A2W efficiency and calculating BTUh per SqFt....

@CWHudson said:

"This shows my structure needing between 1 and 2.5 BTUh per square foot to maintain my 70F interior temp."

As you know, that's your electrical usage converted directly to BTU, before applying the COP. If your average COP is, say 3.5, then your actual heat loss was between 3.5 BTU/hr/sq ft and 8.75 BTU/hr/sq ft. That doesn't sound unreasonable to me given your highly insulated, tight envelope and your relatively mild climate.

"an average of 1.1 BTUh per sq ft for every hour of February."

Again, that's before applying the COP. So if your average COP was 3.5, your envelope lost an average of about 4 BTU/hr/sq ft. Again, very good, but believable for your envelope and climate.

For a rough comparison, here in the Boston area, in a 100-year-old house with some envelope upgrades, we lost about 10 BTU/hr/sq ft average, with about 70% more HDD's than you. After correcting for HDD's, your envelope in my climate would have lost about 7 BTU/hr/sq ft, so your much better insulated envelope loses 30% less heat than mine, which is believable.

If you say what make/model heat pump you have, that would help to know what actual COP you are probably getting.

When trying to calculate your home's heat loss, you also have to consider your total electricity used, not just what the heat pump uses. All the other electric energy (fridge, microwave, heat pump dryer, vampire losses, lights) also goes into heating the house. This "wasted" heat energy becomes proportionly larger factor for a tight home like yours. At your low levels, even wasted body heat becomes a nonsignificant factor. If I'm home all day I add about 2.5kwh+ of heat to the house, depending how many trips to fridge I make.

Refrigerators, computers dishwashers, lights, pets, people, solar gains, all add BTUs to the space. With loads in the single digits (btu/sq ft) you barely need a heat source:)

It looks like you did a good job with insulating your house and your heat pump is running well.

How are you measuring electricity usage?

Other manufacturers will provide more information about performance at different temperatures. The key factor is the temperature delta between outside and water temperature. This is a chart I made using Chiltrix performance data. The x-axis is temperature delta and the y-axis is COP.

These are all at 100% output. Generally COP improves as output is reduced, modulating heat pumps tend to have higher COP at their minimum output than their maximum output.

So there's a triple whammy on COP as temperature declines:

1. Lower temperature means higher temperature delta.
2. If you're using outdoor reset, lower temperature means you need hotter water to meet the heating load, which means even higher delta.
3. Lower temperature means higher heating load, which means that the heat pump runs at a higher output, which also means lower COP.

So in your calculations you have to have someway of accounting for the variation in COP with temperature to have any accuracy in measuring output. My heat pump will tell me flow and temperature rise on the console, from which I can calculate instantaneous BTU/hr.

Finally, a semantic note: BTU and Watt are not measuring the same thing. BTU is a unit of energy, Watt is a unit of work, which is energy divided by time. Watts are commonly multiplied by time to get energy, which is where kWh come from. BTU's are frequently divided by time to get work, which is where BTU/hr come from.

So 1W= 3.412 BTU/hr and 1 Wh=3.412 BTU. There really isn't a unit called a BTUh.

100% of the energy in the electricity used by the refrigerator is released into the interior of the house as heat. Where else would it go?

All of the heat that the refrigerator extracts from the interior of the refrigerator gets returned to the house through the condenser coils. All of the heat that the refrigerator extracts from the interior came from the house originally. The net flow of heat from a refrigerator is the electricity flowing into it.

Contrast with a heat pump water heater, which extracts heat from the house and then sends the water holding that heat down the drain, never to be seen again. So it removes heat from the house. A refrigerator isn't sending heat anywhere, it's heat neutral except for the electricity consumed.