LED Power Calculator

LED Power Calculator
Calculate LED Power Consumption, Efficiency, and Energy Cost
Vf LED Forward Voltage
V
If LED Forward Current
N Total Number of LEDs
Ns LEDs per Series String (optional, default: 1)
Vs Supply Voltage (optional — for resistor loss and efficiency calculation)
V
Optional: Energy and Cost Estimation
H Daily Usage (hours per day)
h
£ Electricity Cost (per kWh)
/kWh
LED Power Analysis
Power per LED
—
Total LED Power
—
Total LEDs
—

LED Power Distribution

In a resistor-driven LED circuit, power is split between the LED (useful light output) and the resistor (wasted as heat). The closer the total LED forward voltage is to the supply voltage, the less power the resistor wastes and the higher the circuit efficiency.

Supply Psystem Vs × Itotal Resistor Heat Pwaste = VR × I LED Light Output PLED = Vf × I Efficiency PLED / Psystem Higher Vf / Vs ratio = higher efficiency = less heat wasted

Psystem = PLED + Presistor — minimise the gap between Vf(total) and Vsupply to maximise efficiency.

LED Power Calculator

The LED Resistor Calculator tells you which resistor to use. This calculator answers the bigger questions: how much total power does the circuit consume, how much is useful light versus wasted heat in the resistors, how efficient is the design, and what will it cost to run. Enter LED specs and supply voltage — get a full power breakdown, efficiency percentage, and running costs per day, month, and year.

Core Formulas

PLED = Vf × If — power per LED
PLEDs(total) = PLED × N — total LED power
Presistor = (Vs − Vf × Ns) × If — power wasted per resistor
Psystem = Vs × If × Nstrings — total power from supply
Efficiency = PLEDs(total) / Psystem × 100%

Ns is the number of LEDs per series string. Nstrings is the number of parallel strings (total LEDs ÷ LEDs per string). Each string has one resistor wasting Presistor watts as heat.

Worked Example — Single Indicator LED on 5 V

One red LED: Vf = 2.0 V, If = 20 mA, supply = 5 V.

PLED = 2.0 × 0.020 = 40 mW
Presistor = (5 − 2.0) × 0.020 = 60 mW
Psystem = 5 × 0.020 = 100 mW
Efficiency = 40 / 100 = 40%

60% of the power is wasted as heat in the resistor. For a single indicator LED this is negligible in absolute terms (60 mW), but the ratio shows why resistor-driven designs become inefficient at scale.

Worked Example — 30-LED Strip on 12 V

30 white LEDs (Vf = 3.3 V, If = 20 mA) arranged as 10 parallel strings of 3 LEDs in series.

Vf(string) = 3 × 3.3 = 9.9 V
PLEDs(total) = 3.3 × 0.020 × 30 = 1.98 W
Presistor(each) = (12 − 9.9) × 0.020 = 42 mW
Presistors(total) = 42 mW × 10 strings = 420 mW
Psystem = 1.98 + 0.42 = 2.40 W
Efficiency = 1.98 / 2.40 = 82.5%

Three LEDs in series on 12 V uses 9.9 V of the available 12 V for useful light — only 2.1 V is dropped across each resistor. This is a well-matched design. Compare this to running the same LEDs on 24 V: the resistors would drop 14.1 V each and efficiency would plummet to ~41%.

Key design insight: The closer the total series LED forward voltage is to the supply voltage, the less power the resistors waste. Match your supply and series count to maximise efficiency.

Worked Example — 6 High-Power LEDs on 24 V

Six white LEDs (Vf = 3.2 V, If = 700 mA) in two parallel strings of 3 LEDs each.

Vf(string) = 3 × 3.2 = 9.6 V
PLEDs(total) = 3.2 × 0.700 × 6 = 13.44 W
Presistor(each) = (24 − 9.6) × 0.700 = 10.08 W
Presistors(total) = 10.08 × 2 strings = 20.16 W
Psystem = 13.44 + 20.16 = 33.60 W
Efficiency = 13.44 / 33.60 = 40%

20 W wasted as heat in the resistors — more than the LEDs themselves consume. At 700 mA, each resistor dissipates over 10 W and needs a large heatsink. This is why high-power LED lighting almost always uses a constant-current LED driver instead of resistors. A driver operating at 90%+ efficiency would draw ~15 W total instead of 33.6 W.

