On Thursday, January 29, 2015 at 9:13:24 PM UTC-8, gary wrote:
I have 2 water heaters, plumbed in series.

1 is turned off, as 1 is normally adequate.

Would like to be able to turn heater "ON", remotely via internet/wifi,
when I will need more that the usual amount.

What you want, is an on-demand water heater; one that sees
the flow in the hot water outlet and instantly responds. No
internet or wifi required, the hot water tap is the only control
required. They're available, and will save you money by not
having a full-tank leak of heat to ambient air during OFF phase.