AC Dimmer Power Calculator
Calculates the actual RMS voltage, current, and power delivered to a load through a phase-cut TRIAC dimmer (the type used in RobotDyn, Krida, and generic Arduino-compatible dimmer modules) at any dimming level from 0% to 100%.
Useful when you're sizing wiring, picking a fuse, choosing a heatsink, or just want to know if your 60 W bulb at 30% brightness actually draws 18 W (it doesn't — it draws less; see why below).
What this actually tells you
The dimming percentage on your Arduino sketch
(dimmer.setPower(50) for example) is the phase-conduction
percentage — not the power percentage.
At 50% dimming, the TRIAC conducts for half of each AC half-cycle. But because the sine wave is curved (not flat), and power scales with voltage², the actual power delivered is closer to 50%, not what you'd intuitively expect from chopping the waveform in half. The calculator above does the math properly.
This is why incandescent bulbs feel non-linear when you
use a dimmer — at low dimming levels small changes feel huge, while near
full brightness the change is barely perceptible. The math:
VRMS = Vmains × √(1 − α/π + sin(2α)/(2π))
where α is the firing angle in radians.
Practical applications
- Sizing a fuse: use the worst-case RMS current at 100% dimming. Don't undersize based on the dimmed value — power-up surge hits the full-on number.
- Picking a TRIAC heatsink: TRIAC dissipation is
approximately
Vdrop × IRMSwith Vdrop ≈ 1–1.5 V across the TRIAC junction. At 8 A RMS, that's ~10 W of heat — needs a real heatsink. - Checking lamp compatibility: "dimmable" LED lamps typically need ≥ 50% mains voltage to start conducting. Below ~30% dimming the lamp may flicker or refuse to light.
- Heater PWM (slow toggling, not phase-cut): for thermal loads, prefer slow on/off cycling (cycle skipping) over phase-cut to reduce EMI. Same average power, much less radio noise.
Limitations
This calculator assumes a purely resistive load (incandescent lamps, heating elements, hair-dryer-style fans). For inductive loads (AC motors, transformers, magnetic ballasts), the actual power factor and current waveform differ — real RMS values will be off by 5–15 % and the current will lag the voltage. For dimmable LED bulbs, behavior is highly nonlinear and depends on the driver — treat this calculator as a rough upper bound.
Related
- RobotDyn AC Dimmer 1-Channel — 4–8 A, the module this calculator was built around
- RobotDyn 16/24A High-Load Dimmer — for big loads above 8 A
- Best AC Dimmer Modules for Arduino 2026 — buying guide