Energy Consumption and Running Costs

Multiply total system power by daily usage hours to get energy consumption. Multiply by electricity cost to get running costs.

Edaily = Psystem × hours per day
Emonthly = Edaily × 30
Eyearly = Edaily × 365
Cost = Energy × price per kWh

Cost Example — 30-LED Strip, 8 Hours/Day

Psystem = 2.40 W, usage = 8 hours/day, electricity = £0.28/kWh.

Edaily = 2.40 × 8 = 19.2 Wh = 0.0192 kWh
Eyearly = 0.0192 × 365 = 7.01 kWh
Costyearly = 7.01 × £0.28 = £1.96

Under £2 a year for 30 LEDs. But scale this to 100 strips in a commercial installation and the numbers matter — especially if inefficient resistor matching wastes 40–60% of the power as heat.

Cost Example — 6 High-Power LEDs, 12 Hours/Day

Edaily = 33.60 × 12 = 403.2 Wh = 0.403 kWh
Eyearly = 0.403 × 365 = 147.1 kWh
Costyearly = 147.1 × £0.28 = £41.19

£41.19/year for one fixture with resistors. With a constant-current driver at 90% efficiency, the same light output would cost ~£18.40/year. Over 10 fixtures and 5 years, that is a £1,140 difference — more than enough to justify the driver hardware. For a full analysis of power dissipation in the resistors, use the Electrical Power Calculator.

Improving LED Circuit Efficiency

Match Supply Voltage to Series String Voltage

The single biggest efficiency gain. If your LEDs total 9.9 V forward, use a 12 V supply (82% efficient) rather than a 24 V supply (41% efficient). The resistor only needs to drop the difference.

Maximise LEDs Per Series String

More LEDs in series means a higher total forward voltage and less voltage left for the resistor to waste. Three white LEDs on 12 V (9.9 V used, 2.1 V wasted) is far better than one white LED on 12 V (3.3 V used, 8.7 V wasted).

Use Constant-Current Drivers for High Power

Above 1 W per LED, resistor-based circuits waste serious power. A switching constant-current driver converts supply voltage to the LED string voltage at 85–95% efficiency, keeping heat and running costs low. The upfront cost pays for itself within months at high power levels.

Reduce Operating Current

Many LEDs are rated at 20 mA but produce acceptable brightness at 10–15 mA. Running at lower current reduces total power proportionally and extends LED lifespan. The calculator shows you the exact power savings.

Frequently Asked Questions

How much power does a single LED use?
P = Vf × If. A standard red indicator LED (2.0 V, 20 mA) uses 40 mW. A high-power white LED (3.2 V, 700 mA) uses 2.24 W. The resistor adds more on top — enter your supply voltage in the calculator to see the full system power.
Why is my circuit efficiency so low?
The supply voltage is too high relative to the total LED forward voltage. The excess voltage drops across the resistor as heat. Fix it by adding more LEDs in series, switching to a lower supply voltage, or replacing the resistor with a constant-current driver.
Does the resistor waste more power than the LED?
It can. A single 2.0 V red LED on a 12 V supply: the LED uses 40 mW and the resistor wastes 200 mW — five times more. The resistor always wastes more power when the supply voltage is more than double the total LED forward voltage.
How do I calculate running costs?
Multiply total system power (watts) × hours per day × 365 to get yearly Wh. Divide by 1000 for kWh. Multiply by your electricity rate. The calculator does this automatically when you enter usage hours and cost per kWh.
When should I stop using resistors and switch to a driver?
When total resistor power loss exceeds ~1 W, or when efficiency drops below ~70%, or when you are running high-power LEDs (350 mA+). At that point, a constant-current driver saves more energy over its lifetime than it costs to buy.
Can I use this calculator without entering the supply voltage?
Yes. Enter just Vf and If to see per-LED and total LED power. The supply voltage, efficiency, resistor losses, and running costs are all optional — the results build progressively as you add more inputs.

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Last updated: March 2026