Vicaya GlobalCalculator · Solar · bifacial
Is bifacial worth it on your roof?
A bifacial module collects light on the back as well as the front. How much depends almost entirely on what is behind it — which is the part the datasheet cannot tell you.
What do kWp and kWh mean?
- kWh — kilowatt-hour
- A unit of energy, and the thing your electricity bill charges you for. A 1,000-watt heater running for one hour uses one kWh. A typical home uses somewhere between 2,000 and 10,000 kWh a year depending on where it is and how it heats.
- kWp — kilowatt-peak
- A unit of capacity — how big the solar array is, not how much it makes. It is the output the panels would produce under standard test conditions: bright, cold and perfectly aimed. Real roofs rarely see those conditions, which is why a 4 kWp array does not generate 4 kW for most of the day. One modern panel is roughly 0.4 kWp, so 4 kWp is about ten panels.
- Putting them together
- kWp is the size of the system; kWh is what it produces over time. The ratio between them — kWh generated per kWp installed, per year — is the honest way to compare locations, because it strips out how big the system happens to be. It runs from roughly 700 in cloudy high latitudes to over 1,800 in sunny deserts.
12 × 450 W at 21% is 2.14 m² per panel — about 26 m² of roof.
Mounted this way, the rear face adds 0.3% — and the premium takes more than forty years to repay.
A bifacial module's rear face is rated at 70% of the front, and that rating is honest. What it does not tell you is that the rear face has to be looking at something. Move the same panels to carport or canopy and the gain becomes 14.5%; leave them flat against a roof deck and there is nothing behind them to collect.
Where the gain actually comes from
| Mounting | What it means | Rear gain | Extra output | Worth per year | Premium repaid in |
|---|---|---|---|---|---|
| Flush on a pitched roof — yours | panels a few centimetres off the tiles — the usual house install | 0.3% | 18 kWh | £4 | beyond 40 years |
| Flat roof, low frame | tilted frames roughly 0.3 m above the surface | 5.7% | 310 kWh | £62 | 10.5 years |
| Flat roof, raised frame | roughly 1 m of clear space beneath | 8.9% | 487 kWh | £97 | 6.7 years |
| Ground mount | standard height and row spacing | 11.3% | 619 kWh | £124 | 5.2 years |
| Ground mount, elevated | high mounting, wide rows, little self-shading | 13.7% | 752 kWh | £150 | 4.3 years |
| Carport or canopy | fully open underneath | 14.5% | 796 kWh | £159 | 4.1 years |
Gain through the year
| Month | % |
|---|---|
| Jan | 0.29 |
| Feb | 0.30 |
| Mar | 0.30 |
| Apr | 0.31 |
| May | 0.35 |
| Jun | 0.36 |
| Jul | 0.35 |
| Aug | 0.33 |
| Sep | 0.29 |
| Oct | 0.28 |
| Nov | 0.26 |
| Dec | 0.29 |
How the rear face is modelled
The rear of a module is a plane like any other, so it is transposed like any other: same beam, sky-diffuse and ground-reflected terms as the front, at a tilt of 145° facing the opposite bearing. Turning a plane over swaps its two view factors — the front's small ground view becomes the rear's large one — and that swap is the entire physical basis of a bifacial module.
Annually the front face of this array receives 1,177 kWh/m² and the rear plane 272 kWh/m² before anything is taken away for the rear face's lower efficiency or for whatever is standing behind it.
What this does not model
- How much of the ground is lit is a declared assumption, not a simulation. Rear irradiance is transposed with the same view factors as the front face. How much of the ground beneath is lit rather than shaded is a declared assumption chosen by the mounting option, not a simulation of your array layout. Real bifacial modelling ray-traces an actual array layout — row pitch, mounting height, module self-shading — and this does not.
- No row-to-row geometry. In a multi-row array the row behind shades the ground the row in front is collecting from, and the front and back rows of a field genuinely differ.
- Reflectance is one annual number. Snow is the largest bifacial effect there is and it is seasonal; an annual average cannot express it in either direction.
- The gain is applied as an uplift on annual output. The extra rear irradiance is not passed back through the temperature and low-light chain, which would shave a little off it — so the figure here is marginally optimistic.
- No rear-face soiling or structural obstruction. Rails, junction boxes and cable trays sit behind real modules and collect nothing.
Frequently asked questions
Are bifacial solar panels worth it?
It depends on what is behind them. On a flush pitched roof the rear face sees almost nothing, so the gain is close to zero; over bright ground or on a raised frame it can be worthwhile. This calculator accounts for what is actually behind the panels rather than assuming an open field.
How much more do bifacial panels produce?
The rear-face gain ranges from near-zero on a roof to a meaningful uplift over reflective ground such as snow or light gravel. The tool computes it from the surface behind your array, so the figure is yours rather than a best-case headline.
Do bifacial panels work on a pitched roof?
Barely — the roof surface blocks most of the light that would reach the rear face, so the bifacial premium rarely pays off there. The calculator shows the gain for your mounting so you can decide whether it is worth the extra.