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?

  • 1
    Something is off because you can't have more than 100% efficiency. I don't totally understand what you're doing here, but multiplying that 0.3 * 4.5 can't be right. Also, not sure what COP is.
    – JPhi1618
    Mar 21, 2018 at 19:24
  • 2
    @JPhi1618 You can't have more than 100% efficiency if you don't know what COP is. Mar 21, 2018 at 19:44
  • Apparently you can have snark if you do, though. @JPhi1618, heat pumps aren't described by total efficiency, but another standard that compares work and heat exchange. Link added to question.
    – isherwood
    Mar 21, 2018 at 19:48
  • Ok, well that will be the thing that I learned today. Interesting.
    – JPhi1618
    Mar 21, 2018 at 19:49
  • Even if the calculation is not right, the next question would be what percentage of the time you’ll be running this way, and how many years (or decades) it would take to pay back the cost of a complete backup heating system.
    – Mark
    Mar 21, 2018 at 21:45

2 Answers 2


I agree with your math. Ground sources are quite useful for achieving reasonable heat pump efficiency year-round.

Also, it's possible to scavenge some of the 70% loss at the generator by collecting radiator or exhaust heat. This is called cogeneration and Honda has offered this system for awhile.

  • 1
    Honda utilizes waste heat from a spark ignition generator, but not from a compression ignition generator. Spark ignition engine have a lot more waste heat than compression ignition engines so it is worth the expense of recapturing the energy that would just go out the exhaust, but it is not worth it for compression ignition engines which have a much higher compression (and expansion) ratio. The fact that diesels are much more efficient than gasoline engines means there is much less energy in the exhaust. Mar 21, 2018 at 21:23
  • Intriguing idea. The generator in question is liquid cooled with a radiator and fan, so in theory one could just move the radiator into the house .... In practice, I suspect it's painful with setback requirements, insulation, burying them underground etc ;-)
    – lunix
    Mar 21, 2018 at 21:42

It's a good deal, but a natural gas unit will be by cheaper to run. In your case you can legally use oil #2 (aka off-road diesel aka heating oil) that is the same as diesel instead of road-diesel to avoid the road taxes.

  • Sadly, running natural gas to the property isn't feasible, or I'd definitely use that. I've read mixed reports on using #2 oil - apparently the make up varies quite a bit and it invalidates the warranty. Off road diesel should be fine though (it's a farm). Fortunately the power doesn't go out for long periods of time too often.
    – lunix
    Mar 21, 2018 at 21:35
  • So use red-diesel and it'll be fine (just don't put it in your truck).
    – DDS
    Mar 21, 2018 at 22:14
  • Do not put red diesel in a highway vehicle. The dye is very potent and will take about 10 fill-ups to purge the dye... Mar 21, 2018 at 23:21

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