These numbers vary greatly from application to application/vehicle to vehicle and are from various sources.
Tensioner: ~0 kW
Actually some resistance (negligible)
AC: 4 bhp
Many numbers much higher (especially on demand)
Alternator: 5-6kW (7-10bhp)
10 bhp full load
Water Pump: 10kW (13hp) Davies Craig number.
6.2 to 6.4 torque
1.1 bhp at 2000rpm, 8.6 bhp at 4000.
Power Steering: 2-4kW
6.8 to 8.3 in torque
2 to 4 bhp
Vacuum pump: 3-5kW
5.8 to 6.5 in torque
3 to 5.2 bhp.
Resulting in a loss of between 16 and 27bhp and of course weight is another consideration.
The following numbers are after subtracting the replacement part weight.
Water pump at 6lbs (2.72kg)
Vacuum pump at 5lbs (2.17kg)
Tensioners at 2lbs (.9kg)
Brackets and bolts 3.5lbs (1.59kg)
Pulley and belts at 2lbs (.9kg)
Air conditioning at 51lbs (23.18kg)
Total of 66.5lbs (28.91kg)
1hp per 10lbs.or 6.65bhp gain or the feel of it.