Sun getting through the windows: if you don't have solar protection on the windows (blinds, louvred shutters, etc) then the sun can pump close to 1kW/m² of incident surface into the room. In winter, this is free heating and reduces your bills. In summer, it's the opposite.

Heat leaking through the insulation: this depends on deltaT across the insulation. In winter, with 20°C indoors and 0°C outdoors, deltaT is 20°C. However, in summer, sun baked roof tiles can reach 60-70°C. If the loft is well ventilated has air temperature close to outdoors air temperature, good. However, if temperature on the outside face of the insulation reaches 60-70°C because it's right under the tiles and not ventilated, then your deltaT can be 40-50°C, which means heat leaking through the insulation is 2-2.5x more than in winter.

Dehumidification: some ventilation flow is necessary to keep the house healthy. Depending how airtight your house is, more or less outside air can get in. This outside air is hotter and usually has higher absolute humidity than indoors conditioned air, which raises absolute humidity indoors. So the AC has to condense all the humidity brought in with outside air.

Each liter of water condensed delivers 0.68 kWh of heat to the AC evaporator, and the AC has to pump this heat outside.

For cooling a whole building, or even a big room with lots of windows you definitely need to deal with the sun and the windows before you try and fight it with air conditioning unless you particularly enjoy burning money. Ditto to some extent a room in roof where putting solar on top of the roof (but not in it)) works wonders and is about the only parasol you can get as permitted development.

I think that this is very much in @ashp-bobba's lane.

@clockworks Hi, So there is a big difference between steady state heat loss and dynamic heat loss / gain, rather than bore you with the details we can say they are not the same and give you a solution. Take the size of each room and X 95w and you would be very close to the correct loading of a room with small equipment like TV or computer and a reasonable window of say 1.5m2, if you have very large windows or bifold doors and they face south add 240w/m2 per window.

1 example is a 12m conservatory needs 14kW but the same size room is only 2kW, its all about the glass and fabric insulation.

You do not need the height of the room the 95 accounts for up to 2.5m high

Example:

Room 1 = 4 x 3 =12. 

12X95 = 1140w (or 1.14kW) - unit selected will be a 2kW

Room 2 = 6 x 4 = 24.

24X95 = 2280w (or 2.28kW) - unit selected will be a 2.5kW 

Room 3 = 4 x 3 =12. 

12X95 = 1140w (or 1.14kW) + 3m/2 additional glass = 720w - unit selected will be a 3kW

Room 2 = 6 x 4 = 24.

24X95 = 2280w (or 2.28kW) + 3m/2 additional glass = 720w - unit selected will be a 3.5kW

The above is way over simplified are there are around 30 other factors to take in to account but its a very rough rule that covers most, you would be better off getting a certified dynamic heat calculation but no one pays for them or really does them on AC, its just always been that way and it is mostly the grant that made everyone do it for ASHP heating as I don't ever remember anyone calculating anything for boilers or heating system on domestic before. In commercial its always done for both heating and cooling as there has always been design contracts.