The way that i sized my mini split was almost dead on for me . Way i sized mine . I picked 3 days with close to the same temp as i could. I put my kw meter on it. Left it on 70. Then checked the kws at 3 days . That was a day like our average cold night nothing major I checked the main room i stay in. 234 sq ft . Ran it for few days . 162 hours.
heres the figures
234 sq ft
average nights
162 hours 73.95 kw hrs
10.95 kw hrs day heating room
457 watts hour
1560 btu per hour whole room
6.666667 btu per ft per hour
7000 btu per hour whole house
---------------------------------------
So on average nights it keeps my whole house at 68-70 degrees . I used the a kw to btu conversion calculator.
Kilowatts (kW) to BTU/hr conversion calculator
457 watts was 1559.348894 avgd to 1560
And i used a percentage caclulator
Percentage Calculator
Did 7000 what percent of 13000 (btu of the heat kozy kool) Avg 54% of its capacity it has to run on avg night.
Do keep in mind thoughI sized mine for the coldest nights we would see in the teens and keep 560sq ft at 70 degrees. And not work at full blast to maintain it i have the figures somewhere but i will have to find them. The figures i have is the average cold night that i get here. above freezing temps
That i think is the best way to size the mini split from all the calculators out there being it is your house your checking and not someone elses figures..