A much better way to do this is to get an electric tank water heater, remove the heating elements, and wire the thermostat to run a pump on a loop to the tankless heater. Incoming cold and outgoing hot are from the tank iteslf.
This way, the standby losses are that of an electric tank, which is less than a gas tank due to the lack of a flue down the middle. An advantage over tankless only is that the delivered hot water pressure is higher, because the pressure drop from a tank is noticeably less than from a tankless.
When I get around to installing solar hot water, this is probably the way I'm going to go; the solar can be on another loop from the tank.
Cheers, Wayne