20 years day number HSPF levels power bills wind energy other device startup costs Long lifespan minimum rating cooling supply cooling devices supreme comfort low maintenance inner operations primary benefits cooling solutions electrical heaters conventional devices exterior temperature heating organization Air Source Heat Pump commercial properties geothermal heat pumps relative affordability majority other heating energy alert households Minimal power consumption long-term energy suspension essential little maintenance high energy efficiency grade extensive electric consumption Season Energy Efficiency Ratio Heating Season production Factor