The word retrofit may sound technical or a little dated, but the meaning is simple. A retrofit bulb is a replacement bulb designed to fit a light string or socket you already own.

For example, you can remove older incandescent C9 bulbs from a compatible stringer and replace them with energy-efficient C9 LED bulbs. You keep the existing wire and sockets while upgrading the bulbs.

Understanding Bulb Base Sizes

The bulb's base is the threaded metal portion that screws into the socket. You may see it called an Edison screw, but that is simply industry jargon for the familiar screw-in base used on typical light bulbs.

The letter "E" stands for Edison screw. The number indicates the base's approximate diameter in millimeters.

Important: Bulb shape and base size are different. In bulb names, C means cone or candle-shaped, G means globe-shaped, and S means straight-sided, which is commonly called a sign-style bulb. The number that follows the letter identifies the bulb's nominal size. Because the numbering system can vary by bulb family, it should not be used to determine the socket size. E12, E17, and E26 describe the screw base that must match your stringer. Some bulb shapes, including G50, are available with more than one base size, so always check the product specifications.

C9 bulb on the left and C7 bulb on the right

How to Find the Right Replacement

1. Unplug the stringer and remove one bulb.
2. Look for a label on the plug, wire, socket, bulb, or original packaging.
3. Measure across the threaded metal base. About 12 mm (0.47 in) is E12, 17 mm (0.67 in) is E17, and 26 mm (1.02 in) is E26.
4. Check the electrical ratings. Confirm the voltage and make sure the total load stays within the stringer's wattage and amperage limits.

Retrofit bulbs are intended for stringers with removable, screw-in bulbs. They generally do not apply to sealed mini-light sets with permanently attached bulbs.