On this page
- 01Before you start
- 02Fundamentals
- 03Where it sits in the project
- 04Architectures: on-grid, off-grid, hybrid
- 05Core components
- 06Sizing calculations
- 07Performance challenges
- 08Applications
- 09Installation and commissioning
- 10Check that it works
- 11Common problems and how to fix them
- 12Frequently asked questions
- 13Talk to PSS about solar
Solar on a site is not a catalogue product; it is an energy balance you have to close. Commercial buildings, factories, farms, telecom sites and remote infrastructure use PV to cut energy cost, improve resilience and support sustainability — but panels in the sun are not enough. Design needs loads, irradiance, losses, space, inverter limits, battery autonomy, protection and utility rules.
Pre-sales: accurate sizing avoids underperformance and wasted CAPEX. Site: mounting, strings, cable sizing, grounding, protection and commissioning decide safety and life.
Before you start
- Monthly/hourly load and daytime share; existing board SLD and spare breaker capacity.
- Roof/ground area, orientation, tilt, shading survey; structure loading and wind zone.
- Architecture: on-grid / off-grid / hybrid; backup circuits, hours, chemistry, location.
- Net-metering eligibility vs customer max load / caps; approvals (distribution company, EgyptERA, NREA as applicable).
- Monitoring: plant-level or string-level with alarms; expansion and motor/AC surge loads.
Fundamentals
PV modules convert sunlight to DC. An inverter converts DC to AC for loads or an approved utility connection. Energy may be consumed on site, exported where permitted, stored in batteries, or run dedicated loads (e.g. irrigation pumps).
| Unit | Meaning | Example |
|---|---|---|
| W / kW | Instantaneous power | 500 W appliance; 10 kW inverter |
| Wh / kWh | Energy over time | 50 kWh/day facility use |
| kWp | Rated DC array capacity at STC | 20 × 550 W = 11 kWp |
kWp ≠ daily kWh. Production depends on sun, temperature, shading, orientation, soiling, inverter losses and more. Size the array from energy demand and site resource; size inverter, cables, protection and batteries from their power, voltage, current and duty needs.
Where it sits in the project
Mechanically on roof or ground mounts; electrically on the LV main board via a dedicated PV breaker — or at MV via a step-up transformer on larger sites. Connected in parallel with the utility, not in series with the load — so utility protection settings and a bi-directional meter belong in scope.
Architectures: on-grid, off-grid, hybrid
On-grid (grid-tied)
Runs in parallel with the utility. Daytime PV serves loads; surplus may export if authorised; shortfall comes from the grid. Usually lowest CAPEX and no battery in a conventional layout. Limitation: a standard grid-following inverter shuts down on utility failure (anti-islanding, e.g. IEC 62116) so it must not be assumed to provide blackout backup. Backup needs island-capable equipment, isolation/transfer and usually batteries.
Off-grid
Independent of the utility — remote farms, buildings, telecom, monitoring sites. Typical: PV, inverter or DC power gear, batteries, charge controller or hybrid inverter with MPPT, protection and monitoring. Batteries cover night and low-sun periods. Challenges: autonomy, inverter surge/motor start, cloudy spells (generator or load management), temperature and lifecycle cost.
Hybrid
Grid + PV + batteries. Can prioritise self-consumption, charge storage, import, and back up selected loads when designed for islanded operation. Not every hybrid inverter can carry every load in an outage — backup kW, surge, SoC, PV availability and load selection decide.
| Aspect | On-grid | Off-grid | Hybrid |
|---|---|---|---|
| Utility | Yes | No | Usually yes |
| Battery | Usually not required | Generally essential | Usually included |
| Without grid | Normally no (IEC 62116) | Yes | Yes if designed for islanding |
| Best fit | Urban / daytime load | Remote / no feeder | Savings + selected backup |
Export, net metering and billing depend on regulation and utility approval — never assume they are automatic.
Egypt net-metering / self-consumption notes (verify current rules)
| Item | Position (as commonly applied) |
|---|---|
| Capacity vs customer load | May not exceed the customer’s max load in the prior fiscal year (EgyptERA Decision 2/2020) |
| Caps | 20 MW/project; 25 MW aggregate/customer; 300 MW national under net metering (Decision 2/2020) |
| Merger fee / exemptions | Fee bands by voltage (Circular 3/2022); exemptions and self-consumption rules updated in Circular 3/2023 — confirm before quoting |
| Battery in self-consumption | Often capped (e.g. ~20% of project capacity under Circular 3/2023) — verify current text |
Licensing can set the real capacity ceiling before any engineering calculation.
