Site Survey and System Sizing: Loads, Shading, Tilt, Array, Battery, Cable and Inverter
This section covers site survey and system sizing: load assessment, shading analysis, tilt and orientation, array and battery sizing, and cable and inverter sizing. The exam tests whether you can turn a customer's electricity bill and roof into a safe, correctly sized design.
Load and energy audit
List each appliance with wattage, quantity and hours per day. Convert to Wh/day, add a loss allowance, typically 20–30% for off-grid, and include future load. For grid-tied systems, use the electricity bill's monthly kWh and daytime load profile.
Shading, tilt and orientation
In Punjab, latitude is about 30–32°N; fixed tilt near latitude is a good starting point, adjusted for winter or summer yield. Face modules true south, allowing for magnetic declination. Avoid shading between 9 a.m. and 3 p.m.; check trees, parapets, water tanks and future construction. Row spacing must prevent one row shading the next in December.
Battery and array sizing
Off-grid array: Pdc = daily Wh ÷ (peak sun hours × derate factor). Battery capacity: Ah = daily Wh × autonomy days ÷ (DoD × inverter/controller efficiency × battery voltage). Use C10 or C20 ratings consistently and check depth of discharge.
Cable and inverter
Keep DC voltage drop under about 3% and AC under 3%. Cable area A = 2 × length × current × resistivity ÷ allowable voltage drop. Inverter DC/AC ratio is often 1.1–1.3; check MPPT window and maximum input current.
Worked example
Daily load 3 kWh, 4.5 peak sun hours, derate 0.75: array = 3000 ÷ (4.5 × 0.75) = 889 W, so two 450 W modules. Battery at 24 V, one day autonomy, DoD 0.5, efficiency 0.9: 3000 ÷ (0.5 × 0.9) = 6667 Wh, or 278 Ah at 24 V. For a 10 m, 10 A DC run at 30 V with 4 mm² copper and resistivity 0.0175, drop = 2 × 10 × 10 × 0.0175 ÷ 4 = 0.875 V, about 2.9%, close to the limit.
Traps
Mixing W and Wh; forgetting inverter standby and cable losses; sizing battery without DoD; assuming south and latitude tilt always optimal; sizing cable by current only and ignoring voltage drop.