Solar on every roof, BOI solar correction, open source section

Solar (user, 17 Oct 2026: "use as much roof space as possible")
- PV on every roof and canopy: ~640 kWp on the Payap scenario plot
  (factory roof ~523, canopies ~54, flat office roof ~65 kWp around the
  AC units); ~300 kWp on the Mueang Chi hall. Calculator: pvKWp 640,
  new pvSelf 85 % (zero export, hourly sketch), PV cost per kWp times the
  array, office-roof PV in the iso view, roofs drawn full.
- 12.8M to 16.6M THB at 20 to 26 THB/W; CSV baseline 34.37M to 56.34M;
  launch plant ~168M / 231M / 294M THB with contingency; Plan B ~216M
  (label ~215M), ~39M above Plan A; Plan A unchanged (it leases the
  array), its solar lever ~17.5M. PV payback ~4.2 to 5.9 years.
- Permits (rooftop solar no factory licence since 28 Dec 2024, ERC
  exemption under 1,000 kVA, PEA zero export) and PPA discounts sourced
  in sustainability.md.

BOI correction
- Announcement 3/2568 (in force 1 July 2025) ended BOI promotion for
  solar installations; only the battery may qualify (12M THB per MW cap).
  Removed the "BOI solar measure" from boi.md, total-investment,
  revenue-model, sustainability, site-candidates and both plan versions;
  Plan B's case for owning the array is the PPA bill it avoids
  (~0.07M USD a year).

Open source (user, 17 Oct 2026)
- digital-factory.md "Open source throughout, and working with other
  plants": everything we run and build is open source on NixOS, our code
  published (licence to decide), vendor firmware the stated exception;
  later the stack and factory-automation services for other pressing
  plants, not in the revenue.
- Business plan section 5.6 plus an executive-summary bullet, reading
  guide and not-modelled mentions; draft 0.3 rebuilt (A 60, B 61 pages),
  renders re-made with the roofs full.

docs/sustainability.md

@@ -47,8 +47,10 @@ process cooling (up to ~650 kW), drawn in [steam and cooling](steam-cooling.html
   [facility systems](facility-security.md)) adds a major tropical-climate
   load on top of process cooling and must be part of the PV and battery
   sizing.
-- System sizing (kWp, storage, grid-connection model) is open until the site
-  is chosen.
+- **Array size, decided 17 October 2026: every roof and canopy carries PV**
+  ("use as much roof space as possible"), so the kWp follows the site:
+  ~640 kWp on the Payap scenario plot, ~300 kWp on the Mueang Chi hall
+  (below). Storage beyond the 271 kWh ride-through battery stays open.
 
 ### Roof design for PV (candidate Mueang Chi site)
 
@@ -108,8 +110,49 @@ the electric-boiler route is chosen.
 
 On the Payap scenario plot the 128 m factory roof takes ~901 modules
 (~523 kWp) and the car and scooter canopies another ~54 kWp, ~577 kWp in
-all; what that size means for export, the battery and BOI is in
-[site candidates](site-candidates.md#roof-pv-and-parking-canopies-part-and-draft-model-october-2026).
+all ([site candidates](site-candidates.md#roof-pv-and-parking-canopies-part-and-draft-model-october-2026)).
+The flat office roof adds ~65 kWp around its three AC units (estimate
+from the [calculator](fleet-calculator.html) drawing), so covering every
+roof and canopy gives **~640 kWp**, the size the investment and both
+business plan versions carry since 17 October 2026. On the Mueang Chi
+hall the same rule gives only ~300 kWp (the 247 kWp roof above plus the
+canopies); more there means ground mount on the plot behind the building
+or the phase 2 roof.
+
+**What ~640 kWp means on the grid** (October 2026):
+
+- **Zero export.** A factory on PEA's grid cannot sell its surplus: the
+  interconnection needs a zero-export controller or reverse-power relay,
+  and net billing is limited to small users
+  ([CapSolar, rooftop solar regulations 2026](https://capsolar.co.th/en/knowledge/rooftop-solar-regulations-thailand)).
+  An hourly sketch (two shifts on 300 days, ~60 kW off-shift base load,
+  the 271 kWh battery; estimate) puts **~85 % of the ~900 MWh yield to
+  use on site**; the rest, mostly on Sundays and holidays, is curtailed.
+  With electric steam the launch plant draws ~2,000 MWh a year, so the
+  array covers ~38 % of it.
+- **Permits.** Since Ministerial Regulation No. 3 B.E. 2567 (in force
+  28 December 2024) rooftop solar outside industrial estates is not a
+  factory at any size, so no Ror.Ngor.4 licence; at ~500 kVA of inverters
+  the array stays under the ERC's 1,000 kVA line and needs an exemption
+  notification, not a generation licence; PEA's interconnection review
+  takes 3 to 6 months and is filed alongside procurement
+  ([CapSolar, factory solar and the RG.4 permit](https://capsolar.co.th/en/knowledge/factory-solar-rg4-permit-thailand)).
+  Ground mount is not covered by the rooftop exemption and is checked
+  case by case.
+- **No BOI benefit for the panels.** Since 1 July 2025 (Announcement
+  3/2568) the BOI no longer promotes solar installations; the battery and
+  its inverters may still qualify, capped at 12M THB per MW
+  ([BOI](boi.md)). The array is justified by the bill alone.
+- **Leasing instead (Plan A).** Under a solar PPA the developer funds,
+  installs and maintains the array and sells its power at a discount to
+  the PEA tariff: usually 10 to 15 % over 15 to 20 years at MEAei
+  ([MEAei](https://meaei.com/en/best-solar-rooftop-businesses-thailand)),
+  10 to 30 % over 15 to 25 years for bills of 100,000 THB a month and up
+  at CapSolar ([CapSolar PPA](https://capsolar.co.th/en/services/ppa)).
+  The calculator's 3.0 THB/kWh is ~21 % below the ~3.8 THB/kWh blended
+  rate, inside CapSolar's range but above MEAei's usual discount (estimate,
+  to quote); the launch plant's bill (~0.75M THB a month before solar, calculator)
+  clears the threshold.
 
 ### Energy efficiency architecture: recover before generating
 
@@ -252,7 +295,10 @@ not a cost one.
 (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 **~3.8 to 5.4 years** at 20,000 to 26,000 THB/kWp
-installed (estimate; Thai market prices checked October 2026 in the
+installed. The full ~640 kWp array uses ~85 % of its yield on site, saves
+~2.8M to 3.0M THB a year and pays back in **~4.2 to 5.9 years**; the
+last ~340 kWp, more of whose yield is curtailed, take ~4.9 to 6.3 years
+(estimates; 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