We are looking to upgrade from electric  bb heat to a mini split with both air & heat. Location Langley BC.
We live in a in law suite above a garage and 2rooms beneath us. So the mini split will heat only the 2nd floor open concept kitchen& family room including a small 8×9 bathroom, 

Total square footage  about 800. There is a 120 square  foot bedroom which we don’t  heat ,it stays at about 58-65 F during the  winter

So instead of a manual J, I have an efergy meter installed in rhe sub panel and I have been monitoring  my hydro for the past 2 years.

I have 2 -500 watt bb &1 1250watt bb (2250×3413=7679 btu) to heat the upstairs.
This is enough to heat the upstairs unless it gets  really cold15-20F , then I need to supplement  with  a portable 1500 watt heater to get the night  time set back temperature from 62  to 68 F.

The mean temperature in January and February is 33F night  and 36 F days.
It rarely  goes much below freezing here. 
When looking at internet web sites  for mini split sizings they recommend  9000 -12000 btu for 400 square  feet  18000 for 800 square feet. 
Am I missing something  or will a 9000 btu or even a 12000 btu model heat our square  footage  in this milder validate?

Am looking at a gree mini split: 9000 btu model specs:cooling  cap 1535-12966 , heating 2388- 13648, seer 38, hspf 15.

Gree 12000 btu cooling cap 3100-13000, heating 3071- 18766 , seer 30,5 , hspf 14.

10 year parts & compresser warranty.

I think either  one would  work without  short cycling.

Any thoughts ?