This Osram QT-ECO 1x18-24/220-240 L is a non dimmable matchbox ballast to run: 1 x 24w T5 fluorescent tube 1 x 22w T5 Circular tube 1 x 15w or 18w T8 Fluorescent tube 1 x18w or 24w Double turn CFL's 1 x 18w or 24w Dulux F CFL's

This product is now obsolete please click on the link below for viable alternative  to 18W  and 24W  

https://www.easy-controlgear.com/products/tridonic-pc-1x18-24w-basic-linear?_pos=6&_sid=199dc1f00&_ss=r

If it is a 21W tube please order the 

https://www.easy-controlgear.com/products/philips-hfmblue121lh-ph-1x21w-tl5-hf-linear-m-bx-blue-ballst?_pos=1&_sid=88515baf7&_ss=r

If you want the same but square 

https://www.easy-controlgear.com/products/qt-eco-1x18-24-220-240-s?_pos=1&_sid=167c82a4e&_ss=r