Passive and active solar technologies helped us meet our goal of a net-zero energy home.
Based on a detailed appliance-by-appliance analysis, including a charging station for a future electric vehicle, we calculated that a 4 kW PV system would take us to net-zero energy. For appliances not yet owned, we found the actual models we wanted and used consumption data from the Energy Star database. We used the actual data for the well pump, air-source heat pump, and the energy recovery ventilator. Then, we padded the load analysis a little for things like data monitoring and logging equipment. Working with a system installer, we decided upon 20 Sharp ND-224 PV modules, each matched to an Enphase M-190 microinverter—4.48 kW total.
We predicted hot water consumption based on our actual usage patterns, and converted gallons per day to Btu of heating energy per day. Our goal was to meet 100% of demand year-round with solar hot water. We used NREL’s PVWatts calculator to determine the solar energy available at our location. Then, using efficiency data from the Solar Rating & Certification Corporation, we determined the system components and size (see Access). For simplicity and to avoid the need for a heat-dump loop for overproduction in the summer, we chose an unpressurized drainback system using three Sun Earth EC-40, 4- by 10-foot, flat-plate selective-coating collectors and a 120-gallon storage tank with dual heat exchangers and an electric resistance backup heater.