It is sometimes hard to determine exactly what wattage LED light you need to replace your existing lamps.
Typically, halogen and incandescent lamps have a very low luminous efficacy at around 15 lumens/watt. Compact fluorescents are more efficient at around 60 lumens/watt.
However, when comparing these lamps to LED equivalents there are a number of things that need to be considered before making your purchase.
1. Beam angle. If your existing lamp has a 60∞ beam obviously you should choose a LED lamp with a similar beam angle. A narrower beam angle will look brighter but may not spread the light properly. If the beam is wider the effect will be softer and what you are illuminating can be compromised.
2. Colour. The colour of a LED lamp can affect its lumen output. Normally, for an equivalent wattage, cool whites are 10%+ brighter than warm whites.
3. Lumen output. In the real world there is a lot of over-stating of lumen output. For example, most halogens are rated at 850-900lm, but in practice they may only output 650-750lm. The same applies to LED lamps, where unbranded, cheap lamps may be stated at 600lm but only output 400lm. Lighting Matters recommends using well-known branded products from trusted manufacturers such as CREE, Philips, Verbatim and Brightgreen.
4. Colour Rendering Index. The colour rendering index has a big impact on how well colours show up under the artificial light. Typical colour rendering is 80 Ra, but higher colour rendering (90+) will provide much better colours, closer to natural sunlight.
5. Wattage. When choosing LEDs, try not to pay much attention to the wattage and more on the lumen output, colour rendering index, beam angle and colour temperature as each of these characteristics has an impact on the wattage.