What Power Supply Do I Need for My LED Project? A Complete Sizing Guide

You’ve bought your LEDs, you’re itching to solder, but there’s one decision that trips up everyone: what power supply do I actually need? Pick one too small and your LEDs will flicker, reset, or catch fire. Pick one way too big and you’ve wasted money on a heavy brick you didn’t need. This guide walks you through the exact calculation so you get it right the first time.

Step 1: Count Your LEDs

Everything starts with the number of individual LEDs in your project. Not meters of strip — individual pixels. Manufacturers advertise “60 LEDs per meter” and that number is what matters.

• A 16×16 matrix = 256 pixels
• 5 meters of 60 LEDs/m strip = 300 pixels
• 5 meters of 30 LEDs/m strip = 150 pixels
• A custom sign with individually placed pixels — count each one

Write this number down. You’ll need it for every calculation below.

Step 2: Find the Current Per LED

Now you need the maximum current draw per LED at full white (red + green + blue all on). Always use worst-case — your patterns may use less, but the PSU needs to handle anything your code throws at it.

Note WS2815 draws only 15 mA per pixel — its power consumption is similar to a 5V WS2812B (12V × 0.015A = 0.18W vs 5V × 0.06A = 0.3W).

Step 3: Calculate Total Current

The formula is dead simple:

Total Current (mA) = Number of LEDs × mA per LED

Then convert to amps (÷ 1000).

Let’s run two common examples:

Example A — 300 WS2812B pixels (5V): 300 × 60 mA = 18,000 mA = 18A

Example B — 300 WS2815 pixels (12V): 300 × 15 mA = 4,500 mA = 4.5A

Same number of LEDs, vastly different current. This is why choosing the right voltage matters.

Step 4: Apply the 80% Rule

Here’s the golden rule: never load a PSU beyond 80% of its rated capacity. If rated for 10A, draw no more than 8A. Three reasons: power supplies run hotter near their limit, many trigger over-current protection during surges, and headroom handles inrush current on power-up.

Minimum PSU Rating = Calculated Current ÷ 0.80

• 18A ÷ 0.80 = 22.5A → buy a 25A (or larger) PSU at 5V
• 4.5A ÷ 0.80 = 5.625A → buy a 6A+ PSU at 12V

Step 5: Choose Voltage (5V vs 12V vs 24V)

You can’t pick a PSU without first deciding the voltage. Here’s how they compare:

5V — Best for dense matrices and short runs. WS2812B and SK6812 are the kings here. No voltage conversion needed for the pixels. Downside: voltage drop hits hard after 2-3 meters, requiring power injection.

12V — The sweet spot for long runs. WS2815 strips handle 12V natively, and voltage drop is manageable for 5-6 meter runs without injection. Most analog RGB strips are 12V too.

24V — Used for constant-current LED strips (often in architectural lighting). Not common for pixel projects, but excellent for very long runs (10m+) with minimal voltage drop.

Quick guide:

• Under 200 pixels, short runs → 5V (simpler, cheaper)
• 200+ pixels, long runs → 12V (less power injection, lower current)
• Architectural / permanent installs → 24V

Step 6: Pick a Quality PSU

Here are the go-to options:

Mean Well LRS is the gold standard — reliable, efficient, and metal-enclosed with proper cooling (~$20-40). Generic PSUs are cheaper but many overstate ratings by 20-30%. A “10A” generic brick may only deliver 7A.

Most LRS units use screw terminals — strip wire and screw down. For 5V builds pulling 20A+, use 14 AWG or thicker. LRS units don’t include an AC cord — you’ll need a standard IEC C13 cable.

Quick Reference Table

Final checklist before you buy:

• Counted individual LEDs
• Used full-white mA per LED (worst case)
• Divided by 0.80 for headroom
• Chosen voltage that matches your strip
• Selected a Mean Well or quality brand PSU
• Got an IEC C13 power cord

A properly sized PSU is the difference between years of reliable operation and flickers, resets, or worse. Do the math, buy quality, and enjoy your bright, stable LEDs.