How to size a power supply for an LED strip

Multiply the strip’s length by its maximum power consumption per metre. Then add any controller load and wiring losses supplied by the same unit, and allow appropriate capacity above that total. For example, six metres of strip rated at 12 W/m require 72 W before those additions.

Start by matching the voltage. A 24 V strip needs a suitable 24 V supply; a 12 V strip needs a suitable 12 V supply. Extra wattage is useful capacity. Extra voltage is a different electrical condition and may damage the strip. This guide covers strips designed for a specified low-voltage DC supply, not mains-voltage strips or bare LED assemblies that require constant-current operation.

Read the strip specification before doing the maths

You need three figures: rated supply voltage, maximum watts per metre and installed length. Check the units carefully. A five-metre reel described as 48 W already has a total rating; multiplying 48 W by five would count its length twice.

LED count is not a substitute for a power specification. Two strips with 60 LEDs per metre can have different operating currents and power consumption. The LED design and the strip circuit both matter. Likewise, the labels SMD and COB do not identify a particular wattage.

Use the strip’s electrical input power, not its light output. Lumens and watts answer different questions: lumens describe light output, while watts tell you the electrical load that the supply must support.

The three calculations you need

Use watts for power, volts for voltage and amperes for current. A 72 W load draws 3 A at 24 V. A different strip designed for 12 V and consuming the same 72 W draws 6 A. See our watts-to-amperes examples for more on the calculation.

The 25% allowance is an example used here, not a universal requirement. The power-supply manufacturer’s loading limits and operating conditions determine the final choice.

Worked example: six metres of 24 V strip

Assume a strip rated at 24 V and 12 W/m, with six metres installed. Its maximum specified load is 6 × 12 = 72 W. For this example, allow another 3 W for the controller and wiring losses. The total output load becomes 75 W, or 75 / 24 = 3.125 A.

Adding 25% capacity gives 75 × 1.25 = 93.75 W. A suitable 24 V, 100 W supply could meet that requirement, provided it can deliver the necessary output under the actual installation conditions. A 24 V, 5 A supply, rated at 120 W, may also be suitable.

Check that the quoted wattage is a continuous output rating. A peak figure or an unqualified sales description is not enough. The controller, connectors and cables must also carry the required current. Our guide to selecting adapter voltage and current ratings covers these compatibility checks.

The final column excludes controllers and wiring losses. These are power calculations, not permission to feed the entire stated length from one end. The manufacturer’s maximum run length must be checked separately.

Why “20% extra” is not the same as “80% loaded”

Adding 20% to a 100 W load gives a 120 W supply. That supply operates at approximately 83.3% of its rating. To operate at 80%, divide the load by 0.80: 100 / 0.80 = 125 W. An 80% loading target therefore corresponds to 25% capacity above the load.

Capacity allowance also cannot replace a temperature check. If a nominal 100 W supply is limited to 70 W at the installation temperature, it cannot support a 75 W continuous load there. Check the derating curve, mounting requirements and airflow. Understanding why adapters get hot helps explain why an enclosed cupboard can change an otherwise reasonable selection.

How much strip can an existing supply support?

Work backwards from the available output. A 24 V, 5 A supply is rated at 120 W. Using an illustrative 80% loading target leaves 96 W. If the controller and wiring losses account for 3 W, there are 93 W available for the strip.

For a strip rated at 12 W/m, the power-based limit is 93 / 12 = 7.75 metres. Select an actual length that respects the strip’s cutting intervals, wiring arrangement and maximum feed length. The same method applies when deciding how much current a 12 V supply needs.

Choosing between 12 V and 24 V

At equal power, a 24 V system carries half the current of a 12 V system. With the same cable resistance, that reduces cable heating and voltage drop. This is a useful reason to consider a 24 V strip for a longer installation, provided the rest of the system is designed for it.

It does not mean a 12 V strip can be connected to 24 V. Using the wrong supply voltage can damage the strip and controller. By contrast, an otherwise compatible supply with a higher current rating can be suitable: its rating describes available capacity, not current forced into the load.

