Report:
Time-Dependent Pricing for Rooftop Solar Surplus to Incentivize BESS in Vietnam
Time-Dependent Pricing for Rooftop Solar Surplus to Incentivize BESS in Vietnam
Under both Decree 57/2025 (governing Direct Power Purchase Agreements, including private-wire RTS sales to large consumers) and Decree 58/2025 (governing self-production and self-consumption RTS systems), rooftop solar (RTS) owners selling surplus electricity to EVN under receive a price equal to the previous year’s average Vietnam Wholesale Electricity Market (VWEM) price. Both Decrees currently limit the total excess to 20% of RTS output; however, MOIT is considering raising the surplus export cap from the current 20% to 50% of total RTS output.
While a flat-price annual average approach provides simplicity, it eliminates the intraday price signal that is fundamental to i) incentivizing investment in behind-the-meter battery energy storage systems (BESS) and ii) supporting the grid in meeting the evening peak demand. A flat annual average price pays the same rate for a kilowatt-hour exported at midday—when the VWEM price is at its lowest—while for one exported during the evening peak, when prices are 30–45% above the daily average.
A new XanhTerra Report evaluates an option in Vietnam to support behind-the-meter BESS adoption, using a market-based approach. We examine how other countries have transitioned from flat to time-granular export pricing for distributed solar surplus, and the measurable impact on BESS uptake. The international evidence is unambiguous: when export compensation reflects real-time or time-of-use wholesale prices, it creates a powerful commercial signal for RTS owners to invest in battery storage.
The sections below provide a high level summary of the Report (full report only available by sending an email with your name and contact information to contact@xanhterra.com)
The Problem: Flat Pricing Suppresses Investment Signals
Under Decrees 57 and 58, surplus rooftop solar (RTS) sold to EVN is priced at the previous year's annual average VWEM price—a single flat rate regardless of when electricity is exported.
But the Vietnam Wholesale Electricity Market (VWEM) tells a very different story. XanhTerra's analysis of five years of half-hourly price data (2021–2025) from Vietnam's National System and Market Operator (NSMO) reveals a pronounced and widening "duck curve" in wholesale prices.
40-50%
Midday Trough
FMP drops 40–50% below the daily average during solar hours (9:30 AM–2:30 PM) due to solar saturation
30–45%
Evening Peak
FMP rises 30–45% above average during evening demand (5:00–8:00 PM) when thermal generation fills the gap
60-80%
Intraday Spreak
The midday-to-evening price spread reaches 60–80% of the average price—and is widening over the years
Zero!
BESS Signal
Under flat annual pricing, RTS owners have zero financial incentive to shift exports from midday to evening peak
Average VWEM FMP Averaged by Half-Hour Interval, 2021-2025
Source: VWEM data from NSMO, XanhTerra analysis
Percent Difference Relative to Average VWEM FMP Price, 2021–2025
Source: VWEM data from NSMO, XanhTerra analysis
The core problem: A flat annual average price pays the same for a kWh exported at midday—when the grid doesn't need it—as one exported at 6 PM when demand peaks. This applies to a household with a 10 kWp system under Decree 58 and a factory with a 500 kWp system under Decree 57 alike.
Vietnam's grid faces its most acute stress during evening peak hours (5:00–8:30 PM), when air conditioning loads remain high but solar generation has declined. Surplus RTS exported at midday provides limited system value. A time-dependent pricing mechanism would align RTS behavior with grid needs.