A Variable Speed Pump Battery element represents multiple variable speed pumps that meet the following criteria:

1. the VSPs are parallel with each other (not in-line)
2. the VSPs are sharing common upstream (inflow) and downstream (outflow) nodes
3. the VSPs are identical (have the same pump definition)
4. the VSPs are controlled by the same target node and the same target head.

Parallel variable speed pumps (VSPs) are operated as one group and led by a single VSP, the so-called lead VSP, while the other VSPs at the same battery are referred as to as lag VSPs. A lag VSP turns on and operates at the same speed as the lead VSP when the lead VSP is not able to meet the target head and turns off when the lead VSP is able to deliver the target head or flow.

From the standpoint of input data, Variable Speed Pump Batteries are treated exactly the same as single pump elements that are defined as variable speed pumps of the Fixed Head Type with one exception; number of Lag Pumps must be defined in the Lag Pump Count field.

When simulating a Pump Battery in a transient analysis, the pump battery is converted to an equivalent pump using the following conversion rules:

1. The Flow (Initial) of the equivalent pump is the total flow of all the running pumps in the pump battery.
2. The Inertia of the Pump and Motor of the equivalent pump is the sum of all the inertia values for all the running pumps.
3. The Specific Speed of the equivalent pump is the Specific Speed value that is closest to the result of the following equation: sqrt(number of running pumps) * Specific Speed of pump battery