A power shower should use very little electricity. It is heating the water for the shower that may do so. Is it from the immersion tank? Is it well insulated? Do you need to switch on the immersion heater for the shower or is water heated from the central heating system, backboiler, etc?Check the wattage of each and that should give you a good idea of which is the heavier user of electricity. E.g. a 1kW device run for an hour uses one unit of electricity (ignoring thermostatically controlled or < 100% duty cycle devices for simplicity).
cheaper to use a power shower for 5 minutes