Core components
Modules — mono-/polycrystalline silicon, thin-film where justified. Check Pmax, efficiency (modern crystalline often ~20%+ — use datasheet), Voc/Vmp/Isc/Imp, temperature coefficients, mechanical load, warranties, size/weight. High cell temperature cuts voltage and power vs STC.
Inverters — string, central, micro, hybrid, off-grid. Check AC kW, DC voltage/current/MPPT ranges and channel count, efficiency, environment, grid code. MPPT tracks the max-power point; use separate MPPTs for different orientations/shading — do not mix dissimilar strings on one tracker against OEM rules.
Batteries — LFP (LiFePO₄): high usable energy, deep cycle with BMS, higher CAPEX. Lead-acid (AGM/gel/flooded/OPzS): often lower CAPEX, more conservative DoD, more volume/weight, ventilation/maintenance as type requires. Preliminary DoD assumptions (~50% some lead-acid; ~80–90% some LFP) are not universal — datasheet rules. Size kWh (energy) and kW (power) separately.
Mounting — aluminium/galvanised steel for wind uplift, dead load, corrosion, waterproofing, access, tilt/row spacing. Egypt ~22–31° N: fixed arrays often face south near latitude tilt; east–west trades peak for a flatter daily curve. No single tilt is optimal everywhere.
BOS — DC/AC isolators and breakers, SPDs, string fuses where required, PV-rated DC cables/connectors, AC distribution, earthing/bonding, monitoring, labels, emergency shutdown. Select for real V/I/fault/environment — not every project needs the same combiner.
Typical standards in specs: modules IEC 61215/61730; inverter IEC 62109 / 61727; array wiring IEC 62548; lithium safety IEC 62619.
Sizing calculations
Use location-specific irradiance (peak sun hours as a planning aid — equivalent hours at 1,000 W/m²). Egypt specific yield often ~4.76–5.74 kWh/kWp/day (~1,700–2,100 kWh/kWp/year) before site losses — confirm with a site assessment/simulation.
Daily energy
Daily kWh = Power (W) × hours ÷ 1,000
(use realistic duty cycles). Prefer meter data.
Illustrative schedule → e.g. ~31.7 kWh/day (lighting, PCs, AC, pump, other). Proceed with a worked 50 kWh/day site:
Array capacity
kWp ≈ Daily kWh ÷ (PSH × PR)
PR captures temperature, inverter, wiring, mismatch, soiling, etc. Do not double-count the same losses.
Example: 50 ÷ (5.5 × 0.80) ≈ 11.4 kWp. Egypt design often uses PR 0.75–0.80. A 100 kWp roof in Upper Egypt at PR 0.78 ≈ 148,000 kWh/year; unmanaged dust can cost ~5% more.
Energy-first thumb: monthly kWh × daytime share ÷ net specific yield. 20,000 kWh/month with 60% daylight → ~12,000 kWh from PV ≈ ~90 kWp at ~1,450 kWh/kWp net.
Module count and area
11,400 W ÷ 550 W ≈ 20.73 → 21 modules → 11.55 kWp. Count must also satisfy string V/I and layout.
Module ~2.2 × 1.1 m ≈ 2.42 m² → 21 × 2.42 ≈ 50.8 m² of module area — not total footprint. Add walkways, setbacks, fire access, row spacing, obstacles. Rough planning ~5–6 m²/kWp for some layouts only (≈57–68 m² for 11.4 kWp) — not universal.
Inverter and DC/AC ratio
DC/AC = array kWp ÷ inverter AC kW
. Example 130 / 100 = 1.30. Ratios >1.0 are common; clipping occurs when DC available exceeds AC conversion limit — can be an intentional yield trade-off if OEM limits are respected. Egypt often uses 1.1–1.3. Also check simultaneous loads, motor start and backup kW — array kWp ≠ inverter AC capability.
Battery
Nominal kWh ≈ (Battery-supplied kWh/day × autonomy days) ÷ (usable DoD × battery-to-load efficiency)
Example: (10 × 2) ÷ (0.80 × 0.90) ≈ 27.8 kWh. If PV covers daytime, the battery may not need full daily site demand. Off-grid thumb: 80 kWh/day × 2 days ÷ (0.8 × 0.9) ≈ 222 kWh — that number, not panel count, drives the argument.
String configuration
Series raises voltage; parallel raises current. Check cold Voc against inverter max DC (Voc rises as temperature falls). Example: 550 W module Voc 49.6 V → ~52 V at 5 °C (coeff. typically −0.25 to −0.35 %/°C); 18 modules ≈ 940 V — OK for 1,000 V inverter, not for 900 V. Hot Vmp must stay above MPPT min. Verify Imp/Isc per MPPT, string count, connectors, overcurrent protection. Never assume every inverter accepts 1,000 V or 1,500 V.
