UPMSP High School Solar Repair cheat sheet

Standard Test Conditions (STC): irradiance 1000 W/m², cell temperature 25 °C, AM 1.5 spectrum - the basis for nameplate ratings.

Open-circuit voltage V_oc rises ~2.2 mV per cell per °C of falling temperature; a cold sunny morning gives the highest array voltage, so size string voltage for record-low site temperature.

Maximum power point: current falls roughly linearly with irradiance, while voltage falls only logarithmically - a 50 % irradiance drop cuts current about half but voltage only a few percent.

Off-grid (stand-alone) system = array + charge controller + battery + inverter; grid-tied has no battery and uses an anti-islanding inverter; hybrid combines both.

Energy yield estimate: E(kWh/day) = array kWp × peak sun hours × system derating factor (typically 0.70–0.80 for battery systems, 0.75–0.85 for grid-tied).

Series connection adds voltages, current stays at the lowest module's value; parallel connection adds currents, voltage stays at the lowest string's value.

Bypass diodes limit hot-spot heating when a cell is shaded; blocking diodes stop reverse current from a stronger parallel string into a weaker one.

Typical crystalline-silicon cell: V_oc ≈ 0.6–0.7 V, I_sc ≈ 8–9 A for a full-size 156–166 mm cell; a 60-cell module gives roughly 30–40 V V_oc.

Fill factor FF = (V_mp × I_mp) / (V_oc × I_sc); higher FF means a squarer I-V curve and better-quality cell (0.75–0.82 typical for c-Si).

Temperature coefficients (c-Si, per °C): P_max ≈ −0.35 to −0.45 %/°C, V_oc ≈ −0.30 %/°C, I_sc ≈ +0.05 %/°C.

Module bill of materials: front glass, EVA encapsulant, solar cells, EVA back layer, backsheet or rear glass, aluminium frame, junction box with bypass diodes and IP-rated cables/MC4 connectors.

IEC 61215 covers design qualification and type approval; IEC 61730 covers safety (insulation, mechanical, fire). A module needs both for sale in most markets.

Nameplate values to record on commissioning: P_max, V_oc, I_sc, V_mp, I_mp, maximum system voltage (usually 1000 V or 1500 V), and maximum series fuse rating.

Hot-spot endurance test (IEC 61215) confirms bypass-diode function; a failed diode shows as a burnt junction-box spot or a module section that stays cold in thermal imaging.

Lead-acid charging stages: bulk (constant current to absorption voltage), absorption (constant voltage, current tapers), float (lower constant voltage); equalisation is an occasional controlled overcharge for flooded cells only.

Absorption voltages at 25 °C (per 12 V block): flooded ≈ 14.4–14.8 V, AGM ≈ 14.4–14.7 V, gel ≈ 14.1–14.4 V; apply a temperature-compensation slope of about −3 to −5 mV/°C/cell.

Depth of discharge governs cycle life: lead-acid at 50 % DoD gives far more cycles than at 80 %; LiFePO₄ tolerates 80–90 % DoD with a BMS handling cell balancing and cutoff.

PWM controller: array V_mp must be close to battery voltage; it simply switches the array to the battery, losing the voltage difference.

MPPT controller: converts surplus array voltage into extra charge current; usable array power ≈ battery charge power, so MPPT gains most when array V_mp is well above battery voltage and in cold weather.

Sizing: battery capacity Ah = (daily Wh × autonomy days) / (system voltage × DoD × inverter/round-trip efficiency).

Never mix batteries of different age, chemistry, or capacity in one bank; a weak block drags the whole string down and reverses the healthy blocks' current.

Inverter key ratings: continuous AC output (W/VA), surge rating for motor start (often 2× for a few seconds), DC input window (e.g. 42–60 V for a 48 V battery inverter), and peak efficiency.

Stand-alone inverters are usually pure sine wave; modified sine wave distorts output and can overheat motors, trip some chargers, and buzz audio equipment.

Grid-tied inverter protection functions: anti-islanding, over/under voltage and frequency trip, DC insulation monitoring, and grid-code-compliant ramp rates.

Idle/no-load draw typically 10–50 W for a home inverter; leave it off when no AC load is needed on a small off-grid system.

MPPT is a DC-DC converter; a DC-DC buck converter steps voltage down while boosting current, a boost converter does the opposite - power in ≈ power out minus losses.

Inverter derating: output falls above ~40 °C ambient; ensure clearance around the unit and avoid direct sun on the enclosure.

A hybrid inverter can charge from PV, discharge to loads, and export or limit export; verify its zero-export/CT clamp is installed and calibrated if export limiting is required.

Voltage drop: V_drop = 2 × L × I × ρ / A (copper ρ ≈ 0.0175 Ω·mm²/m); keep PV DC drop < 2–3 % and AC drop < 3–5 %.

PV DC cable must be double-insulated, UV-resistant, and rated for the array's maximum system voltage; typical sizes 4 mm² and 6 mm² with MC4 connectors.

String fuse rating: 1.25 × I_sc, and the fuse must not exceed the module's maximum series fuse rating; fuses protect cables, not modules.

Earthing: bond module frames, mounting rails, and inverter chassis to a common earth electrode; use a single earth point for the array and size the conductor per local code.

DC and AC must be in separate conduit or have a physical barrier; PV DC switches must be rated for DC and clearly labelled, with an accessible DC isolator near the inverter.

