Its not just the water usage,you need to consider the energy taken to heat the water.
At our old place we had a combi gas boiler,no shower,I used to run a few inches of water for a bath. Yet here our water is heated by oil,so to heat an entire tank of water by oil for me to run a few inches off for a tub is madness

so we all shower now,that uses electric but far less electric energy is used to have a quick shower than my few inches of bathwater if that makes sense? Theres nothing power about our shower either
If your water tank is under the roof another thing worth being aware of is that even on the cloudy days

it will be warmer up there than in the winter when the wind was howling around,so if you use oil/electric to heat water then you should be cutting the time its running now,we have got it on for 10mins n the morning & 10 in the late afternoon(smalls still have a bath~when they can cope with the shower the afternoon 10mins will be offhurrah!)as the pre heated water is warmer it will take less energy to warm it
(have been on the sauce so hope that makes sense
