There are a number of slightly different units for comparing how much light a light bulb appears to put off, but the one that is often used to compare compact florescent (CFLs) and incandescent bulbs, for example, is lumens. Fortunately, most manufacturers of efficient bulbs put on their packages what the equivalent inefficient bulb is. For example, a 13 watt CFL is comparable to a 60 watt incandescent bulb. The best thing is to look at what wattage bulbs you currently use and then go to the store and find efficient bulbs that claim to have similar outputs. You can alternatively use this website
to compare incandescent and CFL bulbs.