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.