Gas califont AND hot water cylinder?

We're off grid solar and in the middle of our build. We're currently using a gas califont for hot water. I've been offered a hot water cylinder with a pressure booster pump. I'd like to install it so that when we have the electricity, the solar heats the hot water but if there is not sufficient solar power or we run out of water, the gas califont will kick in (or I can turn it on manually). Does anyone have any ideas on how to do this or even if it's possible?

So would the Califont be tied to a separate water source?

Also, using a solar water heater can be done with little to no electrical input needed. You can use the sun's power directly and the power of convection to create a siphon!

Check out this surprisingly helpful link from energy star

https://www.energystar.gov/products/water_heaters/water_heater_solar/how_it_works

However if you wanted to use the collection tank and pump with the California as a back up during times of no power or water. I would probably try to make it as easy as possible and just use the gas instant heater for the guest bathroom  and your off grid setup for the primary then you only have to turn on the gad when you have friends over that aren't familiar with your systems and it's nuances or in times with lots of clouds and little rain as a back up. This will allow you to just go turn it on for a shower and you can use the cold water plumbed in to flush if you can't wait on a primary bathroom to open up and it will be less confusing that trying to set up and automatic switch that turns on gas when you don't have power in your bank and having to remember to change the valve over to the new system.

Good luck I hope someone with radically cooler ideas pops in here and shares something better!

Thanks Clay Bunch. I probably should have explained the system more clearly.

This is a temporary solution until the house is finished. When the house is finished we'll have a wetback and mega hot water cylinder. I think I've got that part under control.

Until then I have a single gravity fed water source. I had imagined I could feed the water through the hot water cylinder and then through the califont. The califont would only be switched on if there is insufficient solar power to heat the hot water cylinder.