What is the best BTU rating for a room 6×5×3. The room, bedroom, is on the least sunny side of the house.

The normal calculation is 600 BTU/square meter. So that would be 18000 BTU. Since it's on the least sunny side of the house you might get away with a little smaller, but the ceiling seems high, 3 meters, is it insulated or just bare roof tiles?

Ceiling is about 2700mm. I'm not sure if going for a higher BTU, cost a bit more, would save on the electric bills to off-set the price difference. It wouldn't have to work so hard but there is obviously a break point in those calculations.

Strange thing is that we both enjoy the hot weather but love a freezing cold bedroom for sleeping.

Are you cooling only during the night, or daytime also?

If night only, 600 BTU/sqm is a good baseline.

For daytime usage, add 100-300 BTU/sqm - the higher end if the room gets direct sunlight on windows, or the ceiling insulation is poor.

If you have an ensuite or walk-in closet, add up their sqm and divide by 2 as well (assuming the doors to them are normally closed), so the AC is able to overcome the heat they will introduce when those doors are opened.

If you want delta temps of more than 12°C l (i.e. <25°C inside @ 37°C ambient), you will also need to step up the BTU (50BTU/sqm/1° delta)

As it's a bedroom, additional people and other heat sources can probably be ignored

One more thing, as it seems you're doing energy calcs

The "Label No 5" rules for 2015 are based on SEER (Seasonal Energy Efficiency Rating), rather than EER (Energy Efficiency Rating) used for the 2011 rules, so the ratings for 2015 AC models cannot be directly compared to models from previous years (of which there are still lots). Some manufacturers are also re-rating older units using the 2015 rules, so the numbers look better, and on top of that, some are still labelling the spec as "EER" but then explaining in the asterisk text that it's a seasonal rating.

It's getting messy.

As a rule of thumb:

SEER = EER / 0.9

EER = SEER * 0.9

e.g. the Daikin Smart Inverter R32 18K BTU has an SEER of 15.14, which is ~13.6 in the previous standard.

The normal calculation is 600 BTU/square meter.

The normal calculation is 600 BTU/square meter.

600BTU/sqm is a baseline for a bedroom, cooling only at night. Other applications might need up to double that.