Cumulative Solar Insulation

The summation of the measured solar irradiance over a given period of time.

Design Month

The month in which the daily ratio of the energy generated by the array to the energy consumed by the load is minimized. This may not necessarily be the month with the minimum number of sun-hours.

Discharge Head

Vertical distance from center of pump to surface of the water storage tank water or point of free discharge.

Draw Down

When the pump draws water from the bore, the level of the surface may drop depending on the ability of the surrounding earth to replenish the bore. The draw down is the distance from the surface of the water when it is being pumped to the surface level when it is static. This can amount to many tens of feet, and depends on rate of pumping. The higher the rate, the greater the draw down.


The rate at which water is delivered by the pump, usually measured in gallons/minute or liters/second.

Friction Head

There is a loss of energy as water moves through a pipe. The smaller the pipe diameter and the faster the flow, the greater the loss. The pump must be able to supply enough energy to overcome these losses. The Friction Head is the equivalent distance the pump must be able to push water to have enough energy to overcome these losses.

GPM: Means Gallon Per Minute

Low Voltage Disconnect (LVD) Set Point

The voltage set point at which the load is disconnected from the panel.

Open-Circuit Voltage (Voc)

The voltage at the output terminals of a device when no current is flowing in.


The equivalent number of hours of peak (1000W/m2) sunlight received per day at a particular location in the plane of the array.

Total Dynamic Head

The final total head the pump must be able to deliver at the desired rate of pumping, including all the previous heads.


The total amount of water needed daily. Usually given in gallons/day or cubic meters/day.

Plane-of-Array (POA) Irradiance

The solar energy measured in the same plane as the PV array

Pressure Head

If a final discharge pressure is desired, the pump must be able to supply the flow with the needed energy. The Pressure Head is the equivalent feet of head the pump needs to be able to pump to supply this final pressure.

Recharge Rate

The quantity of water per unit of time that replenishes or refills an aquifer.



Solar based Water Supply Schemes

Short-Circuit Current (I sc)

The current flowing between the shorted terminals of a PV device

Standard Test Conditions (STC)

The accepted conditions under which PV devices are commonly rated.

Static Head

Vertical distance from surface of water to surface of storage tank or point of free discharge.

Suction Head

Vertical distance from surface of water to center of pump when pump is located above water. There is no suction head for a submerged pump.