LED Driver Efficiency Calculator
Estimate LED driver input power, output power, efficiency, loss, input current, thermal rise, annual energy use and practical supply rating.
Actual driver efficiency depends on topology, voltage ratio, current, switching frequency, duty cycle, dimming level, magnetics, MOSFET and diode losses, PCB layout, thermal conditions and manufacturer test points.
Engineering tool
LED Driver Efficiency Calculator
Estimate LED driver input power, output power, efficiency, loss, thermal rise, supply rating, annual energy use and efficiency curve behavior.
DC input voltage.
Measured or available input current.
LED string or driver output voltage.
LED load current.
Separate standby/control power for measured mode loss breakdown.
Dynamic Driver Summary
12 V input at 1 A gives 12 W real input, 9 W LED output, 75% efficiency and 3 W driver loss.
Result console
- Input power
- 12W
- Output power
- 9W
- Efficiency
- 75%
- Driver loss
- 3W
- Input current
- 1A
- Output current
- 1A
Driver efficiency varies with input voltage, load, dimming level, temperature and manufacturer test conditions.
Formula reference
LED Driver Efficiency Formulas
Use real input power for efficiency. For AC drivers, watts, VA and power factor are separate quantities.
Pin = Vin x IinPin real = Vrms x Irms x PFPout = Vout x IoutEfficiency = Pout / Pin x 100%Ploss = Pin - PoutPin total = PLED / Efficiency + PquiescentTemperature rise = Ploss x RthetaAnnual energy = daily energy x days per yearWeighted efficiency = sum(efficiency x weight) / sum(weight)Variable definitions
- Pin
- real driver input power
- Pout or PLED
- useful LED output power
- PF
- AC power factor
- Ploss
- driver heat dissipation
- Rtheta
- driver-to-ambient thermal resistance
- Quiescent or standby power is control power not delivered to LEDs
How to Use
- 1. Use measured efficiency mode when real input and output measurements are available.
- 2. Use input requirements when sizing the upstream supply for a known LED load.
- 3. Use output capability when input power and assumed efficiency limit the LED load.
- 4. Use thermal rise to convert driver loss into estimated temperature rise and margin.
- 5. Use comparison and curve modes when choosing between drivers or reviewing datasheet curves.
Worked Examples
Measured driver: 12 V x 1 A input gives 12 W. A 9 V x 1 A LED output is 9 W, so loss is 3 W and efficiency is 75%.
Input requirements: 9 W LED load at 90% efficiency plus 0.1 W quiescent power needs 10.1 W input, 0.8417 A from 12 V, and 12.12 W with 20% supply margin.
Linear driver: 12 V input, 9 V LED string and 700 mA gives 8.4 W input, 6.3 W LED power, 2.1 W heat and 75% theoretical efficiency.
Thermal rise: 20 W input at 90% efficiency loses 2 W. With 15 °C/W thermal resistance, temperature rise is 30 °C and estimated driver temperature is 70 °C at 40 °C ambient.
AC input: 230 V RMS, 0.25 A RMS and 0.90 PF gives 57.5 VA apparent power, 51.75 W real power, 45 W output, 86.96% efficiency and 6.75 W loss.
Engineering Notes
- Driver efficiency is output power divided by real input power.
- Driver power loss usually becomes heat.
- Efficiency varies with input voltage, load, dimming and temperature.
- Quiescent and standby power matter more at light load.
- Power factor and efficiency are different quantities.
- A high-efficiency AC driver can still have poor power factor.
- Buck, boost and buck-boost topologies have different voltage relationships.
- Manufacturer efficiency curves are preferable to a single headline value.
- Driver thermal ratings and derating curves must be respected.
- AC mains LED drivers may have isolation, harmonic and regulatory requirements not covered here.
Common Mistakes
- Using apparent power instead of real input power.
- Confusing power factor with efficiency.
- Ignoring driver quiescent power.
- Assuming efficiency is constant at every load.
- Sizing the input supply equal to LED output power.
- Applying no supply margin.
- Ignoring driver thermal loss.
- Comparing drivers only by headline efficiency.
- Using a buck driver when output voltage exceeds input voltage.
- Assuming PWM dimming reduces LED peak current.
Support reference
FAQ
How do I calculate LED driver efficiency?
Divide real output power by real input power and multiply by 100%. For DC systems, power is voltage times current. For AC drivers, real input power also depends on power factor.
What is the difference between input power and output power?
Input power is the power drawn by the driver. Output power is delivered to the LED load. The difference is driver loss.
Where does LED driver power loss go?
Most driver power loss becomes heat in semiconductors, magnetics, resistors, wiring and control circuitry.
Is power factor the same as efficiency?
No. Power factor relates AC real power to apparent power, while efficiency compares output power to real input power.
How do I calculate AC driver input power?
Use Pin = Vrms x Irms x PF. Apparent power alone, in VA, is not the same as real input power in watts.
Why does driver efficiency change with load?
Switching loss, conduction loss, control power and magnetic loss do not scale equally with load, so efficiency curves usually vary across operating points.
How large should the input power supply be?
Estimate total driver input power, then add design margin. This calculator also suggests a practical supply wattage using existing ECParts supply-size steps.
What is quiescent current or quiescent power?
It is power consumed by driver control circuitry even when it is not directly delivered to the LED load.
Why are linear LED drivers less efficient?
A linear driver drops the difference between input voltage and LED string voltage as heat, so efficiency is mainly limited by VLED divided by Vin.
What is the difference between buck, boost and buck-boost drivers?
Buck drivers step voltage down, boost drivers step voltage up, and buck-boost drivers can support either voltage relationship depending on topology.
Does PWM dimming reduce driver power?
PWM dimming reduces average output power, but LED peak current may remain high and driver efficiency at reduced load can differ from full-load efficiency.
How do I estimate driver temperature rise?
Multiply driver power loss by the driver-to-ambient thermal resistance, then add the result to ambient temperature.
Should I operate an LED driver at full rated power?
Continuous operation near maximum rating can reduce thermal margin. Review datasheet derating curves and real enclosure temperature.
How do I compare two drivers by annual energy use?
Compare input power, operating hours, standby power, days per year and electricity rate. The calculator estimates annual kWh and operating cost for each driver.
Documentation
Related Engineering Guides
Design notes, guides, and engineering articles linked to this tool.
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Disclaimer
This calculator provides engineering estimates for LED driver power and thermal design. Final designs should be verified against the driver datasheet, efficiency curves, derating curves, thermal measurements, power-factor data and applicable safety requirements.