Retorno de inversión. Analizando los costos y beneficios de la energía solar en edificaciones

The cost of solar photovoltaic energy has experienced the most accelerated reduction of any energy technology in history. The global weighted average LCOE (Levelised Cost of Energy) for commercial-scale photovoltaic installations fell from 0.381 USD/kWh in 2010 to 0.044 USD/kWh in 2023, a reduction of 89% in 13 years (IRENA, Renewable Power Generation Costs in 2023). The cost of monocrystalline silicon photovoltaic modules dropped from 2.00 USD/Wp in 2010 to 0.15-0.20 USD/Wp in 2023 (learning curve: 24% reduction for every doubling of cumulative installed capacity). Global installed photovoltaic capacity surpassed 1,600 GW by the end of 2023, with annual additions of 420 GW (75% more than in 2022), of which 240 GW were in China. In Spain, installed photovoltaic capacity reached 25.5 GW by December 2023 (Red Electrica), generating 14.4% of the country's total electricity production.

The building self-consumption segment has experienced explosive growth following the repeal of the so-called "sun tax" (Royal Decree-Law 15/2018) and the approval of Royal Decree 244/2019 regulating self-consumption with surplus compensation. Spain installed 2,649 MW of self-consumption capacity in 2023 (APPA Renovables), of which 45% was residential, 35% commercial, and 20% industrial. The cost of a residential self-consumption installation (3-10 kWp) ranges from 0.90 to 1.40 EUR/Wp installed (including modules, inverter, mounting structure, wiring, protections, permits, and labour), equivalent to 4,500-7,000 EUR for a typical 5 kWp installation. VAT of 21% must be added to this cost (or 0% if the temporary exemption in effect until December 2024 for installations ≤ 10 kWp in dwellings is applicable). Subsidies from the self-consumption incentive programme (Royal Decree 477/2021, NextGenerationEU funds) offer grants of 300-600 EUR/kWp for residential installations, reducing the net cost by 25-40%.

The annual production of a photovoltaic installation in Spain varies according to the site's solar irradiation, module orientation and tilt, and system losses (temperature, soiling, wiring, inverter). Average global horizontal irradiation in Spain ranges from 1,300 kWh/m²·year (Cantabrian coast) to 2,000 kWh/m²·year (southeastern peninsula, Canary Islands). For a 5 kWp installation with south orientation and optimal tilt (30-35° at latitudes 37-43°N), the annual specific production (Yield) is 1,400-1,700 kWh/kWp·year, resulting in a total generation of 7,000-8,500 kWh/year. Typical losses include: temperature (5-10%), soiling (2-5%), wiring (1-2%), inverter (3-5%, efficiency of 96-98%), and mismatch (1-2%), totalling a Performance Ratio (PR) of 75-85%.

Economic savings depend on the self-consumption ratio (energy consumed directly from solar production without passing through the grid). Without a battery, the typical self-consumption of a household with partial daytime occupancy (present in the morning on weekends and evenings during weekdays) ranges from 25% to 40% of total production. With a 5-10 kWh battery, self-consumption rises to 60-80%. Surplus energy fed into the grid is compensated financially under the simplified compensation mechanism (Royal Decree 244/2019): the retailer deducts the value of surplus energy (at the average market price, approximately 0.05-0.08 EUR/kWh) from the monthly bill, provided the compensation does not exceed the value of the energy consumption term from the grid. For a 5 kWp installation in Madrid (production: 7,800 kWh/year, self-consumption at 35% = 2,730 kWh, surplus: 5,070 kWh) with an electricity price of 0.18 EUR/kWh (average indexed tariff 2023), the annual saving is: 2,730 x 0.18 = 491 EUR (direct self-consumption) + 5,070 x 0.06 = 304 EUR (surplus compensation) = 795 EUR/year. With a 10 kWh battery (self-consumption at 75% = 5,850 kWh, surplus: 1,950 kWh): 5,850 x 0.18 = 1,053 EUR + 1,950 x 0.06 = 117 EUR = 1,170 EUR/year.

