Coming Soon
Passive Solar Heating.
Passive Solar Heating.
Forever.
No active electronics. No controller. Nothing to fail. Ever.
No active electronics. No controller. Nothing to fail. Ever.
80 silicon diodes do all the work — no controller needed
Any standard PV panel connects directly to the PassoHeat element. Works with 60, 66 or 72 cell panels. No inverter, no MPPT controller needed.
A chain of 80 silicon diodes converts solar voltage directly into heat. The I-V curve of the panel naturally matches the diode chain — passive self-regulation at any light level.
Heat transfers directly into your water tank via aluminium bars. Water-cooled from both sides for maximum efficiency. Fits any standard tank flange.
Measured May 2026, Yambol, Bulgaria — LDK 225W panel
| Time | Panel °C | Voc | Isc | Est. Power (4 panels) |
|---|---|---|---|---|
| 30 May 2026, ~14:00 | 50°C | 33.22V | 7.07A | ~790W |
| 31 May 2026, 11:30 | 45°C | 32.95V | 7.31A | ~820W |
| 31 May 2026, 13:30 ⭐ | 45°C | 33.51V | 8.80A | ~986W |
| 31 May 2026, 13:40 | — | — | 3.00A | ~340W (light clouds) |
⭐ Above spec current at solar noon — Bulgarian May sun exceeds 1000W/m²
PV panels are constant current sources — the load determines how much power is harvested
A resistive heating element collapses in efficiency as soon as cloud cover reduces current. PassoHeat's 80 silicon diodes maintain near-constant voltage regardless of current — harvesting almost all available power even on cloudy days.
| Condition | PV current | PassoHeat (80 diodes) | Resistive element |
|---|---|---|---|
| ☀️ Full sun | 18A | 594W100% | 594W100% |
| 🌤️ Partial sun | 10A | 320W~100% | 183W57% |
| ☁️ Cloudy | 1A | 29.4W~100% | 1.83W6% |
Example: 2× 300W 60-cell panels in parallel, Vmp ~30V. 80 silicon diodes in two parallel paths of 40 series diodes. Measured Vf: 1.47V at 1A, 1.60V at 10A, 1.65V at 18A — remarkably stable across 18× current range.
No MPPT, no PWM, no microcontroller. Just 80 silicon diodes doing what physics tells them to do.
Fits any water tank with a standard flange. Works with 60, 66 and 72 cell panels via 4 tap points.
Operating point automatically adapts to sun intensity, panel temperature and cloud cover. No adjustment needed.
Aluminium sandwich construction, waterproof epoxy sealed, stainless flange. Designed for 20+ year lifespan.
Single panel system operates below 50V DC — below the Low Voltage Directive threshold. Simple, safe installation.
Combine with a wood-burning boiler for complete year-round independence. Solar in summer, wood in winter.
Be the first to know when PassoHeat launches. Early supporters get priority access and launch pricing.