CSLB C-46 Solar ContractorPhotovoltaic System InstallationHard
A solar installer is sizing the array for an off-grid PV system that needs to power a continuous load of 1500W at 24V DC for 8 hours per day. The system will use 300W PV modules, and the site has an average of 5 peak sun hours (PSH) per day. Assuming a system derate factor of 0.7, how many 300W PV modules are required?
- A12 modules
- B8 modules
- C10 modules
- D14 modules
Show answer & explanationAnswer & explanation
Correct answer: A. 12 modules
First, calculate daily energy demand: 1500W * 8h = 12,000 Wh/day. Next, calculate required PV array output considering losses: 12,000 Wh/day / 0.7 (derate) = 17,142.86 Wh/day. Then, calculate the required array wattage: 17,142.86 Wh/day / 5 PSH/day = 3428.57 W. Finally, calculate the number of modules: 3428.57 W / 300 W/module = 11.43 modules. Since you cannot have a fraction of a module, round up to 12 modules.
Why the other options are wrong
- B. This option is too low and would not meet the daily energy demand after accounting for losses.
- C. This option is too low and would not meet the daily energy demand after accounting for losses.
- D. This option is an overestimation and would result in an oversized and more expensive system.
Off-Grid PV Array Sizing
Sizing an off-grid PV array involves calculating daily energy demand, accounting for system losses (derate factor), and determining the number of modules needed based on peak sun hours.
- Daily energy demand (Wh/day) is the starting point.
- System derate factor accounts for all losses (temperature, wiring, inverter, dirt, etc.).
- Peak Sun Hours (PSH) represent the equivalent hours of full sun irradiance.
- Always round up the number of modules to meet demand.
Memory trick: Demand, Derate, PSH, Modules: Don't Forget Power System Math!