Calculator Inputs
Large screens show three columns, smaller screens show two, and mobile shows one.
Example Data Table
Sample appliance entries and daily energy calculation.
| Appliance | Watts | Qty | Hours/day | Wh/day |
|---|---|---|---|---|
| LED Lights | 10 | 6 | 5 | 300 |
| Ceiling Fan | 60 | 2 | 8 | 960 |
| Laptop | 65 | 1 | 6 | 390 |
| Refrigerator (avg) | 120 | 1 | 10 | 1200 |
| Total | 2850 Wh/day (2.85 kWh/day) | |||
Formula Used
The calculator sizes panels from daily energy and usable sun hours. It applies efficiency factors to reflect real-world losses.
Wiring suggestion uses a minimum charging margin and a conservative cold-voltage check against the controller limit.
How to Use This Calculator
- Choose Quick total or Appliance list.
- Enter your load and peak sun hours.
- Set panel rating and efficiency assumptions.
- If you use batteries, select off-grid and adjust factors.
- Click Calculate to see panel count and wiring.
- Use the export buttons to save results.
Daily Energy and Peak Demand
Panel sizing starts with daily energy, not nameplate wattage. Add each device’s watts × quantity × hours to get Wh/day, then divide by 1000 for kWh/day. Include standby loads running all day. Peak demand matters for inverter selection: if your highest simultaneous load is 900 W, a 1.2–1.5× safety margin targets roughly 1100–1350 W, especially for motor starts and compressor surges.
Peak Sun Hours and Seasonal Variation
Peak sun hours (PSH) convert sunlight into an equivalent number of full‑power hours. A site averaging 5.0 PSH yields 5 kWh from a 1 kW array before losses. PSH can drop 20–40% in winter, and shading can reduce output disproportionately. Use irradiance averages, not a single sunny week. Consider tilt adjustments, and treat morning and late‑afternoon shading as a serious production limiter.
Derate and Conversion Efficiencies
Real systems deliver less than their rated output. A derate factor accounts for temperature, dust, wiring, and module mismatch; 0.75–0.85 is common in warm climates. Energy passing through an inverter typically retains 90–96%, and higher loading can reduce efficiency at the extremes. Off‑grid systems add controller and battery round‑trip losses, often pushing total usable efficiency to 60–75%. Multiplying these factors produces a realistic overall efficiency for sizing.
Panel Count, Roof Area, and Wiring
Required array watts follow: ArrayWatts = DailyWh ÷ (PSH × OverallEff). Divide ArrayWatts by your panel rating and round up to get panel count. Roof planning uses panel area; twenty 2.2 m² modules need about 44 m², plus spacing. Wiring checks string voltage: series count must be high enough for charging yet low enough that cold‑weather Voc, multiplied by a temperature factor, stays under the controller’s maximum input. Parallel strings increase current, so fusing and cable sizing become critical.
Practical Planning and Validation
Validate results against utility bills and expected growth. Add margin for new appliances, battery aging, and cloudy weeks; many designs include 10–25% headroom. Track production with a simple log or inverter app and refine your PSH and derate assumptions after a month. Inspect connectors periodically. For safety and compliance, confirm conductor sizing, overcurrent protection, grounding, and local electrical rules before purchasing hardware, before final equipment selection.
FAQs
How do I estimate daily energy from a monthly bill?
Use the kWh on your bill and divide by days in the billing period. If you have seasonal variation, repeat for a winter and summer month, then size using the lower sun season and higher consumption.
What peak sun hours value should I use?
Start with long‑term solar data for your city, then choose a conservative average. If reliability matters, use the worst typical month or reduce the annual average by 10–20% to cover clouds and haze.
Why does an off-grid system need more panels?
Off‑grid energy passes through a charge controller and batteries, so round‑trip storage losses reduce usable kWh. Adding panels compensates for those losses and for days when you must recharge after deeper battery discharge.
Should I oversize the solar array?
Often yes. Adding 10–25% headroom helps with module aging, higher temperatures, dust, and future loads. Oversizing can also improve winter performance, but confirm your controller and inverter can accept the extra DC power.
What panel wattage rating should I enter?
Enter the rated power of the panel model you plan to buy, measured under standard test conditions. If you are comparing options, run the calculator with 400 W, 500 W, and 600 W to see how count changes.
Can I rely on the series/parallel wiring suggestion?
Treat it as a starting point. Verify string Voc at your coldest expected temperature, check controller voltage and current limits, and follow local electrical rules. For final design, consult an installer or engineer.