The amount of energy that the fly wheel needs to supply depends on the efficiency of the engine and the amount of time it runs for, as well as the power output required. In this case, the engine must run for 30 minutes and develop a maximum power output of 500W.
The energy required is calculated using the equation Power x Time = Energy. This means that the flywheel needs to supply 500W multiplied by the 30 minute run time, equalling 15,000 Watt-minutes. This can also be written as 15kWh.