The differences between the T5 and T8 fluorescent lights are reflected in their diameter, efficiency and cost. The diameter of a T5 lamp is five-eighths of an inch, whereas the T8 version features a 1-inch diameter. T5 lamps are more efficient, but they are also more expensive.
As of 2014, the cost of a typical T8 lamp ranges between $1.75 and $2.50, while the price of the T5 variant is between $5.50 and $10.50. T5 lamps are considered to be more efficient, although the advantage is not proportional to the cost difference. Based on several factors, such as LPW (lumens per watt) and CRI (color rendering index), a T5 lamp is only 9 percent more efficient than a T8 one.