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.
0.3%gain from the rear face
18 kWhextra per year
£4a year, at your unit value
beyond 40 yrsto repay the £648 premium
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
The same panels, the same roof pitch, the same ground beneath — moved from one mounting to
another. Nothing else changes.
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
The rear-face gain is a percentage of the front, so it stays fairly steady across the year
rather than following the seasons. It rises a little in summer, when more of the total arrives
as light the ground can bounce, and where snow lies the winter figure would rise far more than
this — an effect an annual reflectance cannot express.
Rear-face gain, by month
Month
%
Jan
0
Feb
0
Mar
0
Apr
0
May
0
Jun
0
Jul
0
Aug
0
Sep
0
Oct
0
Nov
0
Dec
0
Total
4
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.