Solar Panel Wattage Estimator
Daily output (Wh)
1500
How it works
Sizing a solar power system requires matching solar panel wattage and battery storage to your energy consumption patterns, location, and seasonal irradiance variation. The Solar Panel Wattage Estimator calculates the minimum panel wattage and battery capacity for a given daily load and location.
**Daily energy budget** List all devices with their wattage and daily usage hours: LED lights (10W × 5h = 50Wh), phone charging (5W × 2h = 10Wh), laptop (45W × 4h = 180Wh), etc. Sum to total daily Wh requirement. Apply a 25% system efficiency factor for charge controller losses, battery round-trip efficiency (typically 85–95% for Li-ion, 80–85% for lead-acid), and wiring losses.
**Peak sun hours (PSH)** Solar panels are rated at Standard Test Conditions (STC): 1000 W/m² irradiance, 25°C cell temperature. In practice, a 300W panel produces its rated wattage only during "peak sun hours" — the equivalent hours of full STC irradiance. PSH varies by location: UK average: 3.0–3.5 h/day. Southern California: 5.5–6.0 h/day. Central Europe: 3.5–4.5 h/day.
**Panel sizing formula** Required panel wattage = Daily Wh load / (PSH × system_efficiency). For 500Wh daily load in a UK location (3.0 PSH) with 75% system efficiency: Panels = 500 / (3.0 × 0.75) = 222W minimum. Round up to the next standard panel size (250W or 300W).
**Battery sizing** Battery Wh = Daily Wh × autonomy_days / DoD. For 2 days autonomy, 500Wh/day, 80% DoD Li-ion: Battery = 500 × 2 / 0.8 = 1250 Wh.
Privacy: all calculations run in the browser. No data is transmitted.
Frequently Asked Questions
- Typical UK home uses 2700–3700 kWh/year. A 350W solar panel in the UK generates approximately 250–320 kWh/year (based on 3.0–3.5 peak sun hours/day, seasonal variation, and 80% system efficiency). For 3200 kWh/year consumption: need 3200/280 ≈ 11.4 panels → a 4kWp system (10–12 × 400W panels). UK average installed system: 3–4 kWp. US average: 6–8 kW for typical household. This is for grid-connected systems — off-grid homes need additional battery storage to cover cloudy days and nighttime, significantly increasing system size.
- Solar panel wattage (e.g., 100W) is rated at STC: 1000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum. Actual conditions differ: temperature coefficient: panel output decreases by ~0.3–0.4%/°C above 25°C. A 100W panel at 50°C cell temperature (common on a sunny day): output ≈ 100 × (1 − 0.0035 × (50−25)) = 91W. Irradiance variation: on a clear day at peak sun, panels may achieve rated output. On overcast days: 10–25% of rated power. Annual average across a full day (including night, clouds, morning/evening): typically 15–25% of rated peak in temperate climates.
- Optimal tilt angle ≈ your latitude. UK (51°N): ~35–40° tilt facing south. Arizona (33°N): ~30° tilt facing south. South-facing at optimal tilt = maximum annual yield. Deviation from optimal: West or East facing instead of South: typically 15–20% annual energy loss. Horizontal flat roof: 10–15% less than optimal tilt. Vertical wall-mounted: 25–30% less annually. North-facing (in northern hemisphere): 30–50% less. Shading is more damaging than non-optimal orientation: even partial shading of one cell in a series string reduces output of the entire string significantly (unless bypass diodes are used per panel).
- For off-grid sizing: determine worst-case daily energy need (W×h/day). Determine minimum sun hours in your worst season (winter for northern latitudes: 1.5–2.5 hours peak sun). Size panels for worst-case: panels_W = daily_Wh / (min_PSH × 0.75). Size battery for 2–3 days autonomy: battery_Wh = daily_Wh × autonomy_days / DoD. Example: 500Wh/day daily need, 2h winter PSH, 80% DoD LiFePO4, 3-day autonomy. Panels: 500/(2×0.75) = 333W. Battery: 500×3/0.8 = 1875Wh. Minimum viable off-grid system: 400W panels + 2000Wh (2kWh) battery storage.