Calculation: How much does it cost to heat water using electricty?

[cortijo]

I am trying to work out the efficiency of my solar hot water system and pay back time by comparing it to electric.

Assuming the cost of electricty is 0.19 units per kwh

How much would it cost to heat 50 litres of water from 5C to 40C ?

This would be using a normal electric water tank heater with typical efficiency.

Does anyone know a formula?

[Naptown]

I work in imperial units but here you go
1 btu will heat one pound of water 1 degree F
Water weighs 8.3 pounds per gallon.
So the formula is weight of water x delta t or how much it is heated in deg F /3413 which is the BTU content of 1 Kwh of electricity.
This will tell you how many KWH it takes using resistance heating elements. A bit more if really coated with calcium.
Then multiply by whatever it costs for electricity.

[cortijo]

Thanks for the formula.

It seems that solar water heating really makes economic sense.

I just bought 100 metres of 32mm black tube and conected it to the water inlet before it gets to the solar water heater.
The tube is in the sun on the roof.

It looks like it will get a payback time of about 5 weeks and according to my figures.

My water supply comes in at about 5C (41F)
In the summer even at night the system will increase the temperature of the inlet water to the ambient temperature.

Pre heating the water even before it gets to a commercial solar heat system seems like a good idea.

See more info on my blog

http://cortijoblog.com/2013/04/23/50-litre-solar-hot-water-heater-for-only-71-euros/