LOL Doc,
being teenagers, your kids probably don't use all that much hot water.
Still, when we replaced our old electric Rheem by a 300L solar HWS, we noticed straight away a significant drop in our power bill. And since installation 2 1/2 years ago, the booster hasn't been running for longer than 5 hours in total. Yet, after a sunny day, the hot water comes out scalding - hotter than we've ever had it from the Rheem, which would've cost us between $25 and $35 bucks a month.
Can I be rude and ask how much the solar HWS cost to buy and install? We're running out of roof space for more panels, but have friends with a heat pump HWS that apparently works well for them. I have heard some can be noisy though, so am hoping in a few years the bugs might have been sorted. As our electric system is inside our garage it may last us several years yet and as our total hot water cost is only approx $400 pa, it didn't make sense to me to spend much to replace it - I'll wait until it finally fails. Off peak rate is only 7.92c/kw for us. We mostly shower in the morning, and I wash in cold water - don't know how much difference that makes but we do seem to use far less hot water than the solar HWS sellers quote as the average.