The answer is different for different situations. It depends on your roof space, energy tariffs and access to natural gas. Does your home have natural gas available? If so, gas boosted solar, 5 Star gas storage or continuous flow may suit. If not, then perhaps an electric boosted solar heater or a Heat Pump might be the answer. Do you have a low off-peak electricity tariff? If so, electric water heating on an off-peak tariff may have the lowest running cost.