Contents
Shared utilities: one water plant and one grid
Status: first draft, 21 Oct 2026, revised 21 Oct 2026 (sports centre
and padel hall loads, stormwater without open water, standby generator;
a water tower instead of the booster station, networks along the ring
road), revised 3 Nov 2026 (four Type C corner houses with private pools,
trail lighting; 14 houses on 16.9 rai). Sizes and energy figures come
from model/cost_model.py
and its hourly energy model model/estate_energy.py;
the generated output is model/results.md. Loads,
equipment prices and plant sizes are estimates until the MEP engineer
and vendors replace them (phase 1b, plan.md).
Bracketed numbers such as
[33] refer to sources.md.
The idea
In a normal Thai estate every house is its own small utility: its own water tank and pump, its own PEA meter, its own PV and battery if it has any. Users report that on one estate of more than 150 houses over half the owners pump straight from the mains into their own tanks when the pressure is low.[44] Because this mooban owns every house and the land under them (operating-model.md), it can run its utilities like one building:
- One water plant with one pump station that fills a water tower, and a looped main from the tower to every house. The tower gives the pressure by gravity. No house has a tank, a pump or its own treatment.
- One private grid. Every roof carries PV, but all of it feeds one estate grid behind one PEA connection, with one central battery and one standby generator. A house's surplus runs its neighbours, the water plant, the sewage plant and the sports centre before anything is curtailed.
Water: one plant, one tower for the whole mooban
| Part | Design (flagship, 14 houses) |
|---|---|
| Source | Two bore wells (duty and standby), or the PWA main where the site has one. Wells deeper than 15 m need a drilling licence and a use licence; in Chiang Mai, one of three pilot provinces, they are issued by the municipality or tambon office.[36] |
| Treatment | Aeration and filtration to suit the water test, ultrafiltration, UV, and a chlorine residual to protect the network |
| Ground tank | 60 m³ of treated water at the plant (14 houses at about 0.8 m³ a day each plus the club and pool: about 13 m³ a day, est.) |
| Transfer pumps | One pump station with two pumps, duty and standby, that lift the treated water into the tower |
| Water tower | A 50 m³ steel tank of the champagne type on a column in the service yard, its floor about 21 m up and its top at 25 m. That gives about 2 bar at the street by gravity (about 1 bar per 10 m of height[71]) and about 1.5 bar at an upper-floor shower before pipe losses. 50 m³ is about four days of the estate's use. A 50 m³ tank at 25 m costs about 540,000 THB and its foundation 165,000 to 196,000 THB; at 30 m the tank is about 60,000 THB more,[70] if the MEP engineer wants more pressure upstairs. About 1.0 M installed with riser, overflow, level control, lightning protection and a caged ladder (est.). |
| Network | Looped HDPE main under the ring road with section valves, so a burst isolates one stretch of road while water reaches every other house round the ring; hydrants on the ring |
| At each house | Smart water meter, check valve, filter, and an automatic leak shut-off valve in the plant room; no pressure-reducing valve, because the tower's head stays below 2.5 bar |
| Leak watch | Plant output against the sum of the house meters, and a night-flow check every day |
| Irrigation | A separate non-potable line from treated effluent and a covered rainwater cistern for the park and the private gardens |
Why a tower. The first draft pressurised the main with a booster station: three pumps on variable-speed drives that had to run whenever anyone opened a tap, day and night. The tower turns that round. The pumps only fill it, in solar hours, and the pressure comes from height: steady, silent, with no controller in the loop. If every pump and the whole grid failed, the houses would still have water for about four days. One pump station remains (two pumps, duty and standby), so there is still one place to service and one set of spares, instead of 20 house pumps cycling on and off.
Energy. The wells, the treatment and the transfer pumps run from 10:00 to 15:00 on solar and fill the tower; nothing pumps at night. The whole plant is on the battery-backed estate grid. In the model, the water plant, sewage plant, pool, sports centre and padel hall, street lights and gate together use 53 MWh a year, about 36 % of the estate's electricity.
