Am I doing these maths correctly?

MothballsWallet

Hi everyone,

I've built my own simple spreadsheet to let me record my gas and electric meter readings and try to work out my annual consumption and I was hoping someone here could help me check my maths for converting my gas readings from m3 to kWh please?

Reading yesterday at 4:55 pm, which was 5304 m3
Reading today at 5:00 pm, which was 5307 m3
5307 m3 - 5304 m3 = 3 m3
Usage per day = 2.99 m3/day
Converting m3 to kWh:
Multiply 2.99 by volume conversion factor of 1.02264 = 3.0580803433314
Multiply that by calorific value of 39.3 = 120.182557492924
Divide that by kWh conversion factor of 3.6 = 33.3840437480345
Multiply that by days per year (365.25) = 12,193.5219789696 kWh

It's just that this figure looks massively large to me, which is why I'd like to check my maths.

Also, I live in the West Midlands and got the calorific value from the UK Power website (the page is here).

For reference: I have my central heating set to come on for 2 hours in the early morning and 2 hours again in the late afternoon. Everything else (cooking, washing machine, shower) is electric.

My boiler is a GlowWorm SpaceSaver RF with a programmable timer (water or heating+water), no thermostat valves on radiators and a thermostat in the entrance lobby. It's an old boiler, but probably not worth replacing as it still works fine.

Boxman

Your maths is basically correct though extrapolating 1 days useage to a whole year and quoting the result to 10 decimal places is fairly pointless I would respectfully suggest. The simplest way to convert cu mtrs to kWh is to multiply by 11.15 - in practice this can vary a little bit but should be close enough for your purpose.

Yes, on the basis of 24 hrs monitoring you will use about 12,200 kWh pa, a little bit lower than the National average, but of course your average daily use will vary wildly according to the season.

MothballsWallet

Thanks for your reply, Boxman.

The actual sheet doesn't display to 10 decimal places, just 2: I just wanted to do the calculations based on 1 day's usage to check my formulas.

I just copied across each calculation step's value and pasted it in without rounding anything because, if anything, I'm a lazy old git

zagfles

That's low usage for the winter. You'll use nowhere near as much in the summer so you can't estimate annual use from a single day in the winter.

From my readings I use about 6 times as much gas Oct-April as I do from April-Oct.

MothballsWallet

I've done other calculations to work out weekly and monthly usage as well, but I just wanted to sanity check that I was using the correct maths to convert cubic metres to kWh considering there are a few steps to the calculation, and I'm getting on a bit, don'tcha know?

scaredofdebt

If you want to be really accurate/pedantic, to convert cubic metres of gas to KWh the equation is:

M3 x 1.02264 x Calorific value of gas / 3.6

The 1.02264 is the correction factor as gas isn't inert ie it's affected by atmospheric pressure etc and the calorific value is the "burn quality" of the fuel as that can vary. This figure is quoted on your bill and it is usually around 39-40 but you don't know it until the bill is produced.

Asleep yet?

But as Boxman says, just multiply the cubic metres by 11.15