Sizing a PV system correctly

Real consumption · Geographic location · Roof type · Energy goals

An oversized system means money spent needlessly; an undersized one will not cover your intended consumption. Sizing starts from the home's real consumption and accounts for several location and technology factors.

Rule of thumb: 1 kWp installed produces roughly 1,100–1,350 kWh/year in Romania depending on location (south > centre > north). A family with an average consumption of 4,500 kWh/year needs about 4–5 kWp.

Steps for correct sizing

1

Calculate average daily consumption

Take your electricity bill (kWh/month) and divide by 30. E.g. 375 kWh/month ÷ 30 = 12.5 kWh/day.

2

Identify the location's specific yield

Using PVGIS or Romania's solar map. South (Dobrogea, Muntenia): ~1,300–1,350 kWh/kWp/year. Centre (Transylvania, Moldova): ~1,100–1,200 kWh/kWp/year.

3

Calculate the required power

Formula: Power (kWp) = Annual consumption (kWh) ÷ Specific yield (kWh/kWp). E.g. 4,500 kWh ÷ 1,250 = 3.6 kWp → rounded to 4 kWp.

4

Choose the number of panels

Divide total power by the power of one panel. E.g. 4,000 W ÷ 415 W/panel = 9.6 → 10 panels.

5

Size the inverter

Inverter power = 90–110% of the installed DC power. Oversizing DC by 10–20% (clipping ratio) is economically efficient.

6

Check the roof

1 kWp needs roughly 5–6 m² of usable roof (standard monocrystalline panels). Check the structural strength if the installed power > 10 kWp.

Estimated yield by region

Dobrogea (Constanța, Tulcea)1,300–1,380 kWh/kWp

Muntenia, Oltenia1,220–1,310 kWh/kWp

Moldova (Iași, Bacău)1,150–1,230 kWh/kWp

Centre (Cluj, Sibiu)1,100–1,190 kWh/kWp

North-West (Oradea, Satu Mare)1,080–1,160 kWh/kWp

Yield correction factors

SOUTH orientation (optimal)coef. 1.00

SE / SW orientationcoef. 0.93–0.97

E / W orientationcoef. 0.78–0.85

Optimal tilt (30–35°)coef. 1.00

Tilt of 15° or 45°coef. 0.95–0.97

Partial shading−10% to −40%

Practical sizing examples

Client profile	Annual consumption	Recommended power	Configuration
2-room apartment	1,800 kWh	3 kWp	7–8 panels · 3 kW inverter
Small house without AC	3,500 kWh	4–5 kWp	10–12 panels · 4 kW inverter
House with AC + heat pump	6,500 kWh	8–10 kWp	18–24 panels · 8–10 kW inverter
Small business	15,000 kWh	15–18 kWp	35–45 panels · 15 kW inverter
Farm / industrial hall	80,000 kWh	80–100 kWp	Three-phase string inverter

Watch the 100 kWp threshold: Systems above 100 kWp require a special connection approval and an ANRE/distributor grid study. Below 100 kWp the prosumer process is simplified.

String inverters vs Microinverters

Detailed comparison · 9 criteria · Use case

The choice of inverter type significantly influences production, cost, maintenance and system flexibility. The two main technologies have completely different performance profiles depending on installation conditions.

Detailed comparison

Criterion	String inverter	Microinverter
Initial cost	Low – Medium 3,000–8,000 RON central inverter	+20–40% vs string ~400–600 RON/unit × no. of panels
Performance under shading	Medium – Poor 1 shaded panel affects the whole string	Excellent Each panel works independently
Monitoring	String level You detect the group, not individual panels	Panel level Precise diagnostics, individual data
Voltage on the roof	DC 300–1,000 V Arc-fault risk; long DC wiring	Only 230 V AC on the roof Greater fire safety
Maintenance	Simple – a single unit Quick replacement, easy access	Complex – N units on the roof Replacement requires climbing the roof
Scalability	Limited Adding panels may require a new inverter	Excellent Each new panel = +1 microinverter
Typical warranty	5–12 years Huawei, SMA, Fronius: 10 years standard	20–25 years Enphase IQ8, APsystems
Peak efficiency	97.5–98.7% Superior performance in optimal conditions	95.5–97.5% Slightly higher losses from individual conversion
Battery compatibility	Excellent – hybrid inverter Native DC integration	Limited – AC-coupled Slightly lower efficiency
Recommended applications	Shade-free roofs, large systems (>10 kWp), with hybrid battery	Complex roofs with shade, multiple orientations, small–medium systems

Choose a string inverter if: the roof is free of shade, you want a hybrid battery, you have a limited budget, or it is a commercial / industrial installation.

