Solar and battery re-priced from Thai market data (October 2026)

- Rooftop PV, 300 kWp (size still illustrative): 20 to 26 THB/W installed
  with the Deye hybrid inverters, 6.0M to 7.8M THB (was 25 to 35 THB/W,
  7.5M to 10.5M). Sources in the utilities CSV: DEDE Thailand PV Status
  report (Mahidol survey: 22 to 25 THB/W for factory systems of 10 kW to
  1 MW, 2023), CapSolar 2026 (20 to 25 under 1 MWp), Saimai Solar's list
  (300 kW for 5.4M THB), Deye hybrid inverter retail (160,500 THB per
  50 kW).
- Battery 271 kWh, now installed: 2.0M to 3.5M THB (was the bare Gobel
  list price, 1.4M to 2.0M, which matches DEDE's 5,000 to 7,000 THB/kWh
  for LiFePO4); adds enclosure, fire protection, controls and installation,
  below CapSolar's 12,000 to 15,000 THB/kWh for a fully installed factory
  BESS because the hybrid inverters do the conversion (estimate).
- Utilities baseline 27.57M to 47.50M THB; launch plant ~160M / 222M /
  283M THB with contingency; Plan B ~208M (~149M to 266M), ~31M more than
  Plan A; Plan A's total unchanged, its solar lever ~9.7M. PV payback
  ~3.8 to 5.4 years (sustainability.md). Calculator, README, roadmap,
  revenue model and both business plan versions (renders included)
  updated.

docs/sustainability.md

@@ -251,8 +251,11 @@ not a cost one.
 **PV, not solar thermal.** Self-consumed PV is worth **~3.7 THB/kWh**
 (weekday daytime at the peak rate, weekends and early mornings at
 off-peak, Ft included), so the 247 kWp roof saves ~1.2M to 1.3M THB a year
-and pays back in **~4.5 to 7 years** at 25,000 to 35,000 THB/kWp
-(estimate). It does not lower the demand charge, because the peak window
+and pays back in **~3.8 to 5.4 years** at 20,000 to 26,000 THB/kWp
+installed (estimate; Thai market prices checked October 2026 in the
+[utilities CSV](finance/utilities-investment.csv): DEDE's PV status report
+gives 22 to 25 THB/W for factory systems of 10 kW to 1 MW in 2023,
+installers quote 18 to 25 THB/W in 2026). It does not lower the demand charge, because the peak window
 runs to 22:00, long after sunset. Solar thermal loses on every count here:
 flat-plate and evacuated-tube collectors reach only the 60 to 90 °C that
 the feedwater heat pump already recovers from the press cooling return;