Why these numbers — climate and orientation in Bilbao
Bilbao sits at 43.26°N. PVGIS v5.3 estimates an annual yield of 803 kWh per kWp installed for a south-facing balcony mounted vertically (90° tilt) with a 14% system loss assumption. That is -38% relative to the Spain country-average vertical yield of ~1,300 kWh/kWp/yr we use in the per-country payback widget.
For the same hardware mounted at PVGIS-optimal tilt (36° from horizontal, south-facing), the same kit would produce 1,170 kWh/kWp/yr — roughly 46% more. The vertical-mount penalty in Bilbao is average; it's the price you pay for being able to use the kit on a balcony railing without a frame and without rooftop access.
The monthly profile above shows the seasonal shape clearly: Oct is the strongest month (82 kWh per kWp installed), Jun the weakest (52 kWh per kWp). For a typical 800 W (= 0.8 kWp) plug-in kit you can multiply these numbers by 0.8 to read directly in absolute kWh per month.
How to read the payback number
The 3.4-year payback assumes a 70% self-consumption rate (a typical 2-3 person urban household running fridge, standby load, and a midday work-from-home dishwasher cycle). The remaining 30% of generation gets exported and credited at the country-default export tariff (6.0¢/kWh in Spain). Higher self-consumption shortens the payback; an empty flat during working hours can stretch it by 1-2 years.
For full per-country regulatory context — registration, tax treatment, tenant rights, condominium rules — see the Spain country guide.
Where these numbers come from
Yield numbers are PVGIS estimates based on a long-run irradiance model; actual production varies year-to-year ±10% due to weather. Payback is a deliberately simplified calculation suitable for orientation, not a financial recommendation. For installer-grade modelling, use the full PVGIS tool directly with your specific tilt, azimuth and shading inputs.