AWTZ14: 6kw heat (max 9kw), 4.44 COP (4.44 units of heat per unit of electricity), $3000
AWTZ18: 6.7kw heat (max 9.7kw), 4.1 COP, $3350
Both are the same size, indoors and out, and either will heat my the area no problems. According to the sales guy the larger one will be better if we want to leave the hall door open and use it to supplement the lounge heat pump, which is a 7kw Daikin which heats the lounge and bedrooms. The Daikin is slightly undersized, it should really have been a 9-10kw model, but the firm who put it in underestimated the size required.
Smaller heat pumps are more efficient than large heat pumps, unless they're overworked, as the COP (Coefficient of Performance) above shows. I don't know how much difference the 6.0 or 6.7 will make.
We don't use the kitchen/lounge as a living area very often, though we may start to use it more this winter. In winter we'd probably have it come on for 20 minutes in the morning so we don't freeze getting breakfast, and the same before we come home. We're also doing up that area as a second living area, it's much brighter than the other living area.
Thoughts on which one I should get? I'm leaning toward the more efficient, slightly smaller, cheaper unit.