Can one use 20W fluorescent tube lamps as a substitute to 40W lamps?

I bought a second hand, mini sun bed to sustain a few bonsai trees, this winter, indoor. The sun bed has a build in ballast to power 6X40W fluorescent lamps, thus a 240W system (220V~50Hz). Unfortunately the UV-lamps do not emit the best spectrum needed for cultivation and growth. The lamps I need to replace the... show more I bought a second hand, mini sun bed to sustain a few bonsai trees, this winter, indoor. The sun bed has a build in ballast to power 6X40W fluorescent lamps, thus a 240W system (220V~50Hz). Unfortunately the UV-lamps do not emit the best spectrum needed for cultivation and growth. The lamps I need to replace the UV-lamps with, has to be 590mm in length, but the highest wattage, available in this length is 20W. I plan to keep 2 of the UV lamps in the system. I want to know if I am going to blow the 20W lamps by I replacing the old 40W lamps. If so, why?
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