How should power sizing be specified for commercial heating elements?

Size commercial heating elements from the actual thermal duty: mass or flow of the medium, start and target temperature, heat-up time, and standing losses, then correct for insulation, duty cycle and ambient conditions. Never size by enclosure volume alone. Give the supplier real water volume or flow, voltage and wiring, immersion length and mounting bore, and require the calculation basis with watt density, sheath diameter, material, and thermostat or protection matching. Accept on measured heat-up time and stable temperature at the specified conditions.