Quick answer
The relative energy increase in a bifacial evaluation compared with an explicitly defined reference case at the same energy boundary and period.
Answer: Bifacial energy gain is the difference between evaluated bifacial energy and a declared reference, divided by that reference. Match the time period, electrical boundary and comparison assumptions. Rear irradiance, module bifaciality and delivered energy gain are different quantities. Site geometry, reflectance, shading, electrical losses and output limits determine how rear illumination affects the result; no universal gain percentage applies.
Declare the reference before calculating
Gain (%) = (evaluated energy − reference energy) / reference energy × 100
Use the same energy units and period. Identify whether the output is array DC energy, inverter AC energy or delivered meter energy. A gain at one boundary need not survive unchanged through conversion or clipping.
Two comparisons can answer different questions:
| Reference | What the comparison evaluates |
|---|---|
| Same modeled array with rear response disabled | Incremental effect of enabling the rear contribution within that model |
| A specified monofacial alternative | Complete change between products/configurations, including differences beyond rear response |
Do not label an arbitrary monofacial-versus-bifacial design comparison “rear gain only” if module ratings, tracking, spacing or inverter capacity also change. Record all differences.
Illustrative denominator example
Assume an annual reference of 100 MWh and evaluated output of 112 MWh, both at the same delivery meter boundary. The increase is 12 MWh, so gain is 12% of the reference.
That increase is 10.71% of the evaluated 112 MWh total, rounded. These percentages differ because their denominators differ. The example is hypothetical; the difference is attributable to rear contribution only if the comparison actually isolates it.
A percentage energy gain is not a permanent addition to front-side nameplate power in W. Never turn an annual 12% assumption into a new unconditional module wattage rating.
Bifaciality is not an energy-gain ceiling
Bifacial panel documentation supplies a reference rear/front response ratio. It does not establish the rear/front irradiance ratio outdoors. Bifaciality alone does not set a universal ceiling on field energy gain.
SAM’s module model weights rear plane-of-array irradiance by bifaciality before applying the module model. For an illustrative instant with 800 W/m² front irradiance, 100 W/m² rear irradiance and a 0.80 response factor, the weighted rear term is 80 W/m², and the simplified combined input is 880 W/m². The weighted term is 10% of the front irradiance, not 80% annual energy gain. This calculation excludes the model’s other losses and operating effects.
Use the specific model’s conventions for effective versus incident irradiance and included optical losses. Do not add a raw rear quantity to a front quantity that has already been corrected differently.
Inputs that control the estimate
| Input | Evidence to retain | Interpretation limit |
|---|---|---|
| Albedo | Source, seasonality and surface-maintenance assumptions | Surface reflectance is not rear POA irradiance |
| Geometry | Tilt, pitch, clearance and orientation | No universal minimum clearance predicts gain |
| Rear shading | Rack, torque tube, cables and nearby obstructions | Obstruction area alone may not represent electrical loss |
| Rear uniformity | Supported spatial or mismatch treatment | Average rear light can hide nonuniform response |
| Module | Correct bifaciality and electrical parameters | A technology label does not supply all ratings |
| Output constraints | Inverter, current, export and storage configuration | Extra available DC power is not necessarily extra delivered energy |
PVsyst’s bifacial procedure documents rear structural shading, mismatch and geometry assumptions, including limitations for certain layouts. Check model applicability before interpreting a gain percentage as accurate for an irregular roof or mixed geometry.
Read the report’s gain categories
PVsyst’s bifacial results documentation distinguishes rear irradiance and electrical contributions. Read the exact quantity and denominator in the report instead of treating each positive percentage as the same energy benefit.
Preserve the weather source, model/version, configuration and loss definitions for both cases. Run a sensitivity comparison for uncertain inputs with stated plausible assumptions. Do not manually lower a result in a customer proposal without documenting the changed assumption or uncertainty method.
Value the result without a promised payback
For the hypothetical 12 MWh increase, a constant assumed value of 0.10 currency units/kWh gives 1,200 currency units before operating costs and other financial effects. The tariff is a worksheet assumption, not a market price.
Use the project’s time-dependent energy value when applicable. Include module/rack/ground-treatment costs, maintenance and financing before judging the investment. Higher modeled gain alone does not guarantee lower cost per kWh, faster payback or lender acceptance.
For operating-system verification, use loss analysis to match weather, period and energy boundary. A production increase between two different years is not by itself a controlled bifacial-gain measurement.
SurgePV publishes this glossary and sells solar software. When evaluating its generation and financial workflow, request a requirements demonstration showing supported rear assumptions and the energy boundary used. This page does not certify automatic rear modeling or bankable yield estimates.
Questions about the calculation
Is gain a fixed percentage for each surface color?
No. Reflectance, geometry, weather and losses interact. A generic surface table cannot establish a project’s annual delivered-energy gain.
Does an 80% bifaciality factor mean 80% extra energy?
No. It describes reference module response. The outdoor rear illumination and full conversion path must also be evaluated.
Can tracking benefit be included as bifacial gain?
A comparison may include both changes, but name it accordingly. To isolate rear contribution, keep the reference configuration aligned and disclose the comparison method.
Sources reviewed September 30, 2026. All calculations are illustrative; no measured customer result is claimed.
Related Glossary Terms
About the Contributors
Editorial contributor · SurgePV
Rainer Neumann is credited as an editorial contributor on SurgePV content. This profile does not assert engineering credentials, project totals, software-testing experience, education, speaking engagements, or media citations because independent verification evidence is not retained in the publication record.
CEO & Co-Founder · SurgePV
Keyur Rakholiya is identified by SurgePV as its CEO and a company co-founder. His SurgePV author page lists only role information that can be tied to the public profile below; credentials, project totals, testing claims, media appearances, and speaking engagements are not asserted without retained evidence.