RCD/GFCI type matters with inverters: use Type B or a type the inverter manufacturer specifies, because DC leakage and high-frequency components can blind Type AC devices.

Cable current-carrying capacity must be derated for ambient temperature and conduit grouping; a 6 mm² cable rated 50 A free-air may only carry 30–35 A in a hot enclosed conduit.

Irradiance check: a solar pathfinder or smartphone sun-path app confirms shading across the year; even a thin cable shadow on one cell can cut a whole module's output.

Tilt rule of thumb: latitude − 10° for year-round yield in the tropics; steeper tilts favour winter, flatter tilts favour summer and reduce soiling.

Roof-mount clearance and wind loading: leave 100–150 mm behind the module for cooling, and follow local wind-load tables for rail spans and anchor pull-out.

Roof condition and structure: verify purlin spacing, roof age, and waterproofing details before drilling; use flashing or butyl seals at every penetration.

Ground mount: concrete footing depth below frost line; leave the lowest module edge ≥ 300 mm above ground for vegetation and snow.

Commissioning record: serial numbers, string V_oc/I_sc, insulation resistance test, earth continuity, torque values, and inverter firmware - hand a signed copy to the client.

Array orientation: true south in the northern hemisphere, true north in the southern; magnetic declination must be corrected when using a compass.

Diagnostic order: visual inspection → open-circuit voltage per string → short-circuit/operating current → insulation resistance → earth continuity → inverter error log.

Open-circuit voltage test: measured string V_oc should be within ~5 % of the sum of module V_oc values corrected to cell temperature; a low reading means a broken series link or failed bypass diode.

Insulation resistance (megger) test: apply 500 V or 1000 V DC between conductors and earth; a healthy array reads > 1 MΩ (often > 40 MΩ); low values point to water ingress or cable damage.

I-V curve tracing: a step or knee in the curve indicates a shaded or failed substring; a shifted curve indicates soiling or degradation; compare against the module datasheet at measured irradiance and temperature.

Thermal imaging finds hot spots, failed bypass diodes, loose terminations, and cell cracks that a voltmeter will not reveal.

Common inverter faults: 'grid fault' = voltage/frequency out of window or loose AC terminal; 'isolation fault' = low insulation resistance; 'no battery' = BMS communication or fuse.

Before touching DC conductors: isolate the array, wait for the inverter's stated discharge time, verify absence of voltage with a meter rated for the array voltage, then earth and short the conductors.

Cleaning frequency depends on soiling rate: dusty or coastal sites may need monthly cleaning; temperate sites with rain may need only 1–2 cleanings a year.

Clean early morning or evening with soft brushes and plain water; never use abrasive pads, high-pressure jets, or cold water on hot glass (thermal shock cracks).

Check torque on module clamps and DC terminations at each service; loose connections show as heat on thermal imaging and voltage drop under load.

Inspect for snail trails, delamination, browning, frame corrosion, and junction-box cracks; these are degradation signs that reduce yield and may justify warranty claims.

Performance ratio PR = actual energy / (array kWp × irradiance kWh/m²); a sudden PR drop of > 10 % signals a fault, not normal soiling.

Battery maintenance: check electrolyte level in flooded cells monthly, clean and grease terminals, and record specific gravity or per-cell voltage at each service.

Keep a monitoring log with daily kWh, inverter error codes, and cleaning dates; trend lines reveal slow degradation that a single reading misses.

PV modules generate voltage whenever light falls on them; the only safe isolation is covering the array or working at night, plus opening the DC isolator.

Use a CAT III (or higher) meter rated for the array voltage; a 1000 V DC array needs a meter rated ≥ 1000 V DC on the relevant range.

PPE for DC work: insulated gloves rated for the voltage, safety glasses, arc-rated clothing where required, and non-conductive tools.

Arc-flash risk exists on the DC side even at low current; never open a DC circuit under load - open the isolator first, then disconnect connectors.

Lockout/tagout: isolate AC and DC, lock the isolator, tag it, verify absence of voltage, and prove the tester on a known live source before and after.

Battery hazards: hydrogen gas from flooded lead-acid (no sparks, ventilate), acid burns (eyewash and bicarbonate nearby), and short-circuit current high enough to weld jewellery.

Working at height: harness and anchor point, edge protection, and never carry modules up a ladder alone - use a hoist or two-person lift.

Solar home system sizing: daily Wh = sum of (appliance watts × hours); then divide by system voltage and DoD to get battery Ah, and by peak sun hours and derating to get array Wp.

DC appliances avoid inverter losses: a 12 V DC fan or LED lamp uses 10–20 % less energy than the same appliance through a modified sine inverter.

Solar lantern and solar home system kits usually use LiFePO₄ or Li-ion with an integrated PWM/MPPT controller and a USB output; check the battery's usable Wh before promising runtime.

Solar water pumping: a DC pump's flow follows irradiance; size the array for the worst-month sun hours and use a controller with dry-run protection.

Solar street lights: panel tilt and battery autonomy (often 3–5 rainy days) determine reliability; a dirty panel or shaded pole is the most common failure.

Solar refrigeration and vaccine cold chain need a controller with low-voltage disconnect set above the compressor's minimum, and a battery bank sized for cloudy spells.

Small-scale inverters for appliances: check surge watts of motor loads (fridge, pump) against the inverter's surge rating, not just its continuous rating.