temperature of array (the effective cell temperature) With the standard controller the only sub-system losses was the battery efficiency and the calculations are undertaken using Ah. When using a MPPT the calculations are in Wh and the sub-system losses in the system include: Battery efficiency (watt-hr) Cable losses MPPT efficiency In order to determine the energy required from the PV array, it is necessary to account for all the sub-system losses. The energy required at the battery in Wh is then divided by these three losses to determine the required energy to be provided by the array. A sample calculation with daily energy requirement from the PV array has been shown in annex 6 of this guideline. 2. Oversize Factor If the system does not include a fuel generator which can provide extra charging to the battery bank then the solar array should be oversized to provide the equalisation charging of the battery bank. It is recommended in Kenya that this is 10%. A sample calculation with oversize factor has been shown in annex 6 of this guideline. 3. Derating Module Performance The PV array will be de-rated due to: a. Manufacturers Output Tolerance The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module. b. Derating Due to Dirt The output of a PV module can be reduced as a result of a build-up of dirt on the surface of the module. The actual value of this derating will be dependent on the actual location; some city locations might have a high dirt derating due to car pollution, some coastal locations might have a high dirt derating due to salt build up and some locations might have long periods with no rain to naturally wash the modules. If in doubt, an acceptable dirt derating value would be 5%. c. Derating Due to Temperature A solar modules output power decreases with temperature above 25°C and increases with temperatures below 25°C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the
temperature of array (the effective cell temperature) With the standard controller the only sub-system losses was the battery efficiency and the calculations are undertaken using Ah. When using a MPPT the calculations are in Wh and the sub-system losses in the system include: Battery efficiency (watt-hr) Cable losses MPPT efficiency In order to determine the energy required from the PV array, it is necessary to account for all the sub-system losses. The energy required at the battery in Wh is then divided by these three losses to determine the required energy to be provided by the array. A sample calculation with daily energy requirement from the PV array has been shown in annex 6 of this guideline. 2. Oversize Factor If the system does not include a fuel generator which can provide extra charging to the battery bank then the solar array should be oversized to provide the equalisation charging of the battery bank. It is recommended in Kenya that this is 10%. A sample calculation with oversize factor has been shown in annex 6 of this guideline. 3. Derating Module Performance The PV array will be de-rated due to: a. Manufacturers Output Tolerance The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module. b. Derating Due to Dirt The output of a PV module can be reduced as a result of a build-up of dirt on the surface of the module. The actual value of this derating will be dependent on the actual location; some city locations might have a high dirt derating due to car pollution, some coastal locations might have a high dirt derating due to salt build up and some locations might have long periods with no rain to naturally wash the modules. If in doubt, an acceptable dirt derating value would be 5%. c. Derating Due to Temperature A solar modules output power decreases with temperature above 25°C and increases with temperatures below 25°C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the