Hi Kesh the cost of a top ashp like the Daikin Altherma is not cheap but they have just been given the acceptance by the EU that Heat pumps are a renewable source of energy and that the UK are i believe, giving tax breaks or grants towards a new installations.

But as for calculating the correct size the only way is to carry out a heat loss calculation to your property.

If your house has good insulation then the size of the heat pump reducers in capacity.