Keep in mind that heating water by electricity is a lot more expensive (at least historically) per litre than using gas. Last time I estimated this for a 8.5kW 10 minute shower it was 73p vs 40p assuming the flow rates were equal. So, with two people each taking a daily 10 minute shower the electric option cost £241 / year more to run.
Please check my figures, the above are from memory and YMMV.
Please check my figures, the above are from memory and YMMV.