A standard electric water heater requires a 30 amp double pole breaker. These are basically two 15 amp breakers in their electrical panel. In some older homes, only a single 15 amp breaker may be available.  Installing a 30 amp model on 15 amp service would overload the breaker. It also does not allow it to run properly due to a lack of electricity.
For this reason and to give ProTerra more versatility a 15 amp model is available. The main difference is the downsized elements. The 15 amp model uses 2,500-watt elements instead of the standard 4,500-watt elements in standard electric water heaters.  The first-hour rating of the 15 amp model is 54 gallons per hour. On the other hand, the 30 amp model's first-hour rating is 67 gallons per hour.  The recovery time for the 15 amp model is 16 gallons per hour compared to 27 for the 30 amp model.
As you can see by the highlighted section of the spec sheet, the first-hour hot water delivery and recovery rate is much less on the 15 amp than the 30 amp because of this.
It’s been many years since homes were built without having a 30 amp service ran for their water heater and is pretty rare to come across.
The 15 amp variant became available during Rheem’s Gen IV launch in 2017.
If you are replacing any standard electric water heater, chances are that it would be a 30 amp model. If you had any doubts, the only way to verify would be to look at the electrical panel and see what size breaker is present for the water heater circuit.
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