What Is Solar Energy

What is solar energy is answered from physics: it is the energy contained in the Sun's electromagnetic radiation, which reaches the Earth's surface at a maximum irradiance of approximately 1,000 W/m² (STC: Standard Test Conditions, 25 °C, AM 1.5). The photovoltaic effect, discovered by Edmond Becquerel in 1839 and explained by Albert Einstein in 1905 (Nobel Prize 1921), occurs when a photon with energy exceeding the semiconductor's band gap (1.12 eV for silicon) frees an electron from the valence band to the conduction band, generating an electron-hole pair that produces electric current upon separation at the cell's p-n junction.

The maximum theoretical efficiency of a single-junction cell is limited by the Shockley-Queisser limit to 33.7% for a band gap of 1.34 eV. Photons with energy below the band gap are not absorbed (transmission loss), while photons with excess energy lose the surplus as heat (thermalisation loss). Multi-junction cells overcome this limit by stacking semiconductors with different band gaps: the laboratory record stands at 47.6% (NREL, 2023, 6-junction cell under concentration).

The photovoltaic market is dominated by crystalline silicon (c-Si), accounting for 95% of global production (ITRPV, 2024). Monocrystalline PERC (Passivated Emitter and Rear Cell) achieves commercial efficiencies of 21-23% at a module cost of €0.15-0.25/Wp (2024). TOPCon (Tunnel Oxide Passivated Contact) technology raises commercial efficiency to 24-25%, and heterojunction (HJT) combines crystalline silicon with amorphous silicon layers to achieve 25-26% with lower temperature degradation (-0.26%/°C versus -0.35%/°C for PERC).

Perovskite cells represent the most promising emerging technology: laboratory efficiency of 26.1% for single cells (NREL, 2024) and 33.9% for perovskite-silicon tandems (EPFL/CSEM, 2024), surpassing the theoretical limit of silicon alone. Their solution-deposition fabrication promises costs below €0.10/Wp, but long-term stability (degradation from moisture and UV) remains the main barrier to mass commercialisation. Thin film (CdTe, CIGS) fills specific niches with efficiencies of 18-22% and advantages under low-irradiance and high-temperature conditions.

Spain receives global horizontal irradiation of 1,400-1,900 kWh/m²·year depending on location: from 1,400 kWh/m²·year along the Cantabrian coast to 1,900 kWh/m²·year in the southeast (Almería, Murcia). With single-axis tracking, equivalent sun hours (ESH) reach 1,800-2,200 hours/year, meaning a 1 kWp system produces 1,400-2,200 kWh/year depending on location and tilt. The PVGIS database (JRC, European Commission) allows expected production calculations for any location and configuration.

Spain reached 25.1 GW of cumulative photovoltaic capacity by the end of 2023 (REE), with 5.6 GW of new installation that year. Photovoltaic generation covered 14.4% of peninsular electricity demand in 2023 (REE). The LCOE (Levelized Cost of Energy) for new utility-scale PV plants stands at €25-35/MWh in Spain (IRENA, 2023), below the marginal cost of natural gas (€50-80/MWh), confirming photovoltaics as the cheapest source of electricity generation on the Iberian Peninsula.

Royal Decree 244/2019 regulates electricity self-consumption in Spain, eliminating the former "sun tax" and establishing two modalities: self-consumption without surplus (all energy consumed on-site) and self-consumption with surplus (excess injected into the grid). The surplus modality under simplified compensation allows deducting the value of injected energy from the electricity bill, with compensation prices of €0.05-0.10/kWh depending on the retailer (2024).

A typical residential self-consumption installation in Spain has 3-5 kWp, occupies 15-25 m² of roof area, costs €4,000-7,000 (without battery) or €8,000-14,000 (with 5-10 kWh lithium battery), and covers 30-50% of annual consumption without battery or 60-80% with battery. The payback period is 5-8 years without battery and 8-12 years with battery (IDAE). Collective self-consumption, regulated by the same RD 244/2019, allows sharing an installation among multiple consumers within the same building or within a 500 m radius (extended to 2 km by RD-ley 29/2021).

Storage is key to overcoming solar intermittency. Lithium iron phosphate (LFP) batteries dominate the stationary market at prices of €150-250/kWh (2024), cycle life of 6,000-10,000 cycles, and round-trip efficiency of 92-96%. Sodium-ion batteries, with projected costs of €40-80/kWh by 2028 and no dependence on lithium or cobalt, represent the most promising alternative for large-scale storage.

At grid scale, green hydrogen produced by electrolysis with solar electricity (PEM electrolyser efficiency: 60-70% LHV) enables seasonal storage. Spain, as the EU's second-highest country for solar irradiation, has a competitive green hydrogen production potential estimated at €1.50-2.50/kg by 2030 (Hydrogen Roadmap, MITECO 2020), compared to a current cost of €4-6/kg. Solar-storage-hydrogen integration configures an energy system where solar energy transitions from an intermittent source to a firm pillar of supply.