What is the difference between demand charge and energy charge?

Energy charge is based on actual kWh consumed, while demand charge is based on the peak kW drawn in the billing month. Energy charge is the largest at 50-60%, demand charge typically 20-35% of the total bill.

What is the Ft surcharge and why does it change every 4 months?

Ft is a variable fuel cost adjusted by ERC every 4 months based on natural gas prices and exchange rates. Current rate is 0.1623 THB/kWh (May-Aug 2026). Solar hedges against Ft risk since self-generated power pays zero Ft.

How low does Power Factor need to drop before a penalty kicks in?

PEA/MEA penalizes when Power Factor drops below 61.97% (lagging) at 56.07 THB/kVAR of shortfall. The primary fix is capacitor banks — solar inverters help partially but aren't the main solution.

How much can solar reduce a factory electricity bill?

Solar can reduce factory electricity bills by 30-60% depending on system size, load profile, and self-consumption ratio. A 500 kW factory with 300 kWp solar saves ~218,000 THB/month (~36%), reducing every cost layer: energy, demand, Ft, and VAT.

What's the difference between Cat 3 and Cat 4 factory bills?

Cat 3 (medium business, 30-999 kW) can choose TOU or flat rate and has demand charge. Cat 4 (large industrial, 1,000+ kW) must use TOU only, has higher demand charges and a 70% ratchet rule. Cat 4 typically saves more from solar due to higher bills.

Factory Bill Guide

Thai Factory Electricity Bill Anatomy: Every Line Item Explained

5 Cost Layers500 kW Worked Example12 min read

Table of Contents

1.5 Cost Layers in Your Factory Electricity Bill 2.Energy Charge — The Biggest Layer 3.Demand Charge — Peak Power Cost 4.Ft Surcharge — Fuel Cost Pass-Through 5.Power Factor Penalty — Reactive Power Cost 6.Service Fee + VAT — Small but Multiplied 7.Worked Example: 500 kW Factory Before/After Solar 8.Pre-Solar Checklist: 3 Things to Prepare 9.Frequently Asked Questions About Factory Electricity Bills

Quick Summary

A Thai factory electricity bill has 5 cost layers: TOU energy charge (50-60%), demand charge (20-35%), Ft fuel surcharge, power factor penalty, and 7% VAT. Rooftop solar reduces every layer — a 500 kW factory example saves ~218,000 THB/month.

OVERVIEW

5 Cost Layers in Your Factory Electricity Bill

EXAMPLE

Worked Example: 500 kW Factory Before/After Solar

Cat 3 TOU Factory · 22-33 kV500 kW contracted demand150,000 kWh/month70% On-Peak / 30% Off-Peak300 kWp installed

Before Solar

~612,000 THB/month

After 300 kWp Solar

~394,000 THB/month

Line Item	Before	After	Savings
Energy Charge	389,000	243,000	-146,000
Demand Charge	66,465	39,879	-26,586
Ft Surcharge	24,345	15,216	-9,129
PF Penalty	0	0	0
Service + VAT 7%	33,600	20,900	-12,700
Total	~612,000	~394,000	~218,000