Sizing a Rooftop PV System Against a NZEB Target with Export Limits
A new 1,800 m² office building near Varna is designed to meet the Bulgarian nearly zero-energy building (NZEB) requirement. Annual demand is estimated at 210 MWh electricity and 60 MWh heat (heat pump, COP 3.2). The roof can host 300 kWp of PV. The local distribution operator caps export at 30 kW due to network constraints. The client wants to claim NZEB status and asks how much PV to install, whether battery storage is needed, and how to present the result in the EPC.
1. Calculate the PV generation, self-consumption potential, and whether the 300 kWp array is appropriate given the 30 kW export cap.
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Generation estimate. In Bulgaria, a well-oriented rooftop yields roughly 1,100–1,250 kWh/kWp/year. Using 1,150 kWh/kWp: 300 kWp × 1,150 = 345 MWh/year. That exceeds the building's 210 MWh electricity demand - but generation and demand are not simultaneous.
Self-consumption. An office building's demand peaks during working hours, which coincides reasonably with PV output. Without storage, typical self-consumption for an office is 40–60% of generation. At 50%, self-consumed = ~172 MWh, exported = ~173 MWh. But the export cap is 30 kW instantaneous, not annual. On a sunny midday, 300 kWp could produce ~240 kW, of which the building might use 100–150 kW, leaving 90–140 kW to export - far above the 30 kW cap. The excess would be curtailed.
Conclusion. 300 kWp is oversized for the export constraint unless storage absorbs the surplus. A smaller array (e.g., 150–180 kWp) or a larger array with batteries would be more appropriate. I would model hourly generation against the load profile to size the array and storage, rather than use annual totals.
2. Advise on battery storage sizing and how to present the NZEB claim in the EPC.
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Storage sizing. The goal is to shift midday surplus to evening or to avoid curtailment. Estimate the curtailed energy from the hourly model (e.g., 60–90 MWh/year). A battery of 100–150 kWh usable capacity, cycling daily, can absorb much of the midday peak. Check round-trip efficiency (~90%) and the value of shifted energy versus battery cost. If the export cap is the binding constraint, storage is justified; if not, downsizing PV is cheaper.
NZEB presentation. The EPC must show the building's primary energy use and the renewable share. Report:
- Delivered energy after on-site PV and storage.
- Renewable energy generated on-site and used on-site (not exported).
- The primary energy factor and the resulting class.
Export does not count as reducing the building's own primary energy in the same way as self-consumption, so the NZEB claim depends on self-consumption. I would present the calculation transparently, state the assumption on self-consumption ratio, and note that the NZEB status is conditional on the as-built PV and storage being installed and commissioned. If the export cap forces curtailment, the EPC should reflect the actual usable renewable contribution, not the nameplate array.