just under 1 unit of electricity per night.
With the hot water on I was using around 4.5 units per night.
So my hot water heating was using about 3.5 units per night.
Estimating that I use 3 bowls of washing up per night at 0.3 units each, then about 1 unit is for the actual hot water. The other 2.5 must be heat loss from the tank.

PasturesNew

tomstickland wrote: »

I've just bought an energy meter from Amazon and plugged it into my kettle.
A brimmed kettle of water (about 8 cups) uses 0.16 units (Kilowatt hours, aka Kwh).

I estimate that it'd take two of these mixed with cold water to create a typical bowl of washing up liquid. Maybe slightly less. Call it 0.32 units.

I also did a theoretical study.
Dimensions of bowl 330x280x100mm, giving a volume of 9,240,000 mm3
which is 9.24l (a litre is 1/1000th of a cubic metre).
Density of water is 1000kg/m3, so bowl has 9.2Kg of water in it.
Sounds about right.
Say a typical bowl of washing up water is 50deg C, that's a 30 deg rise over room temperature.
The specific heat capacity of water is 4.2 KJ/(KG deg C).
Hence it takes 1160KJ to heat the water up.
A KwH is 3600 seconds of 1 KW, hence 3,600KJ.
One bowl of washing up is 1160/3600 units, which is 0.32.

Genius!

At a daytime rate of 12p per unit that's just under 4p for a bowl of washing up.

Say I use 4 bowls per day, then that's 1.3 units per day, which is 16p per day. Over 365 days it's £56.

Now here's the interesting thing for me.
At the moment I use economy 7 to heat a tank overnight. From numbers I've seen on the web, a tank will lose around 3 units per day.

So the daily cost of my washing up water based on a night time rate of 4p per unit is:

(1.3 + 3)* 4 = 5.2p + 12p = 17p.
ie: 17 p per day, of which 5.2p is for the water used and 12p is for heat lost from the tank during the day.

Compare that with heating the water when I needed it:
1.3*12 = 16p.

It's very close. If more hot water is used then night time heating will become the cheaper option.

Good posting. Some tips:
1] Get a girlfriend
2] Put Excel away, it's addictive.

I use my kettle to heat water to wash up. I have an immersion heater and thought it would be uneconomical to heat all that water just to do some washing up.

I use 2 kettles/day though. Maybe you're eating too much!

I'm sure your costs to heat up a tankful must be wrong though.... doesn't seem right. Seems too low.

And/or recalculate that doing all your washing up on the cheap rate hours.