Yes - even though you may have an electric water heater now - the wiring will not be sufficient for an electric tankless water heater. An electric tankless takes typically 240V with 30amps - an electric tankless typically takes 240V with 100-120amps. Often the electricity that they require is equal to the entire electrical capacity of a standard home- which often makes them energy hogs. You would need to consult an electrician on if you exiting electrical panel can support this - or if you would need a service upgrade.