A good approximation is to multiply the LED rating by 9. A 4.5 watt LED bulb is equivalent to a 40 watt incandescent bulb. Obviously, you have to compare like with like - one in obscured glass won't give as much light as one with no glass cover. Since the way the light is emitted is different, you'll need a higher rating if the light is mainly reflected off the ceiling, but a lower rating for table lights etc.
The best type of LED lamp is the corncob design. The others have a small corncob hidden inside an obscured glass case. I've only ever been able to find these on eBay. They are common in China, but you need to buy from someone who imports them. Here's a UK source
http://www.ebay.co.uk/itm/E27-E14-G9-G4-24-27-36-69SMD-3528-5050LED-Warm-White-Spot-Light-Corn-Lamp-Bulb-/350970572246?pt=UK_Light_Bulbs&var=&hash=item51b779edd6 Spanish eBay only seems to have offers from Hong Kong and these will take around 3 weeks to arrive.