Dear Reader,

Yes, Mercury has issued a - possibly - quite helpful list of 62 things in the home, and how much power you'd expect them to use per hour. Well, how much they'd cost to run per hour.

A couple of assumptions that you need to be wary of, though, is that they assume you've got a rate of 20.14 cents per kWh, and that you're running your appliances constantly.

I find it strange, however, that they haven't included the cost of running the hot water cylinder, since they've pretty much got everything else (even what's in the kitchen sink!).

So I'll show you how you can work out what your hot water cylinder should be using. First of all, we need the 'water heating calculation', which is:

(3.966 x (number of litres of water that needs heating) x (difference between start and end temperatures)) / 3.41

This will give us the number of watts we'll need to be able to heat the specified amount of water in one hour.

To find out how many hours it will take if we have a limited number of watts available, we divide the number of watts we'll need by the limited number of watts.

O.K., let's get into the examples:

Let's say you need to heat 135 litres to 65 degrees from 10 degrees, and that you have a 3,000 watt element to do it with. By the way, this is pretty much the standard set-up of residential hot water cylinders.

Anyway, let's plug these figures into our equation:

(3.966 x 135 x 55) / 3.41 = 8,635.645 watts needed to heat that amount of water in one hour.

But since we only have 3,000 watts, it'll take 2.879 hours, since:

8,635.645 / 3,000 = 2.879 hours

So, how much will it cost? Taking the amount Mercury gives us, we'll turn it into a dollar amount, and multiply it with the number of hours taken:

2.879 x 0.2014 = 58 cents every day - assuming the hot water cylinder only needs to be heated once (and it might just need to be heated once, depending on how much hot water gets used).

To find out what you should be paying to run your hot water cylinder, find out what size the element is, how many litres it holds, and what the temperature is at the cold water tap (if you're having a bit of trouble, contact us on 0800 72 83 44). Then multiply it with the rate you're getting for the hot water cylinder (which is probably on a controlled rate, i.e., you're paying less for it); you'll probably quite surprised.

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