1. Understand the cell designs
TOPCon uses a passivated contact based on a very thin tunnel oxide and silicon. HJT combines crystalline silicon with thin amorphous-silicon layers for passivation and selective contacts. These designs aim to reduce electrical losses in the cell. They do not, by themselves, predict winter generation for a Swiss building.
2. Compare complete modules
Record rated power, dimensions, efficiency, tolerance and test conditions. A laboratory cell record is not the efficiency of the delivered panel. On a restricted roof, also compare layouts: a more powerful but larger module may leave unused space around a dormer. Ask for the total capacity that can actually be fitted while retaining required access and clearances.
3. Read temperature and low-light data
Compare the exact models’ power temperature coefficients and available low-irradiance data. Cell temperature differs from air temperature; mounting and ventilation affect the calculation. A passivation property does not establish a fixed cloudy-weather gain. For a north-facing roof or hot location, model both options using the same weather data, shading and method.
4. Separate warranty and service life
Read product and power warranties separately. Record the initial threshold, contractual degradation, duration and exclusions for the proposed model. A guaranteed minimum power is not a measurement of your system’s ageing. Clarify the evidence required for a claim and who pays for transport, roof access and replacement.
5. Connect the datasheet to the installation
Have voltages, currents, connectors, string configuration and inverter limits checked. For a bifacial offer, ask which rear-irradiance assumptions were used: bifacial describes a module property that can accompany either cell design. Keep dated datasheets with the quotation and layout. Then compare installed cost, useful generation and service without automatically ranking every technology.