For a constant-voltage LED strip, calculate load by multiplying installed length by the specified power per unit length. Divide watts by the strip voltage to obtain output current. To apply a chosen maximum loading fraction, divide the total DC load by that fraction to find the minimum nameplate capacity under that assumption.
This is a calculation guide and reusable worksheet for low-voltage constant-voltage strips. It is not a constant-current LED driver selector. If the load specifies a drive current and forward-voltage window, use the driver compatibility guide instead.
Gather the inputs before calculating
Use the selected model’s specified maximum operating power, not LED density or the wattage of a different strip in the same family. Manufacturer selection tables pair particular strip types and lengths with supply choices; their values are product-specific. Flexfire power supply selector
| Input | What to enter |
|---|---|
| Nominal voltage | The required DC voltage for that strip |
| Power per length | W/m or W/ft, using a documented maximum condition |
| Installed length | Actual powered strip length in the matching unit |
| Other DC loads | Relevant controller consumption and distribution losses |
| Loading fraction | A documented project assumption, such as 0.80 |
| Installation limits | Model-specific derating and operating conditions |
Keep units consistent. One meter equals approximately 3.28084 feet. Convert the length or the power-per-length figure before multiplying; do not multiply meters by W/ft.
For RGBW or tunable-white systems, confirm what the datasheet’s maximum power figure includes. Some control arrangements limit simultaneous channel output. Do not invent a total by multiplying one channel value without knowing the permitted operating mode.
Calculate strip load and output current
For each branch:
P = L × w
Here, P is strip power in watts, L is installed length and w is power per unit length. For multiple branches at the same voltage:
Ptotal = P1 + P2 + … + other DC load allowances
Then:
I = Ptotal ÷ V
As a simple example, 5m of a hypothetical 12W/m strip represents 60W. At 24V, that corresponds to 2.5A. At 12V, a different compatible strip with the same 60W load would require 5A. The current difference does not make the two strips interchangeable.
Convert a loading target into a supply rating
If f is the chosen maximum load fraction, calculate:
Minimum nameplate output = Ptotal ÷ f
At f = 0.80, a 60W DC load requires 75W under that planning assumption. A final product selection must also pass its input, thermal and other documented operating limits.
Why adding 20% is not the 80% rule
These two expressions give different results:
| Method for a 60W load | Calculated capacity | Resulting load fraction |
|---|---|---|
| Add 20%: 60 × 1.20 | 72W | 83.3% |
| Target 80% loading: 60 ÷ 0.80 | 75W | 80.0% |
Neither percentage should be presented as a universal rule for every driver. State the project method explicitly. Check manufacturer derating separately: a unit may not deliver its full nameplate output in the proposed installation conditions. MEAN WELL load and temperature guidance
If a manufacturer permits 70% of nameplate output at a particular condition, and your project separately requires operation at no more than 80% of that available output, the combined fraction is 0.70 × 0.80 = 0.56. This is an illustration of two independent assumptions. Do not apply both automatically if the project’s margin already accounts for the same condition.
Worked example: three 24V lighting zones
Assume three compatible constant-voltage strip branches. All numbers below are illustrative and are not TTLL product specifications.
| Zone | Length | Specified power | Calculated strip load | Current at 24V |
|---|---|---|---|---|
| Shelf A | 4m | 9.6W/m | 38.4W | 1.60A |
| Shelf B | 3m | 9.6W/m | 28.8W | 1.20A |
| Cove C | 5m | 14.4W/m | 72.0W | 3.00A |
| Total strips | 12m | Mixed | 139.2W | 5.80A |
Suppose the design adds a documented 4.8W allowance for relevant DC-side consumption and losses. Total planned DC load is then 144W, or 6A at 24V.
At an 80% loading target, minimum nameplate capacity is:
144W ÷ 0.80 = 180W.
A hypothetical 200W candidate would operate at 72% nameplate loading under this calculation. It is only a candidate: its usable output, branch arrangement, controller limits and installation requirements still need to be checked. If zones use separate supplies, calculate each supply’s load independently rather than allocating the combined result arbitrarily.
A quick reference table
The following table uses a hypothetical 10W/m strip and an 80% loading target. It excludes other loads and derating; it does not establish maximum continuous run length.
| Installed length | Strip load | Current at 24V | Minimum capacity under the stated assumption |
|---|---|---|---|
| 2m | 20W | 0.83A | 25W |
| 5m | 50W | 2.08A | 62.5W |
| 8m | 80W | 3.33A | 100W |
| 10m | 100W | 4.17A | 125W |
Copy this blank schedule into your project notes:
| Branch ID | Strip model | Voltage | Length and unit | Maximum W/unit | Load W | Controller/channel |
|---|---|---|---|---|---|---|
| A | Enter model | Enter V | Enter length | Enter value | Length × W/unit | Enter device |
| B | Enter model | Enter V | Enter length | Enter value | Length × W/unit | Enter device |
What a wattage calculation cannot validate
The arithmetic does not select conductor size, branch protection, connector ratings or permissible strip length. It does not prove dimming compatibility or suitability for an outdoor enclosure. These require the actual component documents and installation design.
Supply efficiency also needs the correct interpretation. Divide output power by efficiency when estimating AC input power, not when directly comparing the load with a supply’s stated DC output rating. Building circuit planning then involves additional factors, including input current behavior, that this worksheet does not resolve.
For the remaining product checks, use the power supply selection guide. For the physical distribution arrangement, see how to connect strips to a supply.
Frequently asked questions
Should I divide LED wattage by power supply efficiency?
Not to compare it directly with rated DC output capacity. Use efficiency when estimating input power, then assess input-side requirements separately.
How many meters can a 100W supply power?
Under an 80% loading assumption, it provides an 80W load budget before other allowances. Divide that budget by the selected strip’s W/m. The result is total powered length, not a guarantee that it can be fed as one continuous run.
Can I add 12V and 24V loads together?
They need separate compatible outputs. A combined watt total does not make one fixed-voltage output suitable for both.
Does this method size a constant-current driver?
No. Constant-current selection also requires the correct drive current and an operating voltage window matching the LED load.
Turn the calculation into a power supply enquiry
Browse TTLL’s LED power supplies and send your completed branch schedule. Include the loading assumption, controller models and installation conditions so the proposed supply can be assessed against the same inputs.
