Efficiency is still good for air source heat pumps IIRC. They only go down to 100% at well below freezing which is probably not a problem in Hawaii. Ground sour

Efficiency is still good for air source heat pumps IIRC. They only go down to 100% at well below freezing which is probably not a problem in Hawaii. Ground source only requires a small hole though, of the kind that is done for water. I'm in a granite rich area where basements are very rare but water wells are very common and reasonable cost to drill.