Solar Power System Design: The 5 Numbers You Must Calculate Before Buying Panels

Solar Power System Design: The 5 Numbers You Must Calculate Before Buying Panels

Buying panels first and calculating later is the most expensive order. A solar system is sized from the load, not from the panel price. These five numbers — in order — decide whether your system works, and what it costs.

1. Daily energy demand (kWh/day)

List every appliance, its wattage, and hours of use per day. Sum them into a single daily kilowatt-hour figure. This is the foundation — everything else scales from it.

2. Peak sun hours for your location

Your array output depends on the irradiation your site actually receives, not the panel’s lab rating. Multiply the array kW by your location’s peak sun hours to estimate daily production. Same panels, different city, different output.

3. Array size (kWp) and panel count

Divide daily demand by peak sun hours, then apply the system efficiency derating (wiring, inverter, temperature — commonly 75–85%). That gives the array kW, which determines how many panels and how much roof area you need.

4. Battery bank sizing (Ah) and days of autonomy

For off-grid, size the battery for your daily load plus the number of cloudy days you want to ride through, and respect depth of discharge. Undersizing here means the lights go out exactly when you need the system most.

5. Inverter and cable sizing

The inverter must handle the array’s peak output and the surge of inductive loads (pumps, motors). Cable cross-section must keep voltage drop within limits over the actual run length — a thin cable quietly wastes the power you just paid to generate.

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These five calculations chain together — an error early in the chain makes every later number wrong.

We built a single Excel sheet that walks the whole sequence: daily load, array sizing, panel count, battery bank, and inverter/cable selection, with the formulas already in place. Enter your appliances and location, and it recalculates the rest.

👉 Excel Solar Power System Design Calculator

*For design study and feasibility — always have the final installation reviewed by a qualified installer against local code.*