Nerd alert: heat pumps vs. oil-fired hydronic baseboards analysis

We are heading for a cold snap this week, with a low of -15C forecast for Tuesday. What temperature does it make sense to turn up the thermostat so the oil-fired heat takes over from labouring heat pumps?

Well, I'm glad you asked!

I looked at our oil consumption for 5 years before we installed heat pumps. In the non-heating summer season, we used oil to heat just domestic hot water, to the tune of about 2.8 liters per day. We used a measurable amount of oil annually, so taking 2.8 * 365 from that total gives the annual oil consumption for heating only.

The annual heating degree day total here is 3600, meaning that the daily outside temperature is, on average, 3600/365 (i.e. 10) degrees lower than our desired home interior temperature. My outside temperature sensor agrees, maybe a bit warmer.

Oil is 10.7 KWh per liter, so (mutter, mutter) heating our house with oil consumes 4.48 KWh per heating degree day, or 186 watts per degree difference between inside and outside.

I'm measuring the power consumption by the heat pumps every few seconds, and the outside and inside air temperature. Home Assistant can thus calculate the instantaneous power consumption per delta T for the heat pumps.

It's getting up around 100 watts / delta T. So still much more energy efficient than oil. Oil costs about 13 cents per KWh last time we bought a winter fill up. Electricity is 18.5 cents. Heat pumps still win! The crossover is when the heat pumps are consistently using about 135 watts per deltaT. 

I should add an alert to let me know when to start sipping on oil. I don't think it gets cold enough here. 

Incidentally, I am thinking it makes sense to turn down the heat pump set point overnight, which goes against common lore. I'll ponder this a bit more, but first impression is they recover to our daytime setting in less than an hour, and we use significantly less energy over night.