SOLAR SYSTEM SIZING GUIDE
- How big of a solar array should I install?
- How big of a battery bank should I install?
These questions may seem daunting to one without solar or electrical background, but can be answered by using simple arithmetic calculations in the sections below. Solar array sizing (how many solar panels) will require one to initially calculate the total energy (in watt-hour or Kilo-watt-hour) first and divide by the number of peak sun hours on site. Battery bank sizing also begins the same way with calculating total energy requirement first, then consideration must occur for total back up days in case there’s no sun as well as the maximum DOD (depth of discharge) on battery.
W = E ÷ G ÷ Ksys
Where
W = peak wattage of the array required in Watt
E = daily energy requirement in Watt-hours (Wh)
G = average daily number of peak sun hours on site
Ksys = total system efficiency factor which includes efficiency losses in all system components such as PV, inverter, charge controller, battery, and wiring loss. (varies but may use 0.7 as average)
Q = ( E x A ) ÷ ( V x T x Kinv x Kcable )
Q = minimum battery capacity required in amp-hours (Ah)
E = the daily energy requirement in Watt-hours (Wh)
A = the number of days of backup required
V = the system DC voltage (V)
T = the maximum allowable DOD (Depth of Discharge) of the battery (recommended by battery supplier)
Kinv = inverter efficiency—this is 1 if there is no inverter
Kcable = the efficiency of the cables delivering the power from battery to loads (typically 95-97% based on 3-5% loss)