RGB, RGBW and addressable strips need another check

The load of an RGB, RGBW or addressable strip changes with colour and brightness. A measurement taken with only the red channel on may be far below the system’s maximum demand. Use the manufacturer’s maximum specification and the most demanding operating mode the installation can reach.

If the stated W/m figure already covers all channels, do not multiply it by three or four again. If figures are given separately, add the channels that can operate together. For tunable-white strips, check whether the controller allows both white channels to run at full output simultaneously or limits their combined output.

Controller limits apply per channel and to the unit as a whole. For example, a hypothetical controller rated at 3 A per channel and 6 A total cannot run three channels at 3 A each simultaneously. A larger power supply does not increase those limits.

PWM dimming can reduce average consumption, but the supply still needs to handle startup and changing loads. “It will usually be dimmed” is not a reliable sizing method unless the maximum demand is properly constrained and verified. If a wall dimmer controls the mains input, check dimmer compatibility; an ordinary adapter is not automatically suitable.

A bigger supply does not fix every dim strip

There may be enough wattage at the power supply but too little voltage at the far end of the installation. Resistance in the cables, connectors and strip tracks causes voltage drop. That is why brightness can fall along a long LED strip even with an adequately rated supply.

A voltage-drop calculation considers current and conductor resistance. For a long two-wire DC feed, include both the outgoing and return conductors.

Suitable remedies include a larger cable cross-section, shorter feeds and additional feed points arranged as the manufacturer specifies. A cable can satisfy its current-carrying limit and still produce too much voltage drop for a particular installation.

One suitable supply can feed several strip branches, with their loads added together. Do not join the outputs of separate power supplies unless their manufacturers explicitly support that arrangement. Installations using separate supplies need correctly separated power sections and compatible control connections.

Check the whole current path

A supply rated at 10 A does not make a small barrel connector capable of carrying 10 A. Check the connector’s current rating and contact quality as well as its physical dimensions. Confirm positive and negative polarity before connecting the strip.

Choose suitable terminals and joints. A poor contact can become a local hot spot. If a cable or connector gets hot, investigate that fault before increasing supply capacity. The supply’s own overload protection may not protect every smaller branch cable; branch protection must suit the conductors and connectors it protects.

Where efficiency and operating hours belong

If the DC load needs 75 W, that is the starting point for output-capacity selection. With an assumed power-supply efficiency of 90%, the corresponding mains input is approximately 75 / 0.90 = 83.3 W. The difference becomes loss in the supply. Do not count the supply’s conversion loss twice by adding it again to the DC load. Power-supply efficiency matters for heat and input consumption.

Operating hours do not multiply the instantaneous wattage requirement. They determine energy use, while prolonged operation makes thermal and continuous-duty performance important. At an assumed constant input of 83.3 W, four hours per day for 30 days uses approximately 10 kWh. See kW versus kWh and monthly electricity-consumption calculations for the distinction.

A 100 W power supply does not consume 100 W continuously just because that number appears on its label. Input consumption depends on the load, efficiency and standby losses. For a suitable plug-in installation, a correctly rated energy-monitoring smart plug can measure actual mains consumption.

Choose the right type of supply and enclosure

Common 12 V and 24 V strips with built-in resistors or current regulation generally require a constant-voltage DC supply. This relates to how LED current is limited. A constant-current driver intended for a bare LED string is not interchangeable simply because its wattage is sufficient. Also distinguish an adapter’s AC input from its output: check whether it supplies AC or DC.

When comparing power supplies, include the installation method. Open-frame supplies require suitable protection and enclosure arrangements; DIN-rail supplies may suit a control cabinet. Check the relevant safety and application requirements, ventilation and environmental conditions. Exposed mains terminals require installation by a competent person.

Verify the installation under load

Check the system at its highest intended load. On the isolated low-voltage side, compare the DC voltage at the supply and at the strip input. The difference helps reveal losses along the feed. Use the meter’s COM and voltage terminals for voltage measurements.

For current measurements, observe the instrument’s current and time limits. Never place a meter set to amperes directly across the supply output. Shutdowns, repeated restarts or a substantial voltage collapse under load call for checks that distinguish a faulty adapter from an overloaded one.