The customer's house in Binghamton, NY uses 5750 kWh per year. The full-sun-hours per year in Binghamton is 1496. The system efficiency is 0.75. Use these inputs to calculate the system size and cost. Here is one formula to get you started: Solar electric size in kilowatts (kW) = yearly electric usage in kWh divided by (full-sun-hours multiplied by system efficiency). In formula form: PV size in kW = load in kWh ÷ (Full-sun-hours per year x system efficiency).