How to Calculate Your Factory Electricity Bill Do It Yourself in 5 Minutes (TOU + Demand + Ft)

Read Your Bill First — The 4 Numbers You Need

Don't understand the bill? Read our factory bill anatomy guide first, then come back and follow these 5 steps.

5 Steps to Calculate Your Factory Bill

With your 4 bill numbers in hand, follow these 5 steps in order — plug your own bill values into each one. There is no fixed baht figure here, because your bill depends on the units your factory actually uses.

1. 1

   Step 1 — Split On-Peak and Off-Peak Units

   A factory TOU (Time of Use) bill already splits units into two windows: On-Peak is 09:00–22:00 Mon–Fri, and Off-Peak is 22:00–09:00 plus weekends and holidays. Note both figures — call them A units (On-Peak) and B units (Off-Peak). The energy rates differ a lot — peak is almost double off-peak — so you must separate them first. See full windows and rates on the Thailand electricity tariff page.

2. 2

   Step 2 — Calculate the Energy Cost (units × rate)

   Multiply each window's units by that window's energy rate. For TOU Type 3–4 factories at ≥69 kV in the May–Aug 2026 period: On-Peak = 4.1025 THB/unit, Off-Peak = 2.5849 THB/unit. So energy cost = (A × 4.1025) + (B × 2.5849) THB. (These are the current-period ERC rates; other voltage levels differ slightly — check your voltage level's rate on the tariff page.)

3. 3

   Step 3 — Add the Demand Charge

   On top of per-unit energy, a TOU factory also pays a "demand charge" based on its peak kW (the highest 15-minute power demand in the month) multiplied by the demand rate in THB/kW for your customer type. This is billed separately from your units, which is where many factories are surprised — see the demand rate by type and how to cut it in our Demand Charge / TOU / TOD guide. Add this amount to the energy cost from Step 2.

4. 4

   Step 4 — Add the Ft (Fuel Adjustment Charge)

   The Ft is billed as THB-per-unit multiplied by your total units for the month (A + B). For the May–Aug 2026 period, Ft = +0.1623 THB/unit, so the Ft portion = (A + B) × 0.1623 THB. Add it to the running total from Step 3. The Ft is adjusted every 4 months by the ERC — the next period may differ; read the background and check the latest value on our What is the Ft charge page.

5. 5

   Step 5 — Add 7% VAT → Net Total

   Add everything from Steps 2–4 (energy + demand charge + Ft) into a "pre-VAT subtotal," then multiply by 1.07 to add 7% value-added tax. The result is your net electricity bill for that month. (Some bills add a small fixed monthly service charge too, but it's tiny relative to the three main components.)

Worked Example — Plug In Your Own Bill

Suppose your factory (TOU Type 4, ≥69 kV) in the May–Aug 2026 period has On-Peak = A units, Off-Peak = B units, and peak demand = D kW. The calculation order is below — just replace A, B, D with the numbers from your own bill:

Why Factory Bills Are High — and Where to Cut

Once you break it down, factory bills clearly come from three places, each cut differently: (1) expensive On-Peak units — shifting use to Off-Peak or self-generating daytime power with solar cuts this the most; (2) the demand charge — manage peak kW with load shifting and by not starting motors simultaneously; (3) the volatile Ft — every unit you self-generate with solar is a unit on which you pay neither the energy rate nor the Ft. This page is a calculation method, not investment advice.

Frequently Asked Questions