fuel consumption figures could be useful is if there was a meter recording the amount of power generated. Even with my setup, I am not sure if the 4800W rating of the heater is accurate enough to make an assumption of the overall power drawn.
House meters are not difficult to come by. You have one? Plug in power meters are similarly cheap and easily obtainable, but not perhaps to 5kW.
Get one and use it?
Secondly, use the one you do have.
It should not be difficult to determine the output from your site heater by measuring usage from your mains supply for a very short while. Plug it in, turn it on, allow to get to operating temperature, count revolutions per unit time or time per revolution of disc on meter. Calculate power of heater from results and spec. on the meter. If it is a digital meter, do it for a set time or set power usage indicated. Just remember to turn off the rest of the house while you are doing it. Check it a second time. Job done if both results are compatible..
Now you know the output of the site heater as acurately as you need at mains voltage, you can simply adjust for any small voltage variation on the 'oid.
Regards, RAB