I'm looking at a 40KW (continuous) Kubota Diesel generator for whole house backup in a sustained power outage of a week or two.
Everything in my house is electric, including the heat, and the house is large. The heat is an efficient geothermal pump though, so I'm wondering if feeding this off the generator is more efficient than a secondary boiler. There's no natural gas available or propane on site.
By my calculations, running at half load: 20KW consumes 1.7GPH (per Kubota), $3/gallon = 25c/KWh (vs 12c/KWh electric)
1 gallon diesel = 137,452 Btu ~= 40KWH. So I burn 3.4 gallons of diesel / gallon of diesel energy ~= 30% efficient
However, the heating COP of the Waterfurnace 5 ton heat pump is 4.5.
The way I understand this is approximately I'll get back 0.3 * 4.5 of energy for a total efficiency of 135%. I appreciate there are other small losses in the system, both manufacturers are probably exaggerating a little etc but still it seems better than running a diesel boiler (~90% efficient?) to heat the house.
Is my reasoning correct, or am I missing something?