Quick Answer
Thai factories shut down 20-30 days/year on average (Songkran 7-10 days, New Year 5-7 days, maintenance 1-2 weeks), but solar systems keep producing normally — representing 5-8% of annual generation. Without management, excess electricity is either curtailed (wasted) or exported to the grid via Net Metering/Billing. Four key scenarios: (1) Net Metering — offset next month's bill, (2) Net Billing — sell at avoided cost, (3) Neither — must curtail or use battery, (4) PPA — check Take-or-Pay clause. Using shutdown periods for O&M (panel cleaning, thermal imaging, maintenance) can boost PR by 2-5%.
Factory Shuts Down, Solar Keeps Producing — What Happens
Solar systems produce electricity every sunny day regardless of whether the factory is operating. During extended holidays, factory consumption drops dramatically while solar output remains at full capacity. The excess electricity needs somewhere to go. Planning ahead transforms this 'problem' into an 'opportunity' for additional revenue or savings.
Thai Factory Holiday Calendar
Songkran (April): 7-10 days — coincides with peak solar irradiance month. Maximum generation but factory is closed.
New Year + Christmas (Dec-Jan): 5-7 days — lower irradiance but still 70-80% of average output.
Maintenance shutdown (year-round): 1-2 weeks — schedule during rainy season (Jul-Sep) to minimize solar loss.
Base Load vs Zero Load
A 'closed' factory doesn't consume zero electricity. Base load from security systems (CCTV, alarms), cold rooms/server rooms, emergency lighting, and standby fire pumps typically accounts for 5-15% of peak load. So 85-95% of solar production is excess.
When Solar > Load — Reverse Power Flow to Grid
When solar output exceeds base load, excess electricity flows back to the grid. Without Net Metering/Billing, the inverter auto-curtails (reduces output) to prevent unauthorized reverse flow — meaning the generated electricity is effectively wasted.
4 Scenarios for Managing Excess Solar During Factory Shutdown
How to handle excess solar electricity depends on your enrolled policy and contract type. Here are 4 key scenarios with recommendations for each.
Scenario 1: Net Metering — Offset Next Month's Bill
If your factory is enrolled in Net Metering, excess electricity is exported to the grid and offset unit-for-unit against the next month's bill. During Songkran when overproduction is high, credits accumulate. Limitation: credits cannot be cashed out — if accumulated beyond what you can use in a billing cycle, they're lost.
Scenario 2: Net Billing — Sell to Grid at Avoided Cost
Net Billing pays for excess at the avoided cost rate (~2.00-2.20 THB/kWh), lower than your electricity rate (~4.15 THB/kWh) but better than wasting it. During shutdown, Net Billing revenue helps offset the demand charge you'll pay regardless. Ensure your bi-directional meter is installed before long holidays.
Scenario 3: No Net Metering/Billing — Curtail or Use Battery
Without Net Metering/Billing, you have two options: (A) Let inverters auto-curtail — losing production above base load, or (B) Install battery storage (BESS) — store daytime excess, use at night for base load, reducing off-peak electricity cost. Evaluate whether battery investment makes economic sense given your shutdown days.
Scenario 4: PPA Contract — Check Take-or-Pay Clause
If purchasing via PPA, check the contract for a Take-or-Pay clause: (1) With Take-or-Pay: you pay a minimum regardless of consumption — negotiate a reasonable commitment, (2) Flexible PPA: pay only for actual consumption — no extra cost during shutdown, (3) Negotiate a shutdown period exemption before signing, especially covering April.
How to Maximize Revenue During Factory Shutdown
Instead of wasting excess generation, several strategies can extract value from your solar system during factory downtime.
Shift Loads Before Shutdown
Before closing: fully charge EVs, charge BESS to capacity, pre-cool cold rooms/server rooms below normal setpoint so thermal mass maintains temperature longer without running at full capacity.
Smart Curtailment — Reduce Only the Excess
Configure your EMS to curtail only production exceeding base load — don't shut down the entire system. Modern inverters support zero-export limiters. Set export limit to 0 kW and the system produces just enough for base load.
Use Shutdown as O&M Opportunity
Factory shutdown is the ideal time for panel cleaning, thermal imaging (hotspot detection), I-V curve testing, and cable/connector maintenance — without affecting production since the system is partially shut down anyway. Result: PR improves 2-5% after cleaning and repairs.
Impact on ROI — How Much Do Holidays Affect Returns
20-30 factory shutdown days per year represent approximately 5-8% of annual solar production. The ROI impact depends entirely on how well you manage the excess generation.
200 kWp: shutdown production ~22,000 kWh → Curtail = lose ~91,300 THB | Net Billing = recover ~44,000 THB | Net Metering = save ~91,300 THB
500 kWp: shutdown production ~55,000 kWh → Curtail = lose ~228,250 THB | Net Billing = recover ~110,000 THB | Net Metering = save ~228,250 THB
1 MWp: shutdown production ~110,000 kWh → Curtail = lose ~456,500 THB | Net Billing = recover ~220,000 THB | Net Metering = save ~456,500 THB
Note: calculated using average rate 4.15 THB/kWh, avoided cost 2.00 THB/kWh, 25 shutdown days/year, average irradiance 4.4 kWh/m²/day.
Safety Considerations — Unattended Factory Operation
Solar systems produce electricity even when nobody is at the factory. All safety systems must remain operational throughout the shutdown period.
Fire Protection + Rapid Shutdown
Ensure your Rapid Shutdown system (NEC 2017/2020 or IEC 60364-7-712) remains active — capable of killing DC within 30 seconds. Automatic fire suppression (sprinkler/FM200) must stay active throughout shutdown. Check water levels and pressure before closing.
Remote Monitoring Alerts
Configure alerts for: inverter fault, grid trip/islanding, abnormal DC voltage, communication loss. Push notifications via monitoring app to O&M team via LINE/SMS/Email. Verify data logger SIM card has sufficient credit before holidays.
Theft Prevention
Ground-mount areas: CCTV + motion sensors + solar-powered lighting (self-powered). Rooftop: lock roof access points. Accessible cable trays should have cable locks. Inform security guards that the solar system remains active — no unauthorized access.