Choose microinverters if: the roof has chimneys, dormers or trees that cast shade, you want panel-by-panel monitoring, or panels on multiple orientations (E + S + W).

Intermediate solution: Power optimizers

Optimizers (SolarEdge P-series, Tigo, Huawei SmartLogger) are fitted on each panel and remove the string inverter's shading disadvantage while keeping a lower cost than microinverters. Per-panel MPPT, with DC→AC conversion still centralized.

Advantage: medium cost, panel-by-panel monitoring, safety (shut-down to 1V DC).
Disadvantage: an extra point of failure per panel; DC wiring still present on the roof.

The benefits of batteries in PV systems

LFP storage · Energy independence · Profitability · Backup

Dominant technology: LFP (Lithium Iron Phosphate) batteries are today's standard — safer than NMC, with 4,000–6,000 cycles and a 15–20 year lifespan. Manufacturers: BYD Battery-Box, Pylontech, Huawei LUNA, Deye, Dyness, Felicity, V-TAC, Fronius.

Independence from the grid

Energy produced during the day is stored and used at night. Self-consumption rises from 30–40% to 70–90%. With 10 kWh + 10 kWp, a family can reach 85–95% autonomy over the summer.

Significant bill reduction

Without a battery, the surplus is fed into the grid at 0.10–0.28 RON/kWh. With a battery, the same energy is worth 0.80–1.20 RON/kWh. A 3–8× saving per kWh.

Power-outage protection (UPS)

Instant backup <20ms, faster than traditional UPS units. Sensitive equipment keeps running. Recommended capacity for a full night: 5–10 kWh.

Tariff arbitrage (peak/off-peak)

Night charging at the reduced tariff, discharging at the peak tariff. An extra 0.20–0.40 RON/kWh saving. Ideal for businesses with morning and evening peak consumption.

Protection against energy price rises

Energy prices in Romania rose by 60–80% over the past 4 years. The battery stores energy at the solar cost (~0.05–0.10 RON/kWh amortized), regardless of grid price movements.

Capturing peak production

The solar production peak (10:00–14:00) is captured and delivered in the evening at 18:00–23:00, when consumption is highest. Self-consumption goes from 30–40% to 70–90%.

Electric vehicle integration

Charging exclusively from solar energy via a smart wallbox. The cost per km drops to 0.01–0.03 RON/km versus 0.15–0.25 RON/km from the night grid.

Local grid stabilization

Hybrid inverters (Huawei SUN2000, Victron, SolarEdge) offer Virtual Power Plant and reactive power control, compensating voltage fluctuations and extending the life of sensitive appliances.

When is a battery worth it?

High evening consumption (18:00–23:00): if >40% of consumption is in the evening, the battery pays off quickly.
Frequent power outages in the area, or a critical business where an interruption means financial loss.
Two-zone or dynamic tariff: a night–day difference > 0.30 RON/kWh makes arbitrage profitable.
Electric car at home: the buffer battery allows controlled charging from solar, even after sunset.
Weak prosumer compensation: if the distributor pays under 0.25 RON/kWh for the surplus, storage is more profitable than feeding into the grid.

Indicative ROI: A 10 kWh LFP battery (BYD/Pylontech, ~6,000–9,000 EUR) has an ROI of 8–12 years, with a 15–20 year lifespan. With subsidies (AFM, PNRR) the ROI drops to 5–8 years.

How shading affects production

Types of shading · Quantitative impact · Technical solutions

Shading is the most common factor that drops production below expectations. Understanding the mechanisms and the solutions is essential both during design and when diagnosing an existing system.

