You could, but it would be less than 10W of electricity, so not enough to power a lightbulb. The water tank is insulated, so the water just heats up slowly until it reaches boiling point. That doesn't mean it's a powerful heat source.

So basically if you use up all the hot water in your shower you are going to be in hot water!

Ah I see, so it's about the rate of heating more than the amount of heat it can potentially generate. That makes sense, thanks.