What stays in each house. Hot water, cooling and, at the four Type C corner houses, the private pool: its variable-speed pump and salt cell (about 0.75 kW, est.) run from 09:00 to 17:00 on solar, inside the house's 800 kWh allowance (spec.md §6b). Each house keeps its heat pump water heater (the estate's energy management system runs it in solar hours) and its own inverter AC units; a district hot-water or chilled-water loop across a low-density estate would lose heat in long pipes and add a second single point of failure (engineering judgement).
Electricity: one shared grid
| Part | Design (flagship) |
|---|---|
| Connection | One PEA 22 kV connection with metering, an estate-owned 250 kVA transformer, and the main low-voltage board in the energy centre in the frontage strip beside the gate |
| Distribution | Low-voltage cables in ducts under the ring road, fed from the energy centre with a normally-open point on the far side at the club roundabout, so a cable fault can be isolated and the houses fed from the other end |
| Each house | Revenue-grade sub-meter, house board with RCBO per circuit (spec.md §5) |
| PV | 6 kWp on every south roof slope at opening, roofs built for 10 kWp; a string inverter in the plant room feeds the house board, and whatever the house does not use flows into the estate grid |
| Central battery | 170 kWh LFP with a grid-forming inverter in a fire-rated room in the energy centre, sized by the model to carry the estate's average load from 18:00 to 06:00 |
| Backup | When PEA fails, the estate disconnects and runs as an island on the battery and the roofs: every house, the water plant and the gate, with its barriers, cameras and intercom, stay on. EV chargers and water heaters are shed first. If the outage outlasts the battery, the standby generator starts (below). |
| Standby generator | 200 kVA diesel set in a sound-attenuated canopy with a 500 L base tank, in a fenced, bunded compound by the workshop; feeds the main board through the battery inverter's generator input |
| Energy management | Limits export to zero in the base case by curtailing inverters when the battery is full; moves flexible loads into solar hours: water plant and tower filling, water heaters, the club pool and the four private pools, EV charging, pre-cooling |
| Outdoor lighting | LED street lights on the ring road and low bollard lights about every 25 m along the perimeter trail, on the battery-backed estate grid, dimmed late at night (est.) |
This replaces a home battery of 15 kWh in every house, as the first draft had (210 kWh across 14 houses), with one 170 kWh battery, and the per-house hybrid inverters with plain string inverters.
What the model says
Hourly simulation over 2019 to 2023 for Hang Dong, with PV output and outdoor temperature from PVGIS[42] (1,411 kWh per kWp a year for a 15° south-facing roof, inside the 1,300 to 1,500 range for Thailand[13]). Household and common loads are estimates; the reference family in a Type A house uses 450 kWh a month, as in the first model.
| Flagship | Lean | Pilot | |
|---|---|---|---|
| Houses | 14 | 12 | 2 |
| PV, kWp | 84 | 72 | 12 |
| Central battery, kWh | 170 | 130 | 30 |
| PV generation, MWh/yr | 118.5 | 101.6 | 16.9 |
| Estate use (houses + common), MWh/yr | 147.8 | 100.1 | 17.6 |
| Bought from PEA, MWh/yr | 35.9 | 16.1 | 2.7 |
| PV curtailed, MWh/yr | 3.5 | 14.6 | 1.4 |
| Share of the estate's use met by its own PV | 76 % | 84 % | 85 % |
| PEA bill per house per month, THB | 979 | 511 | 491 |
- The estate makes most of its own power. It covers 76 % of its use from its own roofs and buys about 36 MWh a year, mostly at night and at weekends when the TOU rate is low. The flagship's PEA bill is about 164,000 THB a year, a quarter of which is the demand charge. The four larger Type C houses with their pool pumps use more than Type A houses, so the 14 houses use about 95 MWh a year (82 MWh before the corner houses). The club's loads weigh more on 14 houses, so the bill per house is higher than on the 12-house lean estate.
