The Calypso smart water heater controller is compatible with electric water heaters up to 20.8A (@ 240V). 

To determine the amperage of your water heater, you can use the following formula:

Amperage = Power in watts / Voltage 

(Ex: 4500 W / 240 V = 18.8 A)

If you are still unsure, you can refer to the Giant manufacturer’s brochure (see specifications table on page 6) or the Rheem manufacturer’s brochure (see specifications table on page 2).