EPC Best Practices
Bucharest Villa PV + Storage Design: 8.64 kWp, 89.4% Self-Sufficient
Bucharest villa system design: 18 × 480 Wp modules, a 10 kW Kehua hybrid inverter at 86.4% sizing and DC-coupled LFP storage reaching 89.4% self-sufficiency.
Part 1 recorded the site visit: the roof, its orientation and the load the system has to carry. This part turns that survey into an engineering design — the array, the inverter, the battery and the expected energy flows, all modelled in PV*SOL Premium against Bucharest climate data.
The design pairs an 8.64 kWp array of 18 ZNShine 480 Wp modules, split across two south-facing roofs, with a 10 kW Kehua iStoragE3 hybrid inverter and two DC-coupled lithium iron phosphate (LFP) battery systems. The target is to cover 89.4% of a four-person household's annual consumption from solar.
By the end you should be able to see how array size, string layout, inverter sizing factor and battery capacity combine to produce a self-sufficiency figure — and what to check before you commission a similar system.
Design Inputs: Site, Climate and Load
The villa sits just north of Bucharest in a temperate continental climate, with roughly 1,300 kWh/m² of annual irradiation (about 3.6 kWh/m² per day) and a wide seasonal swing between short, overcast winter days and long summer days.
It connects to the Bucharest-area low-voltage distribution network, so grid connection follows EU requirements for Type-B distributed generators under EN 50549-1.
The load comes from a two-adult, two-child household drawing about 4,308 kWh a year with a 10 kW peak. That peak, not the annual total, is what the storage is sized against.
System Design Summary
Generation, storage and grid exchange are managed by a single hybrid inverter. The array is split across two south-facing roof areas and wired as two independent strings. The table below sets out the design boundary conditions.
Parameter | Value | Remark |
|---|---|---|
PV generator | 8.64 kWp | 18 × 480 Wp ZNShine modules |
PV surface | 36.0 m² | Two south-facing roof areas |
Inverter | 10 kW hybrid (Kehua iStoragE3 10K) | 3-phase, 86.4% sizing factor |
Battery | 2 × LFP, 33 kWh total | Kehua iStoragE B16-S2, 16.5 kWh each, DC-coupled |
Mains | 3-phase, 400/230 V | 230 V phase-to-neutral; cos φ = 1 |
Climate data | Bucharest 2001–2020 | Meteonorm 8.2 |
Grid connection | Low-voltage parallel | EU Type-B, EN 50549-1 |
Table 1: System design boundary conditions.
Array Design: Two South-Facing Roofs, Two Strings
The array uses 18 × 480 Wp ZNShine modules split across the two usable south-facing areas:
1. Roof area 1: 8 modules, 16.0 m², 3.84 kWp, 10° tilt, azimuth 181°, roof-mounted
2. Roof area 2: 10 modules, 20.0 m², 4.80 kWp, 10° tilt, azimuth 182°, roof-mounted
The two groups form two strings: MPPT 1 takes the string of 8, MPPT 2 the string of 10. Each string sits near its optimum voltage, and the two MPPT inputs operate independently so shading on one roof does not drag the other down.
Inverter Selection: Why the Sizing Factor Is Below 1
Conversion and energy management are handled by a Kehua iStoragE3 10K hybrid inverter: 10 kW rated AC output on a 3-phase 400/230 V mains, matched to the 8.64 kWp array at an 86.4% sizing factor.
In this project the sizing factor below 1 is deliberate. It keeps the DC array inside the inverter's working range, so the array is fully used with no clipping — the simulation records 0 kWh of clipping loss at the inverter. The trade-off is that the inverter runs at 86.4% of its rating at array peak.
The inverter's two MPPT inputs match the two strings exactly. For a distributed generator on an LV network, the inverter is certified for parallel operation under the EU Type-B regime (EN 50549-1) that applies to Romanian prosumer installations.
Battery Design: DC-Coupled LFP Sized to the Evening Peak
Storage is carried by two Kehua iStoragE B16-S2 lithium iron phosphate batteries of 16.5 kWh each — 33 kWh in total — DC-coupled to the iStoragE3 10K inverter at a nominal 10 kW.
DC coupling lets the battery charge straight from the array and discharge through the same inverter, so energy passes through a single conversion stage. Two batteries are specified so stored solar comfortably covers the evening load.
The simulation reports the battery delivering 2,684 kWh a year to the household, with a low cycling rate of about 3.6% (annual battery throughput relative to capacity), which supports a design service life beyond 20 years.
Energy Simulation Results
The design was validated in PV*SOL Premium against Bucharest climate data. The table below gives the headline annual results.
Metric | Value | Remark |
|---|---|---|
Specific annual yield (DC) | 1,321 kWh/kWp | Module-side yield before PR conversion |
Performance ratio (PR) | 88.49% | |
Yield loss from shading | 1.2% | |
PV output to grid and battery (AC) | 10,139 kWh/yr | |
Solar self-consumption (direct PV + battery discharge) | 3,851 kWh/yr | Energy delivered from PV and battery to the load |
Grid export | 6,288 kWh/yr | |
Self-consumption rate | 38.0% | Share of PV output used on site |
Self-sufficiency | 89.4% | Share of total consumption |
CO₂ avoided | 4,151 kg/yr | |
Table 2: Simulated annual performance (PV*SOL, Bucharest climate).