The simple payback period of a residential photovoltaic installation without battery in Spain ranges from 4 to 7 years without subsidies, and from 3 to 5 years with NextGeneration subsidies. For a 5 kWp installation in Madrid: investment of 5,500 EUR (VAT included), subsidy of 1,800 EUR (360 EUR/kWp), net cost of 3,700 EUR, annual saving of 795 EUR: payback = 4.7 years. With a 10 kWh battery (additional cost: 5,000-7,000 EUR, additional subsidy: 490 EUR/kWh x 10 = 4,900 EUR): total net investment of 5,800 EUR, annual saving of 1,170 EUR: payback = 5.0 years. The IRR (Internal Rate of Return) over 25 years (guaranteed module service life with degradation ≤ 0.5%/year) ranges from 10% to 18% for battery-free installations, and from 6% to 12% with a battery (the battery has a service life of 10-15 years with 6,000-10,000 cycles, requiring replacement at 12-15 years).

The NPV (Net Present Value) confirms profitability: for the 5 kWp battery-free installation in Madrid, with a discount rate of 3%, energy inflation of 2% per year, and module degradation of 0.5%/year, the NPV over 25 years is 10,200-13,500 EUR, multiplying the initial investment by 2-3 times. Factors that improve profitability include: higher irradiation (Seville: Yield of 1,700 kWh/kWp compared to 1,500 in Madrid), higher electricity prices (each additional 0.01 EUR/kWh increases annual savings by 75-85 EUR), IBI property tax rebates (reductions of 25-50% for 3-10 years, applied by more than 400 Spanish municipalities), and ICIO construction tax rebates (reductions of 50-95%). Factors that reduce profitability include: unfavourable orientation (east-west: -10-15% production), partial shading (-5-25% depending on hours and affected area), and low simultaneity between production and consumption (self-consumption < 25% without battery). The cost-benefit analysis demonstrates that solar energy in buildings is the most profitable energy investment available to residential consumers in Spain, with returns exceeding those of any financial product of comparable risk.

Solar thermal energy for domestic hot water (DHW) production complements photovoltaics with equally attractive returns. The CTE HE-4 requires a minimum solar contribution of 30-70% of DHW demand depending on the climate zone (I to V). An installation of 4 m² of flat plate collectors (optical efficiency η₀ = 0.75-0.80, loss coefficient a₁ = 3.5-4.0 W/m²·K) with a 200-300 litre storage tank produces 1,500-2,500 kWh/year of thermal energy, covering 60-80% of the DHW demand for a family of 4 (160 litres/day at 60°C). The installation cost is 2,500-4,000 EUR, with annual savings of 200-400 EUR (replacing natural gas or electricity) and a payback period of 6-12 years. Evacuated tube collectors (efficiency 15-20% higher than flat plate at temperatures > 60°C) extend the application to low-temperature underfloor heating (35-40°C).

BIPV (Building-Integrated Photovoltaics) systems integrate photovoltaic modules as a building envelope element, replacing conventional facade, roof, or glazing materials. Semi-transparent photovoltaic glass (efficiency of 8-12%, light transmission of 10-40%) replaces conventional glazing in facades and skylights, generating electricity at a cost premium of 30-60% over standard glazing but eliminating the cost of a separate support structure. Solar roof tiles (Tesla Solar Roof: 71.67 W/tile, efficiency of 19.5%; REC Solar: 58 W/tile) integrate photovoltaic generation into the roof with a conventional aesthetic appearance, at a cost of 250-350 EUR/m² (compared to 50-80 EUR/m² for a ceramic tile roof + 90-140 EUR/m² for rack-mounted panels). The global BIPV market reached 4.2 billion USD in 2023 (Market Research Future), with a CAGR of 18% through 2030. The sector outlook points to a solar LCOE below 0.03 USD/kWh by 2030, lithium iron phosphate (LFP) batteries at 50-80 USD/kWh (compared to 150 USD/kWh in 2023), and perovskite-silicon tandem modules with efficiencies exceeding 30%, which will further accelerate the return on investment of solar energy in buildings.