The tankless or instantaneous water heater can do exactly what Syhprum said and since they are all likely to be used simultaneously can add an enormously to the peak load of the building. I don't know if the electricity supply authority in Nigeria dose this but in Australia you can by electricity during periods of low demand for a discount of up to 80%. You can buy storage tanks that have two elements in them. The primary element heats up all the water in the tank and is set to run only during the off peak periods and at the reduced rate. The secondary element only heats 25% of the water in the tank and is run on the normal full price power but is only used if the tank runs out of hot water so you get the best of both worlds.

By using a system like this you are not only purchasing electricity at a greatly reduced rate but spread the load so that you don't have huge peaks when everybody gets up in the morning and wants to take a shower at the same time. You could drop your peak load by as much as 50% and possibly even more so its worth looking at for that reason alone.