Rear Bike Lights
Rather than providing illumination to see by like a super-bright front bike light think of your rear light as a safety beacon. Twenty lumens is a reasonable starting point, but the latest and greatest lights can pump out up to 300 lumens. If you're going to be using your rear bike light during the day the brighter the better. It has to work harder to stand out but be careful not to dazzle other road users at night. Also look for lights with a range of settings, so you can tailor the brightness according to the riding conditions and battery life.