- PV size depends on export. With no export, the
cheapest PV size is small (4 kWp a house without a battery). Given the
battery the estate wants for backup, 6 kWp a house is the base case;
filling every roof to 10 kWp would cost about 40,000 THB a year more and
mostly add curtailment. With the corner houses' larger loads, 6 and 8
kWp now cost about the same with a large battery (at 200 kWh, 8 kWp is
about 13,000 THB a year cheaper in the sizing sweep,
results.md), so the fitted size is worth re-checking in phase 1b. If PEA lets the estate sell its surplus at 2.20 THB/kWh,[9] 10 kWp on every roof becomes the cheapest choice. CapSolar reports that systems under 1 MW for self-consumption can sell surplus through PEA net billing at about 2.20 THB/kWh;[34] whether that applies to this connection is a phase 0 question. Hence: fit 6 kWp, build every roof for 10. - The battery is for resilience, not savings. At 6 kWp a house it costs about 230,000 THB a year more than going without (capital, service, replacement), or about 1,700 THB a month on the rent of a Type A house. It buys whole-estate backup through PEA outages, including the water supply, and raises the share of the estate's use met by its own PV from 52 % to 76 %.
- Sharing beats every house on its own. One battery serves 14 households with different daily rhythms, and house PV also runs the water and sewage plants:
| Design (flagship) | Capital, PV + batteries | Annual cost of energy (THB) |
|---|---|---|
| Every house on its own: 10 kWp + 15 kWh, own PEA meter, no export | 7.91 M | 1,025,000 |
| Every house on its own, with residential net billing | 7.91 M | 798,000 |
| Shared grid: 6 kWp each + 170 kWh central, no export (base) | 6.02 M | 745,000 |
| Shared grid: 10 kWp each + 170 kWh central, no export | 7.70 M | 785,000 |
| Shared grid: 10 kWp each + 170 kWh central, export at 2.20 THB/kWh | 7.70 M | 649,000 |
Annual cost of energy = annualised capital at 5 % (PV 25 years, batteries 15 years central, 12 years home) + service + PEA bills - export income. Common loads in the islanded cases sit on their own meter without PV, because in a normal estate a house's PV cannot run the water plant.
- EVs soak up the surplus. With 7 tenant EVs charged at home under estate control (16 MWh a year, billed to the tenants at no more than the PEA residential rate), the share of the estate's use met by its own PV goes from 76 % to 70 % and curtailment falls from 3.5 to 1.1 MWh a year.
Standby generator: the second layer of backup
The battery carries the estate through the evening and the night; a PEA outage that lasts into a second night, or a run of grey days in the rainy season, would empty it. The generator covers that case.
- Size. The estate's highest hourly load in five years of the model is about 49 kW (57 kW with 7 EVs charging). Recharging the 170 kWh battery at about half its capacity an hour takes another 85 kW, so a set of about 140 kW (200 kVA standby) can carry the whole estate and refill the battery at the same time. The lean estate needs about 150 kVA.
- How it runs. The energy management system starts it only when PEA is out and the battery falls below a set level; it runs at a good load, carries the estate and recharges the battery, then stops, and the roofs and the battery take over again. PEA and the generator never run in parallel.
- Fuel. On an average day the estate needs about 98 kWh more than its roofs make (what it now buys from PEA), roughly 27 L of diesel; with no sun at all about 405 kWh, roughly 113 L (est., at about 0.28 L/kWh). The 500 L base tank therefore lasts from about four days to two and a half weeks; the workshop keeps a fuel supplier on call.
- Cost. About 1.7 M installed: 100 to 200 kVA sets cost 0.4 to 1.2 M THB in Thailand and installation 10 to 15 % of the machine;[68] plus the changeover controls and the compound (est.). Service runs at 2 to 3 % of the machine price a year;[68] with monthly test runs under load, fuel turnover and a replacement reserve over 25 years, about 640 THB per house per month. Without it the rent needed falls by about 1,500 THB a month (costs-and-rent.md).
