Gas heater running cost calculation

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eracode

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#305909 13-Jun-2023 08:07

I am wondering how much our Rinnai mains gas in-wall heater costs to run but unsure how to calculate it.

Specs show that input is 8 to 31.5 MJ/h and output is 1.8 to 7 kW.

Our Genesis Energy bill shows that the gas price (ignoring daily static charge) is 8.7 cents/kWh.

I guess the calculation of maximum hourly running cost is just as simple as 7 kW x 8.7 cents = $0.61? Not sure if (or where/how) the input figure is relevant and what it means.

Sometimes I just sit and think. Other times I just sit.

timmmay

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#3089265 13-Jun-2023 08:54

Could you work backwards from your energy bill to get a per day price that's good enough?

eracode

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#3089266 13-Jun-2023 08:56

timmmay:

Could you work backwards from your energy bill to get a per day price that's good enough?

That won’t tell me the running cost of the heater alone. We also run water heating and cooking hob on gas. Of course I can easily work out the average daily cost of all gas usage but that’s not what I want to know.

Sometimes I just sit and think. Other times I just sit.

mkissin

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#3089269 13-Jun-2023 08:58

Its input power is what your gas meter will see, as that's the gas consumption (but in energy terms). The output will be the heat generated. The difference is all the other crap...leakage, water, general emissions, etc.

8 MJ/h is (multiply by 1 million, and divide by 3600 seconds per hour) 2.22 kW, so that gives you a total efficiency (1.8/2.2 = 82% on low), and your gas usage.

Same on the high setting => 8.75 kW in, 7 kW out, gives 8.75 x 8.7 = 76 cents per hour running costs on high, and 80% efficiency.

edit to fix formatting.

eracode

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#3089272 13-Jun-2023 09:05

@mkissin Excellent answer - many thanks. Not quite as simple as I thought.

Sometimes I just sit and think. Other times I just sit.

Mehrts

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#3089301 13-Jun-2023 10:56

Without any other gas appliances used:

1. Take a gas meter reading.
2. Run the heater for a set period of time. One hour will be easiest to calculate.
3. Read meter again.
4. Subtract first reading from second, that's the volume of natural gas you used in metres cubed.
5. Multiply this number by 11.44754, which converts to kWh. This figure should be represented on your gas bills as a "conversion factor" This is set under NZS 5259.
6. Find the kWh pricing on your bill (unit cost).
7. Multiply the kWh number you calculated by the unit cost. This is how much it actually costs you to run that appliance for one hour.
8. Repeat for different heat settings on heater.