Is the RTG20006DW a tank or tankless heater?

It is a tankless gas water heater. It heats water as it flows through the unit rather than storing it in a tank.

Can it run two showers at once?

It is sized for light demand, such as one bathroom at a time. Running two hot showers plus an appliance at once can exceed its capacity and lower the temperature.

Does it need electricity?

Yes. Like most tankless gas heaters, it uses household power for its controls and ignition, so it needs an electrical connection as well as gas.

Where can it be installed?

It is designed for indoor wall mounting with proper venting. A licensed installer should confirm clearance and venting for your space.

How does it compare with a tank heater?

It saves floor space and avoids standby heat loss, and it does not run out of hot water. A tank heater costs less to install and can serve several fixtures at once.