For each method there are a quite number of parts to consider, but first I am going to assume that each is being watched using the same type of set.
The UK was a television pioneer, and has had a television transmitternetwork since the 1930s. This used to provide two TV channels on "405 line" television using the VHF frequencies from around 80 masts. From the 1960s this was replaced by a five channel, "625 line" colour system: however these higher frequency transmissions did not reach as far as the earlier system and the network was expanded to the 1,129 transmitters we have today.
The total poweroutput of these 1,129 UK transmitters is 69,877,049W (ERP).
Reception of analogue terrestrial
The main benefit of these high power analogue UHF transmissions is that they are, within reason, easy to receive: all that is required is a Yagi-type aerial, some cheap coaxial cable. By today's standards the receiver (built into all televisions) is stone aged and virtually passive (takes no power).
Another useful feature of these high power analogue signals is that they can be split using simply Y-connectors to feed several televisions, or with low-voltage boosters to supply a whole building from a single aerial. Also, many people have found that set-top and loft-space aerials provide an adequate signal.
Distribution of analogue terrestrial
The television pictures are distributed to the main transmitters using a network of underground fibre-optic links, so today these digital (motion JPEG) digital signals arrive as highly efficient light pulses. The relay network takes the transmissions from the main transmitters and reuses them. Consequently the power required for distribution is negligible.
The digital terrestrial system (Freeview) builds on the existing analogue terrestrial network. By using a digital transmission system (COFDM) the transmission pathways that carried the analogue signals can be used to six to eight times as many television channels.
Currently 80 main transmitters (plus, at the time of writing, three near Whitehaven and one in Wales) carry these digital services. However, as they use frequencies that were left in the analogue network plan to prevent interference between transmitters (in particular during High Pressure Inversion); the interim digital transmissions are very low levels.80 transmitters total power 1,739,289W (ERP)
When the switchover to digital is complete, the network will be fully reconfigured to higher output levels. However, with Climate Change in mind, the levels will be below the analogue network.
1,129 transmitters total power 17,430,885 W (ERP)
Reception of Freeview
These low level signals mean that Freeview reception can be troublesome for people some distance from the transmitter. Set-top and loft space aerials either function poorly, or not at all. To compensate for this, larger rooftop aerials have been required and sometime low-power amplifiers (called masthead amplifiers) need to be fitted.
So, when switchover is complete it should be possible to use unboosted rooftop aerial to supply signals to many televisions in the home as is currently possible with analogue. The transmission system will provide perfect reception at much lower levels than was possible with analogue.
However, many people use a Freeview box for reception (unless you are using an integrated digital television), and these require some additional power to operate the digital reception circuits and send the decoded pictures down the SCART cable. The can also be in the form of a small USB dongle. As the reception circuits are passive, or built into the television, the power levels are almost negligible.
Distribution of Freeview
Freeview is delivered using the same fibre-optic network as the analogue system. When the network is expanded to all transmitters, the relay transmitters will, once again, take their signals off-air from the main transmitters. Again this uses a negligible amount of power.
At first glace, compared to the 69,877 kW of analogue terrestrial output, the 450 W of output from the five satellites that are in the 23.2 East orbital position seems to be highly efficient. Even when this is raised to 30kW to include the total power consumption of these satellites, up there 35,786km above the equator this is entirely supplied by the satellites solar panels.
Approx 125 transponders, 450W.
But this rather neglects the need to get the devices up into space in the first place, and the inaccessibility for maintainance of a device that is hurtling around at 11,300km/h. This means that each device has a lifetime of around 20 years, and there is the small matter of needing a rocket (such as the Ariane 5 or Proton) to launch it.The power required for the satellite launch is estimated at 8,148,682kWH, which given the 20 year life of the satellite. This works out at 7124kWh per week for Paysat and 689kWh per week for Freesat.
Reception of satellite signals
The frequencies used for satellite are much higher than those for terrestrial broadcasts (part of the reason you cannot use a Freeview box to watch Freesat). This requires not only a dish to reflect and magnify the weedy signal (as powerful as light bulb on the moon) but a collector device, the LNB, is required to convert the incoming signals to frequencies that can be carried down coaxial cable.
In addition, to prevent interference between the transponders, each signal is 'polarized' too: the other job of the LNB is to switch polarization. Because of this the LNB must be fed power from the set-top box. If more than one receiver (Sky+ has two) is required, a multiple LNB package is required (usually a four-output 'quad-LNB'), each requiring power.
This output power level explains why satellite boxes tend to run hotter than comparable Freeview boxes. This output voltage drain also means a USB-type device is impossible, and is not built into television sets.
Distribution of digital satellite
The services that are multiplexed and sent to the satellites use a similar low-power fibre-optic terrestrial system to get them to the various Earth Stations (as they are called) around the UK. There are several, such as Madley (on the English/Welsh border), London Teleport (North Greenwich) and one in Southampton (Sky's own).
Very large dishes (tens of meters in diameter) are used and these need reasonably high power levels to ensure the signals can be received for retransmission by the satellites. However, as there are less than 150 signals....
Digital cable television
Cable television mirrors the terrestrial system in many ways. However, instead of using a network of 1074 high power transmitters and aerials, instead a large number of low-power transmitters are fitted in each neighbourhood in boxes called 'cable head ends'. These devices transmit their signals down coaxial cables to each home, where they are attached to boxes that are similar to Freeview boxes.
The analogue cable system (which is still there, you can still watch BBC one, TWO, ITV1, Channel 4 and five by connecting a cable TV cable to an analogue television) was totally passive.
This method of distribution is very efficient and uses low power levels. However, as cables are required to each home, installation is difficult, and now impossible if your home is not 'passed by cable'.
Cable television reception
Whilst in theory, digital cable television reception could be passive and low powered, as with Freeview, in practice Virgin Media use some of the capacity for a return signal to their network. This is used in conjunction with the subscriptioncard to restrict access to services, and it is also used to provide the upload part of their Internet service.
So, rather than simply the head end requiring power, each customer also has to supply power to their connection. This is at low levels compared with the satellite LNB requirements, but cables modems and cable set-top boxes do run hot.
On the up side, the cable TV signals are split using passive devices, so to distribute the service around the home uses no additional power.
Digital cable distribution
The provision of the television services to the cable head ends is, once again, done using ultra-low power fibre optics and has a negligible power drain.
Which is the greenest television?
Putting all the figures together, and making the assumption of 28 hours a week per TV, adding in the transmitter network and share of viewing this gives.
Freeview boxes have a typical power consumption of 10W plus the transmitter contribution makes them use 298Wh per week.
Satellite boxes have a typical power consumption of 30W plus the satellite contribution makes them use 840Wh per week.
Cable boxes have a typical power consumption of 50W plus the satellite contribution makes them use 1400Wh per week.
Analogue TV distribution however averages each set at 400Wh per week.
After switchover, Freeview boxes and the changed digital network is calculated at 343.5Wh per week.