Solar PV Array Panel Count Sizing
Solar PV Array Panel Count Sizing
Sizing a solar PV array starts from the daily energy the system must deliver, then works backward through the site’s peak sun hours (an equivalent number of hours of full-intensity sunlight per day) and the system’s overall efficiency to find how many panels of a given wattage are needed.Skipping the efficiency factor is a common sizing mistake — inverter losses, wiring losses, and panel soiling or temperature derating typically remove 10–20% of the panels’ rated output before it ever reaches the load, so an array sized on nameplate wattage alone will underperform its target.
The panel count is N = E_daily/(P_panel·PSH·η), the daily energy need divided by one panel’s daily energy contribution at system efficiency. where E_daily is the daily energy requirement, P_panel is a single panel’s rated power, PSH is the site’s peak sun hours, and η is the overall system efficiency.
Inputs
E_daily := 20 kWh=7.2e+07 Nm
P_panel := 400 W=400 W
PSH := 5 hr=18000 s
eta_pv := 0.85=0.85
Each panel’s realistic daily energy contribution is its rated power times the peak sun hours times the system efficiency, so dividing the total daily need by that per-panel contribution gives the required panel count.
N_panels := E_daily/(P_panel·PSH·eta_pv)=11.76
Results
A result near 12 panels (rounding up from about 11.8) is a reasonable residential-scale array to cover a 20 kWh/day load with 400 W panels at 5 peak sun hours. Because panel count must always be rounded up to a whole number, the actual installed array typically has a small energy surplus over the target, which is a useful buffer against cloudy days and long-term panel degradation.
Assumptions & limitations
- Peak sun hours is a site- and season-specific average, not a guaranteed daily value.
- System efficiency lumps together inverter, wiring, and temperature derating losses.
- Panel count rounded up in practice; fractional panels are not installable.