Sizing an LED strip power supply
LED strips are rated in watts per foot (or per meter). The total power, current and the size of the power supply follow directly:
- Power (W) = length × watts per foot
- Current (A) = power ÷ strip voltage
- Power supply rating = power × (1 + margin)
A margin of about 20% keeps the power supply cool and extends its life.
Example
A 16.4 ft (5 m) reel of 12 V strip rated 4.4 W/ft:
- power: 16.4 × 4.4 ≈ 72 W;
- current at 12 V: about 6 A;
- with a 20% margin: 87 W, so choose a 100 W supply.
At 5 hours a day and $0.17 per kWh, it uses about 130 kWh and costs about $22 a year.
Typical strip power
| Strip type | W/ft | W/m |
|---|---|---|
| SMD 3528, 60 LEDs/m | 1.5 | 4.8 |
| SMD 2835, 120 LEDs/m | 3 | 10 |
| SMD 5050, 60 LEDs/m | 4.4 | 14.4 |
| COB strip | 3–5 | 10–15 |
Always use the figure on your strip’s datasheet; RGB and RGBW strips list the power with all channels on.
Voltage drop and power injection
The thin copper traces in a strip have resistance, so the voltage drops along long runs and the far end gets dimmer, or shifts color on RGB strips. As a rule of thumb:
- 12 V strips: feed power at both ends or every 16 ft (5 m);
- 24 V strips: up to about 33 ft (10 m), because the current is halved;
- 5 V addressable strips: every 6 ft (2 m) or so.
For long runs, 24 V is usually the better choice. Size the feed wires for the current: 18 AWG for a few amps over short distances, heavier for longer runs.
Safety tips
- Use a power supply listed for the purpose (UL or ETL in the US), and a dry-location or wet-location type as needed.
- Do not exceed the strip’s maximum run length from a single feed, even if the supply is large enough.
- Aluminum channels help dissipate heat and extend LED life, especially for high-power strips.
- Dimmers must match the power supply type: PWM dimmers on the low-voltage side, or a dimmable driver.