Sizing from an electricity bill
Sizing a PV System
An electricity bill typically reveals information about a customer’s total monthly energy consumption. You can use this information to figure out the size of a solar panel system (PV system) that can cover your monthly energy needs.
Take a hypothetical monthly energy consumption of 500 kilowatt-hours, which is on the lower end for a household in California. Assuming there are 30 days in a month, an average daily energy use value can be reached by dividing the monthly use by 30.
This daily energy use number is important because it helps us understand how big a solar panel system you'll need to generate enough electricity to meet your daily energy requirements.
Next, insolation values are needed. Since a “full-sun’s” worth of incoming solar energy is approximated as 1 kW/m, insolation values reported in kWh/m-day approximate the hours of full-sun equivalent that a location receives over the course of a day.
For a Californian home, the average daily irradiance value is 5.2 kWh/m-day. By dividing the daily energy usage by hours a day of full sun, the power output required by the PV system is calculated.
This would be the size of the PV system required if our system was 100% efficient. However, no PV system is 100% efficient because of derating factors that take into account the inefficiencies of the overall system, such as soiling of the panels and imperfect electrical connections.
According to the National Renewable Energy Laboratory’s PVWatts calculator, a typical derate factor is 0.84. For the sake of this calculation, we assume the derate factor is 80%, or 0.8. To determine the size of the PV system, divide the required power output by the derate factor.
From this analysis, the approximate size of a PV system required to completely offset the average monthly energy usage of a 500 kWh/month home in California would be about 4 kW.
Next up...
PV System Costs
In order to determine financial returns, it's important to have a solid understanding of the basic economics that dictate PV system costs.