Sounds like they undersized your installation in order to quote you a lower price. The efficiencies of heat pumps are well documented as are the weather pattern

Sounds like they undersized your installation in order to quote you a lower price. The efficiencies of heat pumps are well documented as are the weather patterns in your area. So, they should/could have known better. Two solutions to this problem: 1) get a bigger/better heat pump. 2) improve you building insulation and figure out where you are losing energy.