The WPG rule goes out the window with small tanks and large tanks. There is a minimum threshold of light required by plants. 3 WPG is high light for a 30 gallon but a 3 watt light on a 1 gallon will grow nothing.
Also, reflectors are a factor. A 23 watt CF screw-in with a poor reflector might put out less usable light than an 18 watt with a good reflector.
A Coralife Mini Freshwater (18 watts) works great, you will have to does Excel if your not running any CO2, and make your own mounting legs for it.
Another option is a Current Satellite 12" 18w (great quality), but you will want to swap the bulb out for a dual daylight. Fits perfectly though.
HC/Glosso will need at least 18 watts on a 2.5 gallon.