Performance challenges
Partial shading — trees, tanks, parapets, neighbours. Series mismatch hurts yield; separate MPPTs / optimizers / microinverters help but placement and string design come first.
Temperature derating — STC is 25 °C cell / 1,000 W/m²; real cells run hotter → less than nameplate W. Use power temperature coefficient and yield simulation; ventilate mounts where possible.
DC voltage drop —
Vd = I × R_total
;
R ≈ ρ L / A
with both go and return. Calculate — do not prescribe 4/6/10 mm² by habit. Keep drop within project/standard limits (often aimed ~1–2% on DC). Use PV-rated cable and matched connectors.
Soiling — dust/sand/droppings; losses are site-specific — not a universal 15–30%. Clean from measured yield vs weather-adjusted expectation; follow OEM methods; never cold-wash hot modules or work live without procedure. Dusty sites need frequent inspection — “every 2–4 weeks” is not universal.
Clipping — AC output pinned at rating under strong sun; review logs and DC/AC economics.
Battery shortfall — undersized usable kWh, high simultaneous load, poor charging, temperature/BMS settings, surge demand. Re-check load profile, SoC history and continuous/surge ratings.
Applications
Residential — daytime offset ± selected backup. Commercial/industrial — daytime coincidence, demand, roof structure, export limits, harmonics/protection coordination. Agriculture / pumping — hydraulic duty:
P_hyd = ρ g Q H
; electrical input ≈ hydraulic ÷ overall efficiency; water storage can beat batteries for irrigation. Telecom / remote — critical DC/AC, autonomy, seasonal yield, remote alarms, generator backup if needed. Critical hybrid backup — define which circuits, duration and shed list; backup port ≠ whole building; design transfer, neutral, earthing, fault protection.
Installation and commissioning
Before: loads, structure, shading, utility rules, BOM, drawings/string schedules, method statements.
During: approved mounts and torque, cable support, compatible connectors, DC/AC separation, earthing/SPD/isolation, labels, working-at-height and electrical safety.
Commission: polarity/strings, Voc checks, insulation (method safe for electronics), bonding, protection/isolators, inverter grid parameters, BMS/comms, monitoring/alarms, backup/transfer tests, as-builts. PV is live whenever illuminated — even with the inverter off.
Check that it works
- String Voc/Imp match the design; cold Voc stays below inverter max DC.
- Polarity, insulation, bonding, SPDs and isolators verified; inverter grid parameters set for the utility.
- Monitoring and alarms report expected yield; backup/transfer tested where hybrid islanding is required.
- As-builts, string schedules, datasheets and O&M handed over.
Common problems and how to fix them
String sized on nominal Voc
Trips on the first cold morning. Recalculate cold Voc and respect inverter max DC.
Shading ignored
Series mismatch kills yield. Redesign strings/MPPTs and placement.
DC cable without voltage-drop check
Calculate go-and-return drop; use PV-rated cable and matched connectors.
AC / household connectors on DC
Replace with PV-rated connectors and isolators.
No array DC isolator
Add isolation per design and code.
Soiling assumed at 2% with no cleaning budget
Measure yield vs weather-adjusted expectation; clean per OEM methods.
Battery sized on inverter rating not night load
Re-size usable kWh from autonomy and DoD.
Export assumed without available net-metering capacity
Confirm eligibility and caps before quoting.
Frequently asked questions
Does standard on-grid work in a blackout?
Normally no — anti-islanding. Need hybrid/backup design.
How many panels for 10 kW?
10,000 ÷ 550 ≈ 18.2 → ≥19 for >10 kWp (10.45 kWp) if layout/inverter allow. Distinguish 10 kWp array vs 10 kW AC inverter.
Space for ~10 kWp?
~19 × 2.42 m² ≈ 46 m² module area + setbacks/walkways.
kWp vs kWh?
Capacity at STC vs energy over time.
What is clipping?
DC available > inverter AC limit — may be intentional within OEM rules.
Is LFP always better than lead-acid?
Not always — compare usable energy, cycle life, temperature, CAPEX, maintenance, warranty.
Cleaning interval?
Site-driven from soiling and recovered energy — not universal.
Can PV run AC / motors?
Yes if array, inverter and battery sized for continuous and start/surge.
Is a battery always required?
No for conventional on-grid; yes typically for off-grid; hybrid when storage/backup needed.
What inputs does a quotation need?
Location, architecture, utility rules, daily/peak demand, motors, backup duration, area, shading, schedule, expansion — plus datasheets and drawings for final design.
Talk to PSS about solar
PSS designs, installs, and maintains solar power systems — on-grid, off-grid and hybrid — from energy balance through commissioning. Contact the PSS team to book a first site survey.
