Power factor is about phase, efficiency is about heat, and they are independent. A worked 100 W at 230 V table shows input current and apparent power at PF 0.5, 0.7, 0.9 and 0.95, and why low PF strains the cable, breaker and drop while high efficiency only saves the driver itself. Includes a checklist of which number to specify.
"Low power factor" and "low efficiency" are often treated as the same complaint, but on a wiring drawing they point at two completely different things: one decides whether the cable and breaker must be fatter, the other decides whether the driver cooks itself early. Confusing them leads to either wasted conductor or a wrong life calculation.
The one-sentence rule: PF sizes the wiring, eta sizes the driver
Power factor (PF) measures whether current and voltage are aligned in phase - a purely phase issue, and it does not add to the driver's own heat
Efficiency (eta) measures how much of the input power turns into heat - it directly sets case temperature and life
A driver can be high efficiency but low PF, or low efficiency but high PF; the two are not linked
What it means for buying: when many units share a circuit or runs are long, PF deserves to be named; in sealed or hot installations, efficiency is the number that saves you
How each one is computed
Power factor is the share of apparent power that does real work:
S = P / PF (apparent power, VA; P is real output power, W)
I = P / (V_in x PF) (ignoring efficiency, input current only depends on PF)
Efficiency:
eta = P_out / P_in
P_loss = P_out x (1/eta - 1) (all of this becomes heat inside the driver)
Combine both for sizing:
I_in = P_out / (V_in x eta x PF)
High PF means less current for the same real power. High efficiency means the driver dissipates less for the same output. Note PF does not change P_loss, because the driver burns real power; eta does not change the wiring current except through the same P_out in the I_in formula.
Worked example: 100 W at 230 V, PF changes the current
Hold eta at 0.9 and V_in at 230 V; vary only PF (these currents and VA are what you pay for at real-power billing):
PF
Input current I_in
Apparent power S
Current vs PF 0.95
0.50
0.966 A
200 VA
1.90x
0.70
0.690 A
143 VA
1.36x
0.90
0.537 A
111 VA
1.06x
0.95
0.509 A
105 VA
1.00x
The point is not how much energy PF 0.95 saves over 0.5 - real consumption is nearly identical. The point is that a PF 0.5 driver, at the same 100 W real load, draws 1.9 times the current of a PF 0.95 unit. Double the current means about 3.6 times the conductor heat (I squared R), double the drop, and a doubled breaker size. That is why low PF hurts the cable and not the driver.
Lower PF means more current for the same real power, so the wiring burden (loss, drop, breaker) grows
Why low PF hurts the cable, not the driver
Every wiring burden tracks current, and at fixed real power current is set only by PF:
Copper loss is I squared R: PF 0.5 draws 1.9x the current of PF 0.95, so the same wire runs about 3.6x hotter; over long runs both temperature rise and drop are noticeable
Breaker and cable: both are sized on current; low PF pushes single-unit current up and, with many units on one circuit, the sum climbs faster and forces a bigger breaker
Metering: where industry is billed on apparent power (kVA), low PF makes you pay for current you do not use as real power - the only place PF directly hits the bill
Why high efficiency only saves the driver
Efficiency only decides how much power the driver burns internally:
P_loss = P_out x (1/eta - 1)
Same 100 W out: at eta 0.85 the loss is 17.6 W, at eta 0.92 it is 8.7 W - nearly double the heat
All of it enters case temperature, and by the 10 degrees C rule it drives electrolytic capacitor life (how it propagates is in how much power headroom)
Efficiency is unrelated to wiring current: eta does not change the burden on the supply wire to the driver; that is PF's job
So high-PF units such as indoor H14, waterproof H16 and wide-input waterproof S05 do not solve driver heat; they solve the three field problems of many units per circuit, long runs and kVA billing. To bring case temperature down and extend life, look at eta and at the installation.
Buying and site checklist
Ask the billing basis first: industry kVA billing, or one circuit dragging many units - name high PF (0.9 or above) and prefer active-PFC units like H14, H16 or S05
Many units per circuit: add every unit's input current before choosing cable and breaker; low PF makes the sum spike (how to size the breaker is in sizing the AC breaker and cable)
Long runs: low PF means more current means more drop; past 5 percent, fatten the wire or move to 48 V (see 24V or 48V, when to switch)
Sealed or hot installation: this one reads eta, not PF - every point of eta lowers case temperature and lengthens life
Do not be comforted by 90 percent efficiency: no efficiency level rescues the wiring burden of low PF; verify both separately
Check THD: high PF usually comes with low THD (active PFC); passive PFC often sits at PF 0.6 to 0.7 with high THD and drags down the whole circuit in quantity
Rated PF is usually at rated load and rated input. Under light load (below 30 percent) active PFC PF drops noticeably, so verify input current at your real load point, not just the front page of the spec. Requirements for power factor and efficiency follow the latest local regulations and certifying bodies.
Power and current conversion is done in the LED driver sizing calculator; the link between PF, efficiency and THD is covered in the glossary guides and is outside this article.
Is your site billed in kVA or kWh? Many drivers, long runs, thin wire - power factor matters more than you think. Tell us load power, input voltage and run length and we will say whether a high-PF unit such as H14, H16 or S05 is worth it or a standard unit is enough. Contact →