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How to Size an LED Power Supply for LED Strip Lights

To size an LED power supply, find the strip's watts per foot, multiply by the installed length to get the total load, add headroom so the driver is not run at its limit, and choose a driver whose capacity meets or exceeds that number at the matching voltage. LED Club currently recommends at least 20 percent headroom, though you should confirm the current guidance before finalizing a build. Getting this right keeps the system reliable; getting it wrong leads to a driver that runs hot, underperforms, or fails early.

Here is the calculation, step by step, with an example.

Step 1: find the strip's watts per foot

Every strip has a power draw rated in watts per foot, listed on its product page. This is the figure that drives the whole calculation, so read it for your specific strip rather than assuming, since it varies by product and output level. Confirm the watts per foot on the page before you go further.

Step 2: multiply by the installed length

Multiply the watts per foot by the total number of feet you will install to get the total wattage the run draws. For example, a strip rated at 4 watts per foot across a 20-foot run draws 4 times 20, which is 80 watts. That total load is the baseline the driver has to cover.

Step 3: add headroom

Do not size the driver to exactly the load, because running a supply continuously at 100 percent of its rating stresses it, runs it hot, and shortens its life. Add headroom by increasing the figure. Using LED Club's current guidance of at least 20 percent, an 80-watt load becomes 80 plus 20 percent, which is 96 watts. Why this margin matters in more depth is covered in our guide on driver headroom. Confirm the recommended percentage before publishing, since guidance can change.

Step 4: choose a matching, adequately sized driver

Select a driver whose rated wattage meets or exceeds the headroom-adjusted figure, at the voltage that matches the strip. For the 96-watt example, you would choose a driver rated at 100 watts or more, at the strip's voltage. Remember that the driver voltage must equal the strip voltage: a 12V strip needs a 12V supply, and a 24V strip needs a 24V supply. Browse suitable units in the LED power supplies collection.

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How to size an LED power supply: a worked example

Say you are installing a 24V strip rated at 3 watts per foot over 30 feet. The load is 3 times 30, which is 90 watts. Adding at least 20 percent headroom gives 108 watts. You would then choose a 24V driver rated at 120 watts or more, since that is the next common size above 108 watts. The exact watts-per-foot figure, and the driver options, should be confirmed on the product pages for the strip and the supply you choose.

What else affects the choice

A few practical points sit alongside the math. If the run is long, plan for the strip's rated maximum run and for techniques like dual-end feeds or power injection, which our related guides cover. If you plan to dim, make sure the driver is a dimmable type and compatible with your control. And for a hardwired or junction-box driver, follow the manufacturer instructions and local electrical code, including NEC requirements where applicable, and use a licensed electrician when line-voltage work is involved.

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Common mistakes to avoid

The frequent ones are sizing the driver to exactly the load with no headroom, using the wrong watts-per-foot figure, and picking a driver whose voltage does not match the strip. Read the watts per foot for your strip, add the recommended headroom, and match the voltage every time.

Frequently asked questions

How do I calculate the power supply size for an LED strip? Multiply the strip's watts per foot by the installed length to get the load, add headroom of at least 20 percent, then choose a driver at the matching voltage whose wattage meets or exceeds that figure.

How much headroom should I add to an LED driver? LED Club currently recommends at least 20 percent, so a driver is not run at its limit. Confirm the current guidance, and add more margin if you expect to extend the run later.

What happens if the driver is too small? An undersized driver runs hot, may not power the full run, and can fail early. Always size for the total load plus headroom.

Does the driver voltage need to match the strip? Yes. A 12V strip needs a 12V driver and a 24V strip needs a 24V driver. Voltage matching is required regardless of wattage.

Can I use a bigger driver than I need? A driver with more capacity than the load is fine, since the strip draws only what it needs across a constant voltage. Just match the voltage and confirm compatibility with any dimmer or controller.

If you want help running the numbers for a specific strip and run, LED Club offers free project support, so you can send us your project details before you buy.

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