Question 1

How much roof area does a viable solar + BESS system need?

Accepted Answer

Roughly 8 m² of usable, unshaded, structurally sound roof area per kWp installed — about 8,000 m² per MWp once access walkways, plant, and edge setbacks are accounted for. A 3 MWp minimum viable system needs around 24,000 m² of roof. Most tier-one industrial-park factories in Bắc Ninh, Bình Dương, Đồng Nai, and Long An have 40,000–200,000 m² of single-storey production roof — well above threshold. Multi-storey facilities need a structural review before sizing.

Question 2

What does a factory solar + BESS system actually cost?

Accepted Answer

Under Self-Invest at Vietnamese rooftop scale, all-in installed solar capex runs approximately $0.50/Wp pre-VAT, or $500,000–$555,000 per MWp on the solar layer. The paired BESS is sized at half the average daily overspill and adds a smaller layer at current installed market prices. For a 25 MWp solar + ~10 MWh BESS, all-in total investment cost sits around $13–17 million depending on region and operating days. Typical unlevered project IRR on Self-Invest solar+BESS sits between 17% and 23% in USD terms, with 55% debt gearing lifting equity IRR into the 25–35% range. Under Zero Capex the customer pays no up-front capital and buys electricity from the investor-owned system at a discount to the prevailing manufacturing tariff.

Question 3

Do I also need BESS to get the full benefit?

Accepted Answer

Standalone BESS — charging from the grid at off-peak and discharging at peak — does not yet earn its keep on the manufacturing tariff. The 22 kV peak/off-peak spread is approximately 2.0× (VND 3,398 peak vs VND 1,190 off-peak; rates per Decision 1279/QĐ-BCT, peak window 17:30–22:30 Mon–Sat per Decision 963/QĐ-BCT), materially weaker than the 3.1× spread on the commercial tariff that hotels and data centres enjoy. But a small BESS paired with rooftop solar is a different proposition. Decision 963 cut the share of solar generation that monetises at peak from roughly 25% to 5%, lowering solar-only revenue by ~19%. A battery sized to mop up the daytime solar overspill — charged for free from the array, discharged across the new 17:30–22:30 evening peak — recovers most of that loss and pushes the integrated solar+BESS IRR back above where solar-alone sat before April 2026.

Question 4

What does the calculator assume?

Accepted Answer

Solar sized at 8 m² per kWp. Region-specific yields of 1,100 kWh/kWp/yr (North), 1,300 (Central), 1,450 (South) — P50 specific yield. Daytime overspill of 30% (5 operating days), 20% (6 days), 5% (7 days). BESS sized at half the average daily overspill, charged free from the array, discharged across the 17:30–22:30 evening peak Mon–Sat per Decision 963. Solar offtake split 95% standard rate / 5% peak post-Decision 963. 22 kV manufacturing tariff per Decision 1279/QĐ-BCT (May 2025). Solar life 25 years, BESS life 15 years. Self-Invest assumes EVN tariff escalation (5% Y1–3, 3% Y4+); Zero Capex assumes a fixed PPA price for the contract term. Capex, O&M, insurance, CPI, FX, depreciation and tax all set to current Vietnam market norms. The output is indicative — Arcus models actual roof, structural class, and EVN bill before quoting.

Indicative numbers on a Vietnam rooftop solar + BESS project, in three inputs.

Own the asset. Pay back from day-one savings.

No up-front. Pay only for the electricity, at a discount.

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