Types of shading

Point shading (chimney, antenna)−10% to −60%

Tree shading (deciduous)−20% to −50%

Inter-row shading−5% to −30%

Deposits (dust, moss)−3% to −25%

Partial snow−15% to −80%

Why does 1 shaded panel affect the whole string?

Panels in a string are connected in series — the current through each panel is the same. A shaded panel produces less current, limiting the whole string to the weakest panel's current. It is the chain effect: the weakest link dictates the total performance.

Bypass diodes (3 per standard panel) limit the damage to 1/3 of the panel, but still cause significant losses.

Quantitative impact by system type

String system without optimizers — 1 panel 50% shaded−35% string production

System with optimizers — 1 panel 50% shaded−5% total

System with microinverters — 1 panel 50% shaded−5% total

Uniform dust deposits over the whole surface−8% production

String system — 3 panels fully shaded (snow)−62% production

Technical solutions to minimize losses

Power optimizers (SolarEdge, Tigo, Huawei): per-panel MPPT, reducing shading losses by 70–90%. Additional cost ~200–400 RON/panel.
Microinverters (Enphase, APsystems): the best solution for complex shade — each panel fully independent. Optimal for multiple orientations.
Correct string design: do not mix shaded and unshaded panels in the same string. Separate roof faces = separate MPPTs.
Correct inter-row spacing: shading angle at the winter solstice <15° for ground-mounted panels.
Regular cleaning: spring washing removes 3–8% of losses from deposits — +50–200 kWh/year per 10 kWp.

PVGIS simulation & shade analysis: Before installation, use PVGIS to simulate production including shading from the local horizon. An ELECRO designer performs 3D shade analysis with PVsyst or HelioScope for systems >10 kWp.

Proper maintenance of a residential system

Preventive · Corrective · Recommended schedule · Warranties

A well-maintained residential PV system runs at optimal parameters for 25–30 years. Maintenance is not expensive when planned correctly — neglect can reduce production by 10–25% and can void the manufacturer's warranties.

Recommended maintenance schedule

Daily / Auto

Production monitoring via appCheck daily production against the historical estimate. A sudden drop (>20%) with no weather change indicates a problem (inverter, panel, connection). Apps: Huawei FusionSolar, Fronius Solar.web, Enphase Enlighten.

Monthly

Quick visual inspectionFrom the street or window: check whether any panels look different (discoloration, cracks, heavy deposits). Check that the inverter LED is green.

Half-yearly

Panel cleaningWash with deionized or tap water (low pressure, no abrasive detergent). Avoid washing during peak solar hours (thermal shock risk). Professional service cost: 300–600 RON/session for 10 kWp.

Yearly

Full technical reviewChecking MC4 connection tightness, inspecting cables, testing string voltage, checking inverter firmware, inspecting the mounting structure (corrosion, loose anchors), checking inverter enclosure sealing.

Every 5 years

In-depth IR inspectionInfrared thermography of the panels to detect hotspots. IV-curve testing per string. Insulation resistance check (min. 1 MΩ). BMS recalibration (if a battery is present). Audit cost: 800–1,500 RON for 10 kWp.

At 10–15 years

Inverter replacementString inverter lifespan: 10–15 years. Replacement does not require changing the panels. Cost: 2,500–5,000 RON (inverter + labour). LFP batteries drop to ~80% capacity after 4,000 cycles (15–20 years).

Warning signs

Low production with no weather cause — faulty panel, loose connection or inverter error.
Red / orange inverter LED — error code in the app; technician intervention needed.
Burning smell or unusual noise — immediate shutdown from the DC isolator and an urgent service call.
Panel with a visible black spot — severe hotspot; fire risk; immediate replacement.
Battery that no longer charges fully — accelerated degradation; check the BMS and consult the manufacturer.

Warranties

Panel product warranty10–15 years

Panel performance warranty25–30 years (≥80%)

Standard inverter warranty5–10 years

Extended inverter warrantyup to 25 years

LFP battery warranty10 years / 4,000 cycles

Mounting structure warranty10–20 years

ELECRO maintenance contract: Includes an annual review, active monitoring, priority for interventions and an annual performance report. An actively monitored system produces on average 8–12% more than an unsupervised one, thanks to fast fault detection.

Technical analyses