When it comes to Off-Grid System Sizing, the most important and common questions every system designer faces are as follows:
- *How big of a solar array should I install?
- *How big of a battery bank should I install?
These questions may seem difficult but can be answered by using simple arithmetic operations in the color boxes below. Solar array sizing will requires one to first calculate the total energy (in watt-hour or Kilo-watt-hour) first and divided by the number of peak sun hours on site. Battery bank sizing also starts the same way with figuring out total energy requirement first, but will also need to consider total back up days in case there’s no sun as well as the maximum DOD (depth of discharge) on battery.
Solar Array Sizing Calculation
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)
Battery Bank Sizing Calculation
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)
