Yeah my heat pump is way overkill for my garage, its fully insulated (under the house) and insulated garage doors. It used to be installed in the house but I have since moved to a whole house hybrid heat pump+nat gas modulating 98% furnace so it was a leftover. It barely runs to keep the garage cool once it reaches the set temp. The nicest part about it being overkill is when you open the doors and pull in a hot running car it can quickly get the whole room (and car) back down to pleasant temps in barely 10-15 minutes.

I have no idea of a accurate way to determine the savings aside from taking numerous temp measurements every day and isolating the Daikin from all the other electrical item I use but from when I got it installed (end of July) to now it seems there is a savings.

Usage in May to Jun was 2679 kWh Avg temp 87 Humidity 59%
Jun-Jul was 2841 kWh Avg Temp 90 Humidity 51%
Jul-Aug was 2313 kWh Avg Temp 86 humidity 58%

In July we had a week or so with several days over 100 (some were 110). Jun was also a pretty hot month for us. Aug seems to have been similar to Jun.

So, all that said, seems like the Daikin may be saving me some money and it is also keeping the sunroom much cooler while costing less. Had I kept it at the same temp I suspect the cost savings may have been greater.