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I recently looked up how weak the Voyager signals are when they get to Earth from the 22 W transmitters (as of September 2013, it was roughly -245 dBm). But that got me wondering how many watts of power the Deep Space Network (DSN) needs to send signals the other way.

Any attempts at Google searching always resulted in how much Voyager throws our way....

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    $\begingroup$ For anyone reaching this page wondering how much power DSN receives from Voyager1 (the more distant probe), it's currently receiving 9.33x10^-23 kW, or roughly -160 dBm (July 10, 2015) $\endgroup$
    – bmhkim
    Commented Jul 10, 2015 at 17:38
  • $\begingroup$ As described here, Voyager 2 needs more power (75 kW) than Voyager 1. $\endgroup$
    – Hobbes
    Commented Oct 30 at 8:25

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Check the DSN Now page when it will show any of its stations communicating with Voyager 1 (code VGR1) or Voyager 2 (VGR2), select that dish and then expand the side column on the right to show all the data. On small screens (Notebooks) the box on the right bottom corner may not appear, you may need to zoom out before (ctrl -). It will show transmit power under up signal section. I'll update this answer as soon as I see that happen (see below for updates), but it would be in the 20 kW range and more, depending on transmitter used to communicate with it.

I.e. pretty much at the peak power available to it and the transmitter can handle. For example, farthest range probe currently communicated with is New Horizons (NHPC) at distance of 31.86 AU from the Earth (round-trip light time of 8 hours, 49 minutes and 52 seconds), via the Madrid station 63 (their largest dish), transmitting at 20.63 kW:

DSN Now Madrid station 63, NHPC New Horizons

Still a temporary fix while I update with exactly what you asked for, but since I Googled for images a bit, I also found this screen grab showing Canberra station 34 communicating with Voyager 2. It's not as far as Voyager 1 (see e.g. Where are the Voyagers at NASA Voyager), and the screen grab seems to have been made on March 21, 2014, so over a year ago, but it was then still over three times the New Horizons' current distance away from the Earth:

DSN Now Canberra station 34 VGR2 Voyager 2

The station here transmits at the power of 19.08 kW. To compare, this same station is currently communicating with SOHO (Solar and Heliospheric Observatory) that is stationed at L1 Lagrange point between the Earth and the Sun (SEL1), about 1.5 million kilometers away from the Earth or ~ 1% the distance towards the Sun, and is transmitting towards SOHO with the power of 1.82 kW at frequency of 2.1 GHz.

Note that transmit power, unless you're already giving it all you can, does vary and depends on many parameters, including frequency, distance to target and angle to horizon, i.e. how thick of an atmosphere the transmitted signal has to go through, even wind, temperature and humidity in the lower atmosphere as those transmitters do get terribly hot when operating at near their peak power. And the lower the angle, the stronger the Earth's atmospheric effects, including diffraction, refraction and attenuation in general.

Thanks to OP's tracking of the Voyager's Space Flight Operations Schedule (PDF), we've now managed to capture Madrid station 63 (70 m dish, not the 30 m smaller ones) transmitting towards the Voyager 1 at the distance of over 131 AU away (Roundtrip Light Time from Earth of over 36 hours and 24 minutes). Temperature around Madrid, Spain was at the time of transmission over 31 °C at 11:36 p.m. local time (CEST), and nearly no wind (picking up from stall to only ~ 5 km/h), so it shouldn't be too surprising that its transmit power was under 20 kW (19.08 kW) to keep thermal noise down and the transciever below its peak TDP (Thermal Design Power):

DSN Deep Space Network Madrid station 63 70 m dish

Transmit power was slowly increasing as the environmental temperature dropped somewhat and the wind picked up on speed a bit. Dish's elevation is also increasing as it tracks Voyager 1's position, which will also lower atmosphere's attenuation of the radio signal.

For another comparison, here's Goldstone's same size dish giving it all it can (21.44 kW on image, going up to 21.6 kW) during transmission to New Horizons, but the temperature there was at the time of capture 15 °C and a nice breeze reaching 27 km/h:

DSN Deep Space Network Goldstone station 14 70 m dish

For additional screen grabs see links in the comments below.

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    $\begingroup$ DSN63 is currently receiving data from VGR1, I'll add a comment with tx power when it happens.... imgur.com/iM11s4w $\endgroup$
    – bmhkim
    Commented Jul 10, 2015 at 16:59
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    $\begingroup$ @TildalWave Understood that transmissions are rare. I'll at least be keeping an eye out for a while. A detailed schedule with dishes and scheduled time slots can be found at voyager.jpl.nasa.gov/mission/soe-sfos/sfos2015pdf/… for anyone interested. $\endgroup$
    – bmhkim
    Commented Jul 10, 2015 at 23:02
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    $\begingroup$ Just captured a screenshot of a Voyager 2 transmission at 18.3 kW: imgur.com/hVLfO7L $\endgroup$
    – bmhkim
    Commented Jul 14, 2015 at 17:14
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    $\begingroup$ @TildalWave I have a screenshot of uplink to Voyager1: imgur.com/lUXIxab . Shows 18.6 kW, although it's up to just shy of 19kW right now. Thanks for your help/time! $\endgroup$
    – bmhkim
    Commented Jul 15, 2015 at 20:45
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    $\begingroup$ @bmhkim Fantastic! That it's under 20k shouldn't be too surprising considering it's mighty hot (over 31°C at 11 p.m.!) and no wind currently around Madrid. They're steadily increasing transmit power as the environmental temperature slowly drops (19.06 kW 30 minutes later, temperature dropped about 0.4°C during this time) to keep the transciever under its TDP limit. Mostly to keep the thermal noise down, but also to extend its lifetime. But elevation is also increasing & with it decrease in attenuation of the atmosphere, so from Voyager1's perspective 131 AU away, it will get better and better. $\endgroup$
    – TildalWave
    Commented Jul 15, 2015 at 20:57
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I don't know how much they need, but they use a 20 kW S-band transmitter on a 70 m antenna at 16 bits per second for uplink.

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This document contains some technical details:

http://deepspace.jpl.nasa.gov/dsndocs/810-005/101/101E.pdf

In page 12 you will see that the S-Band transmitter has a 400 kW maximum output power. Others have 20 kW max.

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