i went and looked at the website of the electricity provider for geneva
a water boiler is with VAT 16.36.- francs a month,as for the other cost it varies a lot,but consider that the tariffs are lower over weekends and during the night, so usually the water boilers are programmed to heat themselves up during nighttime when the electricity is cheapest,it's only getting expensive when you switch the heater on manually during day time.
In our flat we have a 300liter water boiler and are five ppl (4 girls/1 boy

) and it's sufficient that all of us can take a shower and i still have hot water for cleaning!
As for the heating,the benefit of electrical heating is you can adjust the thermostats yourself and decide how warm it shall be in the flat,if you have them all the time on top marks of course it'll be very expensive.But given the fact that swiss houses often are very well insulated you might not need to heat a lot.