Firstly, know your shower type.
Power shower = Thermostatic or mixer type, draws cold water from storage tank and hot water from cylinder. Does not heat its own water. Many models have low voltage pumps built in.
Electric Shower: Heats it's own water on demand. there are two types.
1 Pumped Electric shower: Fed from storage tank and to improve head of pressure a pump is installed within the shower housing.
2 non pumped electric shower; fed from a high pressure supply I.E mains, no need for pump.
Sounds like you may have Electric shower type one. best bet is to replace with newer model of one you have, connections will be on the same side etc.