Free electrical planning tool
Solar Panel Size Calculator
Compare your daily electrical consumption with an illustrative solar yield, and estimate the panel wattage needed to offset that consumption.
Your system
UK seasonal values are illustrative starting points. Use local yield estimates for your intended location and season. Near-zero sunlight or severe shading makes solar-only planning impractical.
Your results
- Estimated daily solar generation
- 1,200 Wh
- Generation at nominal battery voltage
- 100 Ah
- Solar wattage to offset daily usage
- 333.33 W
- Energy surplus
- 200 Wh/day
- Potential charging surplus after daily usage
- 200 Wh/day
Charging surplus equivalent: 16.67 Ah/day. 100% of daily usage offset. Actual charging depends on battery acceptance, charge control and further losses.
- Daily energy use1,000 Wh
- Estimated solar generation1,200 Wh
Planning estimates, not installation approval.
How to use this calculator
Enter daily usage and panel rating, choose a seasonal starting point, then adjust peak sun hours, baseline efficiency and additional shading for your situation. Peak sun hours are equivalent full-power hours, not daylight hours.
Calculation method
Daily solar Wh = panel watts × peak sun hours × system efficiency × (1 − additional shading loss). Required panel watts = daily consumption ÷ the same yield factor. Shading is applied once, separately from the baseline efficiency.
What the results mean
Solar Ah is energy divided by nominal battery voltage, not a controller’s maximum current. Charging contribution is the surplus after daily loads, before any extra battery-specific charging losses, limits or changes in state of charge.
Worked example
400W × 4 peak sun hours × 75% efficiency × 90% unshaded factor gives 1,080Wh/day. Against 1,000Wh usage this leaves 80Wh, about 6.67Ah at 12V. Roughly 370.37W would offset that usage under the same assumptions.
Common mistakes
Do not use daylight duration as peak sun hours. Do not apply shading here if it is already included in your efficiency. Daily energy balance does not prove you can meet an overnight load without sufficient storage.
Frequently asked questions
Will a 400W panel generate 400W all day?
No. The rating is under standard test conditions. Actual output changes with sunlight, temperature, mounting and shading.
Can UK winter solar cover all my power use?
Not reliably by assumption. Use conservative local yield data and plan alternative charging and sufficient battery storage.
Does this select an MPPT controller?
No. Controller voltage and current limits, array configuration and panel specifications need separate checks.
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