How many kWp should a Thai factory install? This guide walks the calculation step by step — from your electricity bill → daytime load → target kWp → roof-area check → generation and self-consumption, with a worked example and the common sizing mistakes to avoid, for factories and C&I businesses.
Figures on this page — such as the daytime-consumption share, specific yield, and area per kWp — are rules of thumb for first-pass estimation. Real system size must reference a 15-minute interval load profile, your roof structure, tilt/orientation, and your local utility's requirements. We do not state a price or payback here — use the ROI calculator or consult an engineer for real numbers. This information supports decision-making and is not engineering advice.
A Thai factory's solar system is usually sized to its daytime electricity load, not its total bill — because most factories run on zero-export (no selling surplus to PEA/MEA), every kWh the panels make must be used on-site the moment it is produced. As a rule of thumb, 1 kWp of modern panels needs about 6.5 m² of usable roof and generates roughly 1,300 kWh per year in Thailand (Global Solar Atlas: 1,314–1,534 kWh/kWp/yr). So a factory paying ~500,000 THB/month with a strong daytime shift can typically absorb 800 kWp–1 MWp before surplus starts spilling.
Want to know why no-backfeed is required and how to design for MEA/PEA? See Zero Export Solar Thailand
The single most important sizing principle: in Thailand, C&I solar is overwhelmingly self-consumption / zero-export — you don't get paid for surplus pushed to the grid under the current net-billing regime, and many PEA/MEA interconnections require an export-limiting (zero-export) device. So the sizing target is the daytime baseload (the kW the factory consumes roughly 09:00–16:00 while the sun is up), NOT the monthly bill total (which also includes night shifts).
The daytime-load fraction changes the right kWp: a single-shift (daytime) factory has load that overlaps the sun well and can absorb a lot of solar relative to its bill; a two-shift or 24/7 factory carries heavy night load, so the share solar can offset (versus the bill) is lower — but a steady daytime baseload can still support a large array. What sets the size is the daytime load, not the number of shifts.
Sizing to the bill instead of the daytime load = wasted capex plus generation thrown away (curtailed) at the zero-export cap. Every kWp beyond the daytime load gets throttled down at midday, becoming capex with no return.
Sizing starts from the bill — but you need to pull the right numbers, not just the total cost.
Pull these: monthly kWh, peak demand (kW), the TOU on-peak/off-peak split, and daytime versus night consumption. To understand the bill structure in detail, see Factory Electricity Bill Anatomy, Factory Electricity Cost Thailand, and Demand Charge / TOU / TOD Explained.
Most single-shift Thai factories run roughly 55–70% of consumption in daylight hours (an illustrative range for estimation — verify per site via a load profile / energy audit). This figure is the key, because it is the ceiling on what solar can offset without exporting.
Use the Bill Analyzer to break the bill into its cost components and understand your tariff structure.
There are two ways to calculate — a load-led method (more accurate) and a bill-led quick estimate.
Target kWp ≈ daytime average demand (kW) ÷ a derate factor, kept at or below the midday load so self-consumption stays near 100%. This needs a 15-minute interval load profile but gives the most accurate size.
kWp ≈ (monthly kWh × daytime fraction × 12) ÷ specific yield (1,300 kWh/kWp/yr). Good for a first-pass estimate when a detailed load profile isn't available yet.
Thailand ≈ 1,300 kWh/kWp/yr (conservative; Global Solar Atlas range 1,314–1,534). The Northeast/Central regions are higher; the South/East are lower. Using 1,300 is a safe starting anchor because it sits just below the GSA low end.
Nameplate × PR (~0.78–0.82) is what you actually get — not the full nameplate. The 1,300 specific yield already bakes this in. For the physics, see Solar Yield & Performance Ratio Thailand.
Use the Solar ROI Calculator: input your bill and region → get an estimated system size and payback (real pricing requires a site assessment).
Whether the target kWp from Step 2 actually fits depends on the roof — check it before committing.
