Generator Set Sizing and Selection This worksheet is for sizing the generator. Alternator surge ratio - The alternator surge ratio of the generator is typically 2.5. This means that during the surge, the maximum power surge of the generator will be 250% of its continuous power rating. Genset Oversize factor factor - The generator oversize factor is used to size the generator to make sure that its power rating will be sufficiently large to support the load. Maximum ambient temperature during operation - This is the maximum temperature the engine generator can reach during operation. Note 1: Maximum ambient temperature should account for temperature rise within the genset enclosure (if any), as well as climate. Derating factor for temperature - The output of the engine generator should be de-rated for temperature. The de-rating factor is obtained from the manufacturer's specifications. Reference Temperature - This is the reference temperature of de-rating for temperature. It is obtained from the manufacturer's specifications. Derating for temperature - The de-rating for temperature is calculated with this formula: If the maximum ambient temperature during operation is larger than the reference temperature,, de-rating for temperature = 1 - ( max ambient temperatu temperature temperature re - reference temperature)) * de-rating factor temperature Maximum Humidity - This is the maximum humidity the engine generator can reach during operation. Derating factor for humidity - The output of the engine generator should be de-rated for humidity. The de-rating factor for humidity is obtained from the manufacturer's manufacturer's specifications. Reference humidity - This is the reference humidity of de-rating for humidity. It is obtained from the manufacturer's specifications. Derating for humidity - The de-rating for humidity is calculated with this formula: If the maximum humidity is larger than the referenc referencee humidity humidity,, de-rating for humidity = 1 - (max ( max humidity - referenc referencee humidity humidity)) * de-rating factor / 10 Altitude - The is the altitude of the generator. It depends on the location of the generator. Derating factor for altitude - The output of the engine generator should be de-rated for altitude. The de-rating factor for altitude is obtained from the manufacturer's specifications. specifications. Usually we de-rate 3% for every additional 300m above 300m altitude.
Reference Altitude - This is the reference altitude of de-rating for altitude. It is obtained from the manufacturer's manufacturer's specifications. Derating for altitude - The de-rating for altitude is calculated with this formula: If the altitude is larger than the reference altitude, altitude , de-rating for altitude = 1 - (altitude ( altitude referencee altitude referenc altitude)) * de-rating factor / 100 Total Derating factor for Temperature, Humidity and Altitude - The total de-rating factor is calculated with this formula: Total = de-rating for temperature * de-rating for humidity * de-rating for altitude Series System In the series system, the generator charges the battery through the battery charger. Therefore, Therefore, the generator should be sized so that the maximum power ratings of the generator and the battery charger are matched to allow for maximum power flow. Refer to the section on system configurations for more information. Suggested min genset rating rating Minimum generator rating = battery charger maximum power (W)* (W) * generator oversize factor / de-rating factors Switched System For the switched system, the generator can be sized to provide power to the load and the battery charger simultaneously. Therefore, Therefore, the required generator rating is the sum of the battery charger maximum power and the apparent power to run loads while charging. There is also the generator oversize factor and the de-rating factor that must be taken into account. Apparent power to run loads while charging charging - This is the amount of power needed to run loads while the generator is charging the battery. Note 2: Equals some portion of maximum demand, depending on load management considerations. considerations. In this example, the vacuum cleaner, water pump and television are on (i.e. computer is excluded). Load surge while charging charging - This is the amount of power needed to be supplied to the surge while the generator is charging the battery. Note 3: Equals some portion of surge demand, depending on load management considerations. considerations. In this example, the vacuum has the highest surge and the water pump and television are taken at running demand (computer is excluded). Required genset rating (based on maximum demand calculations) Genset rating (W) = (battery ( battery charger power (W) + load power (W)) (W)) * generator oversize factor / de-rating factors Required genset rating (based on surge calculations) Genset rating (W) = (battery ( battery charger power (W) + load surge power (W)) (W) ) * generator oversize factor / surge ratio / de-rating factors
Suggested minimum genset rating - The suggested minimum genset rating is the larger of the required geneset ratings calculated based on maximum demand or surge load. Parallel System - Not applicable in this example For the parallel system, the sum of power from the inverter and the generator must be equal if not more than the maximum power of the load. Minimum genset rating (based on maximum demand calculations) calculations) Genset rating (W) = (maximum load demand (W) - inverter maximum demand (W)) (W)) * generator oversize factor / de-rating factors / 1000 Minimum genset rating (based on surge calculations, derated) Genset rating (W) = (maximum surge load demand (W) - inverter maximum surge demand (W)) (W)) * generator oversize factor / surge ratio / de-rating factors / 1000 Minimum genset rating (based on battery charging requirements) Genset rating (W) = battery charger power (W) * generator oversize factor / de-rating factors Suggested minimum genset rating - The suggested minimum genset rating is the larger of the required genset ratings calculated based on maximum demand or surge load. GenSet Selection A genset is selected based on the system configuration configurationss required. Selected Genset Genset - The make and model of the selected genset. Genset Rating Rating - The actual rating of the selected genset. This is obtained from the manufacturer's manufacturer's specifications. specifications. GenSet Runtime The calculations for how long to run the genset. Genset Run Time for Equalising charge Genset runtime = Equalisation run-on time * 30 / design equalisation period Worst Month Genset Run time required for maintaining full state of charge - If the worst Nominal Solar Fraction for design month < 100%, genset runtime is calculated as follows, otherwise it is 0. Genset runtime = (1 - worst nominal solar fraction) fraction ) * worst design load energy for array sizing * 30 / (battery (battery charger capacity * nominal battery efficiency * nominal system voltage)) voltage Nominal Genset Run Time - The nominal genset runtime is the sum of the genset runtime for equalising charge and genset runtime required for maintaining full state of charge. Best Month
Genset Run time required for maintaining full state of charge - If the best Nominal Solar Fraction for design month < 100%, genset runtime is calculated as follows, otherwise it is 0. Genset runtime = (1 - best nominal solar fraction) fraction ) * best design load energy for array sizing * 30 / (battery (battery charger capacity * nominal battery efficiency * nominal system voltage)) voltage Nominal Genset Run Time - The nominal genset runtime is the sum of the genset runtime for equalising charge and genset runtime required for maintaining full state of charge.
