Power = energy flow per unit time. So you can talk about radiation output, radiant flux as it's called in physics, in terms of watts: 683 lm of light at 555 nm wavelength equals 1 joules/second (=1 watt) in radiant flux[1]. 20 kW in light output would be about 14 megalumens.

[1] http://www.dfisica.ubi.pt/~hgil/fotometria/HandBook/ch07.htm...