Solar System Size Calculator

Understand Solar PV Size Estimation from first principles

This section is intentionally more detailed than a normal calculator page. The aim is to help a learner move from what to enter, to why the formula works, to what must be checked in a real installation or financial decision.

1. Core relationship

A 20 kWh/day load with 5 peak-sun-hours and a 0.8 performance ratio needs about 5 kW of PV as a first estimate.

Before calculating, define every symbol, unit and reference point. A technically correct formula can still produce a wrong answer when line/phase quantities, annual/monthly rates, one-way/round-trip length, or input/output power are mixed.

2. Physical or mathematical meaning

Solar PV Size Estimation starts with clearly defined inputs and units.

The core relationship used here is: PV kW ≈ daily energy demand ÷ (peak-sun-hours × performance ratio).

The result is only as good as the assumptions supplied by the user.

A robust calculation should include a sensitivity check rather than relying on one input set.

For real decisions, compare the calculated result with manufacturer, lender, utility or official product data as applicable.

3. Step-by-step reasoning workflow

1. Step 1: Estimate daily kWh. Record the value, its unit and where it came from before moving to the next step.
2. Step 2: Enter peak-sun-hours. Record the value, its unit and where it came from before moving to the next step.
3. Step 3: Apply performance ratio. Record the value, its unit and where it came from before moving to the next step.
4. Step 4: Estimate PV kW. Record the value, its unit and where it came from before moving to the next step.
5. Step 5: Check roof area. Record the value, its unit and where it came from before moving to the next step.
6. Step 6: Check inverter and battery requirements. Record the value, its unit and where it came from before moving to the next step.

4. Worked-example method

Use a worked example in three layers: first write the known values, then substitute them into the equation without rounding, and finally interpret whether the result is plausible. After the calculation, change one input at a time and observe the direction and sensitivity of the result. This develops engineering or financial intuition instead of only producing a number.

5. Sensitivity: what makes the result move?

Do not stop after one answer. Increase and decrease the important inputs and observe what changes. If the result is highly sensitive to an uncertain assumption, that assumption deserves better measurement or a larger design margin.

6. Common mistakes

• Mixing units or periods
• Using an optimistic assumption without a lower-case scenario
• Rounding too early in intermediate calculations
• Ignoring fees, losses, taxes, derating or other real-world adjustments
• Treating an estimate as a guaranteed or final design value

7. Field / decision checklist

• Check every input and unit
• Record the assumption source
• Run a conservative scenario
• Compare with an independent reference
• Keep a margin for uncertainty

8. How to read the diagram

Read the diagram from left to right. Each block is a stage in the reasoning chain. In electrical topics this usually follows energy or signal flow from the source to the load and protection. In finance topics it follows cash flow through time and compounding. If a stage is unknown, the final answer should be treated as provisional.

9. Why measured or real values may differ

Calculators deliberately simplify reality. Electrical equipment has tolerances, temperature effects, waveform distortion, voltage variation and dynamic behaviour. Financial outcomes have timing, fees, taxes, changing rates and market variability. A difference does not automatically mean the formula is wrong; first compare assumptions, units and the exact quantity being measured.

10. Deeper questions to ask