The two charts below show the annual energy flow and how the household's consumption is covered month by month.
Electrical Layout and Drawing Set
Beyond equipment selection, the design fixes the electrical layout: the array is wired as two strings into the two MPPT inputs, the AC side is 3-phase with 6 mm² copper cable running 7 m to the distribution point, and a bidirectional meter at the feed-in point registers both import and export.
The drawings below cover the installation overview, the module area dimensioning plan and the circuit diagram.
Grid Connection and EU Compliance in Romania
For a distributed generator on a low-voltage network, connection follows EU Type-B requirements under EN 50549-1. A bidirectional meter records energy flow in both directions, which enables the prosumer arrangement: own use first, export of the remainder.
Connection terms — full export versus surplus export, net metering rules and any capacity limits — must be confirmed with the local distribution operator against the actual load profile before construction begins.
How the Hybrid System Runs Day to Day
During the day the array powers the home first. Once the house is covered, surplus solar charges the battery, and any remaining energy feeds the grid. In the evening, when household demand peaks, the battery discharges into the load — shifting daytime solar into the evening and reducing imports during the peak.
The control strategy keeps battery cycling low at about 3.6%, avoiding unnecessary charging cycles. The simulation shows no battery energy exported to the grid, and a self-sufficiency of 89.4% — a textbook peak-shaving and load-shifting pattern.
How POP Energy Approaches This Design
For POP Energy, equipment is only part of the delivery. A hybrid system that genuinely shifts the evening load is the product of a full chain of engineering decisions: how the roof is split and the strings are laid out, how the inverter sizing factor and battery capacity are matched to the load peak, how string layout and shading risk are checked before anything is mounted, and how yield and self-sufficiency are proven by simulation.
POP Energy supplied the solar and storage systems for the FIFA World Cup Qatar 2022 and the XXIV Olympic Winter Games Beijing 2022, and applies the same design logic from single-family villas to utility-scale plants.
Design matters, but service carries equal weight: complete scheme design, matched equipment selection, standardised installation quality control, on-site technical guidance and staged sign-off are what keep a design performing across its whole service life.
Key Takeaways for a PV + Storage Design
1. Split the array to fit the usable roof, and give each group its own MPPT. Two south roofs and two strings (8 and 10 modules) keep each string at optimum voltage and isolate shading.
2. A sizing factor below 1 was chosen deliberately on this project. At 86.4% the array stays inside the inverter working range with no clipping — worth doing when the peak load and roof layout justify the extra inverter headroom.
3. Size the battery to the evening peak, not to the array. Two DC-coupled LFP batteries, 33 kWh in total, carry the evening load at a low cycling rate of about 3.6%, supporting a design life beyond 20 years.
4. Validate with simulation, not rules of thumb. PV*SOL against local climate data gives a defensible performance ratio (88.49%), shading loss (1.2%) and self-sufficiency (89.4%), with a DC yield of 1,321 kWh/kWp converting to 10,139 kWh AC.
5. Connect to EU requirements. LV Type-B generators follow EN 50549-1, and a bidirectional meter enables the prosumer arrangement.
6. Aim for high self-sufficiency with healthy battery cycling. 89.4% self-sufficiency with no battery-to-grid export means the storage serves the household rather than tariff arbitrage.
Frequently Asked Questions
Why are the modules split across two roofs instead of one array?
The module count on each roof follows the south-facing surface actually available there. Splitting uses both areas (8 and 10 modules) while keeping each string near its optimum voltage, so the two MPPT inputs can absorb different shading independently.
Why is the inverter sizing factor below 1 (86.4%)?
Pairing an 8.64 kWp array with a 10 kW inverter keeps the array inside the inverter's working range. That avoids clipping — the simulation records 0 kWh of clipping loss — and means the inverter is never asked to handle more than its rating.
How was the battery capacity chosen?
From the evening load, not from array size. The DC-coupled storage is sized so the battery delivers 2,684 kWh a year to the household at a low cycling rate of about 3.6%, which supports a service life beyond 20 years.
How much will a system like this actually generate?
Using Bucharest climate data, PV*SOL returns a performance ratio of 88.49%, a self-sufficiency of 89.4% and about 4,151 kg of CO₂ avoided per year. Real output depends on weather and equipment condition.
What does grid connection require in Romania?
A distributed generator on the LV network follows EU Type-B requirements under EN 50549-1. A bidirectional meter registers both import and export, enabling the prosumer model of self-consumption with surplus export.
Planning a PV + Storage System in Bucharest
If you are planning a hybrid solar-plus-storage system for a villa in the Bucharest area, start from the load curve rather than the roof. Send us your household load profile and site details, and we will review array layout, inverter sizing factor, storage capacity and string design for your specific conditions.
This series continues with a construction and commissioning report once the project is built.
