Calculate G coefficient with fioul or gas consumption

POWER OF THE MACHINE:
This formula is empiric: we use the G factor
We don't care about a shape factor. We suppose that the surface of the bulding is proportional to the volume
B(W)= G (W/m3.°C) x V(m3) x deltaT (T ambiant - T base)

We are looking for the G factor, so we need to find the power of the oil or the gas boiler with the annual consomption
G (W/m3.°C) = B(W) / ( V(m3) x deltaT (T ambiant - T base) )

PCI means: lower heating value (LHV)
It means that we don't use the power in the steam
The unit is J/Kg or kWh/kg
The number of kilogram of fioul (oil or gas) = Power(kW) x DJU(°C) / deltaT(°C) x kW/kg
"periodical" and "efficency" are two opposite factors: the use of the building and the efficency of the boiler
The consumption is:
Conso. (kg) = ( 24(h) x B(kW) x DJU(°J) x periodical (%) ) / ( deltaT x efficency (%) x PCI (kWh/kg) )

We extract the power of the boiler from the previous formula:
B(kW) = ( conso.(kg) x deltaT(°C) x efficency(%) x PCI(kWh/kg) ) / ( 24(h) x DJU(°J) x periodical (%) )
B = (1000 x 25 x 0.7 x 1000) / (24 x 2200 x 1 )

We put the previous formula in the first one:
G (W/m3.°C) = ( conso(kg) x efficency(%) x 1000 x PCI(kWh/kg) ) / ( 24(h) x DJU x periodical (%) x V(m3) )

DeltaT has disappeared. The calculation is given for a internal temperature of 18°C

When you decrease the efficency coefficient (poorer machine), you can see the G coefficient decreasing (good insulation): that's correct !
It's because you get the same looses with a worse machine, so it means the insulation is very good.

In the second part of the calculation, we need the "deltaT" to calculate the Power needed.

Enter the surface of the building in square meter e.g: 100 m2
Enter the height of the rooms in meter e.g: 2.5 m
Enter the local DJU e.g: 2200 °J DJU
Enter the rate of use for the building e.g: 1 (always occupied)
Enter the rate of efficency for the boiler e.g: 0.7 (old boiler)
Enter the calorific value (PCI) of the fuel e.g: 10 kWh/l 5kwh/kg
Enter the consumption of the fuel e.g: 2000 l 2500kg
G coefficient:
Enter the local base temperature e.g: -7 °C Base Temperature
Powerful needed (W):