When evaluating low voltage transformers for this type of loading consider the newer NEMA TP-1 designs.
According to a Square D publication "... the TP-1 energy efficient unit has less energy loss at 35% loading than the [normal] low temperature rise units. The low rise units have less losses at 100% loading. The conclusion is that the low temperature rise units show more savings at higher loading. Again, statistics show that the average loading of a transformer is typically 35%..." A NEMA TP-1 37.5KVA single phase unit is designed to have an effciency of 98.2% at 35% load.
