The DHW heater efficiency calculations for baseline are based on Table 7.8. For
Gas storage water heaters, the size >75,000 btu/h, and <4000 (Btu/h)/gal, the
baseline performance is:

80% Et(Q/800+110V^1/2) SL, Btu/h = 80%*(400000/800+100*130^1/2).
The input Q for the heater is 400,000 Btu/h, and V is 130 gal. So the SL
calculated is 3077 Btu/h.

Since the SL is defined as standby loss based on 70F temperature difference
between stored water and ambient requirements, is it right to calculate the
efficiency by Q-SL/Q? The result is 99.6%, which appears to be so high!

Could anyone show me if it is right or the right calculation for this?

Thanks in advance!

Ying