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. 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:
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:
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):
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:
For additional screen grabs see links in the comments below.