How to Size an LED Strip Power Supply Without Guesswork

Every LED strip failure I have been asked to look at traces back to the same root cause: someone bought a power brick by reading the strip box and rounding to a number that felt right. The brick either runs pinned at its limit and dies in a summer, or it can barely light half the run. Sizing a supply is not a vibe. It is two multiplications and a 20% pad, and once you do it on paper the rest of the build falls into place. Here is the whole method, with the arithmetic spelled out so you can check every step.

Start with watts per meter times length

The single number that drives everything is the total power the run draws. LED strips are rated in watts per meter (W/m), printed on the reel or the spec sheet. Multiply that rating by the length of your run:

total watts = watts per meter × length in meters

A 5 meter reel of 14.4 W/m strip draws 14.4 × 5 = 72 W. That is the raw load, before any supply headroom, and it is also the figure you compare against a power brick's rating. If you are joining two reels end to end, add their lengths first, then multiply once — a 3 m piece plus a 2 m piece is still 5 m at 14.4 W/m, so still 72 W.

Watts per meter already folds in the LED density and the voltage, so you do not need to do anything clever here. A denser strip pulls more watts per meter; a dimmer one pulls fewer. Read the real number off the spec, not the marketing line on the front of the box, which often quotes a "peak" figure you will never run at. If the spec sheet only gives you watts per meter at a different voltage, the LED Strip Power Calculator handles the conversion and the multiply in one shot, so you are not juggling units by hand.

Add 20% headroom to pick the supply

Now the part most people skip. You never size a supply to exactly match the load. A brick running at 100% of its rating runs hot, sags its output voltage, and ages fast. The standard fix is to leave roughly 20% in reserve:

supply rating ≥ total watts × 1.2

Take the worked example: a 72 W load wants a supply rated at least 72 × 1.2 = 86.4 W. Power supplies come in standard sizes, so you round up to the next one. There is no common 87 W brick, so you reach for a 90 W or 100 W unit. That supply then loafs along at around 72% of capacity, stays cool, and outlives the strip.

This is also why "bigger is fine, smaller is not." Buying a 120 W supply for a 72 W run wastes nothing except a little money — the strip only pulls what it needs. Buying a 75 W supply for the same run leaves you 3 W of margin, which evaporates the first warm day. When in doubt, round up.

Convert watts to amps before you touch the wire

Watts tell you the supply rating. Amps tell you the wiring. Current is power divided by voltage:

amps = total watts ÷ volts

That 72 W load pulls 72 ÷ 12 = 6 A on a 12 V strip, but only 72 ÷ 24 = 3 A on a 24 V one. Same brightness, same watts, half the current. This matters because wire gauge, connector ratings, and voltage drop all scale with amps, not watts. Spec your hookup wire and your barrel connectors from the amp figure, never from the wattage. A connector rated for 3 A will quietly cook on a 6 A run even though the supply is sized perfectly.

The amp figure is the quiet hero of a clean install. Halving the current by jumping from 12 V to 24 V is the single easiest way to run a longer strip without re-engineering the wiring, and it costs nothing at the supply because the watts are unchanged.

Watch voltage drop on long runs

Here is where a correctly sized supply still gives you a dim, sad far end. The thin copper traces inside the strip have resistance, and current flowing along them loses a little voltage every meter. The LEDs nearest the supply get full voltage; the ones at the far end see less, so they glow dimmer or shift color. Throwing a bigger brick at it does nothing — the problem is in the strip, not the supply.

Three honest fixes:

Feed power at both ends of the run, so current travels at most half the length from each feed.
Split a long run into shorter segments, each wired back to the supply rather than daisy-chained.
Move to 24 V, which halves the current and therefore halves the drop, roughly doubling the clean length you can run.

As a rough field guide: 12 V strip is comfortable to about 5 meters before the far end visibly dims, 24 V to roughly double that, and 5 V addressable strip rarely makes 2 meters without re-injecting power. When a run starts getting long, run the numbers properly with a voltage drop calculator instead of trusting the rule of thumb — copper trace resistance varies a lot between cheap and good strip.

A second worked example: two strips, one supply

Say you want to drive two separate 4 meter runs of 9.6 W/m strip from a single 12 V supply. Add the loads first:

2 × (9.6 W/m × 4 m) = 2 × 38.4 W = 76.8 W

Apply the 20% headroom: 76.8 × 1.2 = 92.2 W, so you buy a 100 W supply. Each strip pulls 38.4 ÷ 12 = 3.2 A, so the combined draw on the supply is 6.4 A — wire each run back to the supply on its own pair rather than chaining one long 8 meter line, and both runs keep their voltage drop low.

This same workflow scales to a sign backlight, a cove, or a desk build. Pick the strip, multiply for watts, divide for amps, pad 20% for the supply, and check the length against voltage drop. If you are also dropping in standalone indicator LEDs alongside the strip, the LED resistor calculator sizes their current-limiting resistors so they match the supply rail you already chose.

The whole method on one card

Total watts = watts per meter × length.
Supply rating = total watts × 1.2, rounded up to the next standard size.
Amps = total watts ÷ volts — spec wire and connectors from this.
Length check — if a 12 V run passes ~5 m, feed both ends, split it, or go 24 V.

Do those four steps and you will never again buy a brick that buzzes hot or watch the end of your under-cabinet run fade out. The math is small enough to do on a napkin, and every step has a reason behind it that the strip box will never tell you.

Made by Toolora · Updated 2026-06-13