how much juice does it take to heat

[OvertonPlumber]

Oki one for the sparkies to answer how much juice does it take for a 3kw immersion side firing bottom in a 170 ltr unvented cylinder of white meter economy 7 tariff. I.e assuming that water is cold 10c and we aim to heat it to 60c how many hours would the immersion be on and what would be the relative cost.
The reason i ask is im weighing up the cost implications of fitting a twincoil ariston solar cylinder, i already have a spare pumpstation and solar controller thanks keston , lol. I assume im gonna need a couple of flat plate panels on the roof. ( roof ) is flat above the cylinder cupboard and there a handy pitched escarpment between the top of the cupboard and the flat roof handy hehe.
My thought are that how much is my direct costing me? and is a solar system worth the financial outlay. My electric bills are only about £40 a mth scottish electric.
1 bedroom first floor flat.No gas hah hah yes in no im a gas fitter lol.Oh and i own the building freehold so i can do wot i like on the roof roofs hehe lets have a pv electric and windfarm up there!

 

[plugwash]

according to wikipedia water has a volumetric heat capacity of 4.184 J/(cm³·K)

Calculating the energy required if the tank was perfectly insulated is pretty easy.

you have 170000 cubic centimeters of water and you are heating by 50 kelvin so you will need a minimum of 35564000 joules of energy. That translates to about 10KWH. That would mean a heat up time of just over three hours.

In reality the insulation on your tank won't be perfect so it will take more than this.

 

[rssteve]

according to wikipedia water has a volumetric heat capacity of 4.184 J/(cm³·K)

Calculating the energy required if the tank was perfectly insulated is pretty easy.

you have 170000 cubic centimeters of water and you are heating by 50 kelvin so you will need a minimum of 35564000 joules of energy. That translates to about 10KWH. That would mean a heat up time of just over three hours.

In reality the insulation on your tank won't be perfect so it will take more than this.

is that correct? my 210lt uses a 3kw heater and seems to heat the fully tank in 2 hours or less as 2 shower seem to empty the thing. Maby it aint heating up to 60c from 10c.

 

[OvertonPlumber]

incoming mains water temperature can vary depending on your location and time of the year. Where i am on a almost freezing day the incoming is about 5c this time of year it can be like 15c.
Oki so assuming my immersion takes 3 hours to heat my tank. That 3 x 365=1095kwh @ ???

 

[Lectrician]

It depends how often you heat the water at present. If you only heat it when you know you are going to use some, it's not a good comparison.

I know a few people who have had dual coil cylinders installed and a couple of collectors on the roof. All have noticed a substatial lowering of their bills. Particularly electrically heated and oil or LPG gas fired. There is still a huge benefit for natural gas too.

In my opinion, having a solar panel for your water is an excellent investment, and I would go for it if I had a roof that pointed the correct direction, and didn't fit a combi boiler 18 months ago!

 

[electronicsuk]

Oki so assuming my immersion takes 3 hours to heat my tank. That 3 x 365=1095kwh @ ???

No, read Plugwash's post again. Your tank consumes approximatelt 10kWh to heat from cold, if you use it every day of the year that would be 10 * 365 = 3650kWh per year. Your calculation seems to assume that the tank is only on for 1 hour each day of the year. Obviously if you take into account varying incoming water temperature, holidays and usage around the year then the figure will vary.

 

[OvertonPlumber]

oki i just found out my nightrate is 8.1 p kwh so if my immersion is on for 3hrs that 9kwh a nite at a cost of 72 pence a day. thats um £262 a year.

Seems quite good.
Working on the assumption that my solar pumpstation was active for 4-5 hrs a day before it reaches stagnation temperature. Say a grundfos alpha pro which will do 208 days at 89kwh usage say 365 days at 100kwh @ peak would cost £15 to run.
So say it saves £200 yr in electric would be about 8.5 yrs for you to recoup cost and thats not allowing for installation. hmm

 

[electronicsuk]

I'm not sure there has ever been a god argument for solar on cost grounds, perhaps with very large installations where prospective bills are a lot higher, but there's still no guarantees with the sun. Depends how much you care about your "Carbon Footprint"