About 6.5 m² of usable roof per kWp (matches CapSolar's Roof Estimator constant; consistent with market data of 5–7 m² module area plus ~70% flat-roof utilization). A 580W mono panel is roughly 2.3 m² each.
Flat roof: usable area ≈ 70% of gross (after gangways, skylights, AC units, and fire setbacks). Pitched metal roof: higher utilization (~80–85%) because panels lie flush with the roof plane.
If roof-allowed kWp < load-target kWp → you are roof-limited (consider adding a carport or ground-mount). If load-target kWp < roof-allowed kWp → you are load-limited (don't fill the whole roof, or you over-build). For structural limits, see Roof Assessment for Factory Solar.
Use the Roof Estimator: input area, roof type, and orientation → get the installable capacity and approximate panel count.
Once you have the kWp, convert it to annual generation and check what fraction you can self-consume.
Generation (kWh/yr) = kWp × 1,300 (the 1,300 already accounts for PR, so it can be used directly). For example, a 500 kWp system ≈ 650,000 kWh/yr.
Self-consumption % = on-site-used kWh ÷ generated kWh. Under zero-export, the design goal is ≥95% (a design target, not a guaranteed measured value). The better the system is matched to daytime load, the higher this rises.
If a zero-export cap would clip midday generation, adding a battery / peak-shaving may pay off better than adding more panels. See Peak Shaving Solar + Battery for Factories.
A single-shift factory closed on Sundays can lose roughly 14% of potential generation (power produced with no load to absorb it). Factor this into sizing. For mitigation, see Solar Factory Shutdown / Holiday Optimization.
Illustrative — not a real CapSolar client. Numbers are rounded for teaching.
| Step | Input / Calc | Result |
|---|---|---|
| Monthly bill (given) | ~500,000 THB/month, single-shift factory | — |
| Monthly consumption | ≈125,000 kWh/month (illustrative, at a representative C&I rate) | 1,500,000 kWh/yr |
| Daytime fraction | 60% runs in daylight (single shift, Mon–Sat) | 900,000 kWh/yr daytime |
| Target kWp (bill-led) | 900,000 ÷ 1,300 kWh/kWp/yr | ≈ 690 kWp |
| Self-consumption guard | keep ≤ midday load → round to | 650 kWp design |
| Roof area needed | 650 kWp × 6.5 m²/kWp | ≈ 4,225 m² usable |
| Roof check | 7,000 m² gross flat roof × 70% util = 4,900 m² usable | ✅ fits (load-limited) |
| Annual generation | 650 kWp × 1,300 kWh/kWp/yr | ≈ 845,000 kWh/yr |
| Self-consumption | 845,000 generated, ~95% used on-site | ≈ 803,000 kWh/yr offset |
Teaching point: had they sized to the full 1.5 GWh bill (≈1,150 kWp), ~40% would have hit the zero-export cap on weekends/nights and been curtailed = wasted capex. The load-led 650 kWp is the right answer. (For real price/payback numbers, use the ROI calculator or consult the team.)
Five mistakes that lead to over-built, under-built, or slow-payback systems.
We don't guess the size from the bill alone — we start from measuring the real load.
The process: an energy audit → 15-minute interval load profiling → a kWp recommendation matched to daytime load → an ROI model. For detail, see Solar Energy Audit for Factories and Solar ROI for Factories. If you're comparing investment structures, see PPA vs EPC and What Is a PPA.
CapSolar has engineered 80+ MWp of commercial solar across 150+ projects for 100+ Thai clients — we size systems to the real load for the best ROI.
This article is prepared by Frank Lee, Founder of CapSolar, with experience designing and installing solar EPC systems for factories and businesses in Thailand. The sizing method references Global Solar Atlas specific-yield data, CapSolar's Roof Estimator constants, and real project experience.
Reviewed by: CapSolar Research Team | Updated: Jun 2026
CapSolar has delivered 80+ MWp of commercial solar across 150+ projects for 100+ clients — we measure your real load and size the system for your best ROI. Free consultation.