Daily Archives: September 16, 2011

Cost of the heat pumps sitting unused for a year

Our Mitsubishi mini-split air source heat pump seems to have a coil heater thingy in the outside compressor unit (I’ve commented on it before here) which runs year-round, even if it’s not cold outside. Seems to hover around 55 to 61 W when it’s on and goes on and off every 1/2 hour roughly. So that’s 28W on average let’s say. On 24h * 365days so…

(28 * 365 * 24) / 1 000 = 245.28 KWh

((28 * 365 * 24) / 1 000) * .15 = $36.79 assuming 15cents/KWh

To put this in perspective, recently I wrote that our actual yearly output from our PVs is currently 8230 KWh (which btw is close to the new england rule-of-thumb of 1.2*6.9KW rating), and since we have 30 panels, that is

8230 / 30 = 274.3 KWh / panel.

In other words… we are using almost a whole panel on our roof to do nothing but keep the heat pumps sitting idling when they are unused. (“net” of course. since we are grid-tied with no batteries for night and clouds)

Don’t get me wrong, I think air-source heat pumps are great and definitely an interesting choice vs a boiler or furnace and AC. I am sure they are continuing to make them better and better each year. The “hyperheat” h2i models from mitsubishi work well way down to something like -5F, whereas ours I think start limping at more like 5F I believe. -5F. That’s sweet. I wonder if there is a catch. I assume they operate more efficiently at more reasonable temps too. I will have to peruse the curves in the service manual PDFs online to see.

See also:
Marc Rosenbaum — Out with the old, in with the new


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