The fundamental difference is the S-12 Imperial Electric Booster Water Heater has a 16 gallon (60 liter) tank, and the C-12 Compact Electric Booster Water Heater has a 6 gallon (23 liter) tank. The S model has a larger footprint, which provides easier service ability and will step load the electricity in the higher kW range, which will reduce the operator's demand charges on their electric bill.