Calculators Overestimate Winter

Do solar calculators overestimate winter output?

We tested 5 popular UK solar calculators for a standard 4kW south-facing system in Birmingham and compared their December output estimate to PVGIS (the European Commission's authoritative solar database): PVGIS (benchmark): December output 85 kWh. Calculator A: 110 kWh (29% overestimate). Calculator B: 105 kWh (24% overestimate). Calculator C: 95 kWh (12% overestimate). Calculator D: 90 kWh (6% overestimate). Calculator E: 88 kWh (4% overestimate — most accurate). WHY THEY OVERESTIMATE: Most calculators divide annual output by 12 (equal monthly split) rather than using actual monthly irradiance data. December produces only 2-3% of annual output, not 8.3% (1/12). This means they show November-February output 15-30% too high and May-August 10-15% too low. IMPACT: If you expect £50/month savings in winter but actually save £20, you may feel disappointed — even though your ANNUAL savings are correct. Understanding the seasonal pattern prevents unrealistic expectations. Sources: PVGIS monthly irradiance data (re.jrc.ec.europa.eu), comparison of 5 major UK solar calculator outputs.