Tankless water heaters

We moved into our first townhouse. My wife a bit appalled by the water heater tank. Apparently water gets cold toward the end of her showers. 🙂

We plan to rent out the second floor of the house so problem will be worst when tenants will use shower as well. We consider to replace out water tank with tank-less heater. There are few options: one big gas tank-less heater in basement or small electric heaters in each bathroom. The second approach sounds nice since tenants will pay for their own hot water. But I am not sure that out 100Amp panel will handle 40Amp water heaters.

Does anybody have any experience with this?