- Neighbours. Sound-attenuated canopy, exhaust above the canopy, test runs on weekday mornings; the nearest house is about 33 m away (site-plan.md).
Tariff and rules (to settle in phase 0)
- Tariff. With one 22 kV connection and its own transformer the estate would most likely be a PEA medium business customer (type 3: 30 to 999 kW demand): at 22 to 33 kV, 4.1839 THB/kWh on-peak, 2.6037 off-peak and 132.93 THB per kW of on-peak demand, plus Ft and VAT; on-peak is 09:00 to 22:00 on weekdays.[33] The model uses this. The pilot's two houses on one low-voltage connection are modelled as a small business customer, on the same tiers as a household.[33] Both classifications must be confirmed with PEA Chiang Mai.
- Solar. Systems under 1 MW for self-consumption need no ERC licence, but they need PEA interconnection approval and a building modification permit.[34]
- Supplying tenants through a private grid. A Thai law firm notes, in the context of an ERC draft on data centres, that operators who distribute power within a campus or sell onsite generation to tenants could need an ERC licence themselves.[35] Whether an estate that includes electricity in the rent and bills only the excess counts as distribution or sale needs a written legal opinion, and if necessary an ERC ruling, before design freeze. Fallback if it does: every house gets its own PEA meter with its PV behind it (the islanded rows above); the water plant stays shared either way.
- Billing the excess. Electricity and water above the allowance may not be charged above official tariffs.[30]
Stormwater: no open water
| Part | Design (flagship) |
|---|---|
| Roofs and roads | Downpipes and kerb inlets to a piped drain under the roads |
| Storage | Closed attenuation tank of about 540 m³ under the west meadow of the park: 1 m³ per 50 m² of site, the on-site retention rate the Bangkok city plan rewards, at about 6,500 THB per m³ built;[64] a benchmark until Chiang Mai's requirement for the site is known |
| Storm peak | Dry meadow, a hollow about 0.5 m deep in the park, for the storm beyond the tank, empty again within a day |
| Outlet | Throttled outlet to the public drain on the soi, with a non-return flap for when that drain is full |
| Irrigation | Treated effluent and a covered rainwater cistern |
Why no pond. The first layout showed the drainage as a retention pond. Mosquitoes need standing water for several days to develop, so a basin that empties within 72 hours does not breed them;[62] a permanent pond does, and the Aedes mosquitoes that carry dengue breed even in containers holding very little water.[63] So nothing on the estate holds water in the open for more than a day, and the team checks gutters, drains, inlets and the tank's access covers every week. The tank, drains and dry meadow cost about 3.7 M (est.) and about 20,000 THB a year to inspect and desilt.
Who runs it and what it costs
The estate's technical team runs both plants (operating-model.md); contractors service the battery, the sewage plant and the lab tests. In the model the estate systems (water plant, sewage plant, transformer, gate and CCTV, battery and PV, standby generator, water tower, sports centre, padel hall, pool and courts, stormwater, perimeter trail, green lanes and fitness stations, park furniture: service and replacement reserves) cost about 6,300 THB per house per month, and the estate's PEA bill about 980. The utilities add about 7.8 M THB to the flagship's infrastructure budget against the first draft, and save about 440,000 THB per house in the house itself: no home battery (225,000), a smaller PV array with a plain string inverter (185,000), no tank, pump or treatment (30,000).
What this changes in the house
- No home battery and no hybrid inverter: a string inverter, 6 kWp fitted, roof ready for 10 kWp.
- No essential-loads board: the whole house is backed up by the estate battery.
- No water tank, pump or whole-house treatment: a meter, a check valve and a leak shut-off valve instead; the tower sets the pressure.
- An electricity sub-meter and a water meter that feed the house dashboard and the invoice.