With 90% of systems on the market your choice here will come down to 2 options for heating your water when there is not enough sun: Instant gas, or electric storage:
An instant gas heater looks like this:
It heats your water as it flows though the heater, on demand, whenever, for example, a hot tap is turned on.
It is a super efficient way to heat your water.
90% of electric boosted hot water systems use a simple element in the hot water storage tank that switches on whenever the temperature drops below setpoint.
Another, much more expensive option if you don't have gas, may be to use an electric heat pump backup.
You can read about heat pumps here.
If your property has natural gas, then I would say that this is a no-brainer: go for an instant gas booster. It is by far the most efficient, clean and cheapest to run system.
If you have LPG: your gas is going to be a lot more expensive, so it may not be quite such a straightforward option.
If reducing your CO2 is your thing then you should know that Electric boosted solar emits about 3-4 times more CO2 than gas boosted solar.
So now you have enough solar hot water panel knowledge to give your installer a hard time, the next question is: Do you actually need a solar hot water panel?