How much hot water can the RETEX-13 deliver at once?

It is sized for one hot-water use at a time, such as a shower or a sink. Running a shower and a washer together will likely drop the temperature. Check the flow rate and temperature rise you need against its rating.

Does it need a special electrical circuit?

Yes. Electric tankless heaters pull high current and usually need dedicated circuits sized to the unit. Many homes need a panel or wiring upgrade, so have an electrician check your service first.

How is it different from a tank water heater?

It heats water as it flows instead of storing it in a tank. You get continuous hot water within its capacity and no standby loss, but the peak flow is lower than a tank's and the electrical demand is higher.

Where can it be mounted?

It is a wall-mounted unit, so it can go close to the point of use, such as a utility closet or bathroom wall. Keep the clearances and mounting requirements in the manual.

Is it good for a whole house?

It fits a small home or a single bathroom well. For a larger house with several bathrooms in use at once, or very cold incoming water, a higher-capacity unit is the better match.