Firstly, the comments made by others are correct. The power used by each of the components listed will vary on a component by component basis and even on an installation by installation basis. The power used by each component will also vary depending on the speed that it is running at. Also the number and type of components will vary from car to car.
With that out of the way, here are some numbers that could be used for rough approximations.
The power used by the tensioner will be minimal and probably negligible. It will depend on how good the bearings are, but if it was using much energy at all, all that energy would be being converted to heat (or possibly noise). So tensioner can be pretty much ignored I think.
According to Wikipedia's Airconditioner Article
In an automobile, the A/C system will use around 5 horsepower (4 kW)
It is marked as "citation needed' though so take it with a grain of salt, but it would be approximately correct for when the aircon is activated. Remember that aircon is electronically clutched so that it's not always on and therefore not always using that much power.
According to Zena Incorporated
... a 150 Amp alternator, operating at full output, the expected diesel engine load would be about 7-8 hp
Like all the others this number should only be used as an approximation and note that a 150 Amp Alternator is a pretty large alternator.
Also note that this figure is when 'operating at full output' it's pretty rare that an alternator is ever running at full output.
According to Davies, Craig FAQ Question No 19
Davies, Craig performed a number of tests which concluded that a normal water pump uses up to 10kW of power to operate at its high speeds.
Be aware that Davies, Craig sell replacement water pumps that require less power to run so assume the numbers are only approximate.
It's also worth reading through Question 2 on the same page where they describe how the power pulled by a mechanical pump that runs from the drive belt will increase as the cube of the operating speed. So if power draw at 1000rpm is 0.1kW at 2000RPM it will be 0.8kW and at 4000rpm it will be 6.4kW.
According to an EE Times article by Dave Wilson
...replacing your three- to five-horsepower steering pump and its
associated bulky hydraulics with an electric motor is one enhancement
that actually improves performance
Be aware that he's selling the virtues of replacing your power steering pump with an electric one, so assume the numbers are only approximate.
So to summarise
- Tensioner: ~0 kW
- AC: 4kW (5hp)
- Alternator: 5-6kW (7-8hp)
- Water Pump: 10kW (13hp)
- Power Steering: 2-4kW (3-5hp)
Bear in mind that all these values are approximate to start with and they are for max draw, an AC will draw close to nothing when it's not active, as will an alternator. Traditional water pumps will draw more power the faster they are spinning (so the higher the engine is revving). I'm not entirely sure about Power Steering pumps but I believe that they would only draw max power when being fully utilised.
The references I've used here are by no means 'definitive' so if anyone finds any references more appropriate then by all means edit this answer to improve it.