How to Prepare an OPEX Calculation for a Process Plant (Practical Engineering Checklist)

Introduction 

Operating Expenditure (OPEX) calculations are an important part of process plant design studies. Preparing an OPEX estimate requires gathering information from multiple disciplines to estimate the ongoing operating costs of a facility, such as raw materials, utilities, labor, maintenance, packaging, and waste handling. This article is not intended to be a detailed financial modeling guide. Instead, it is a practical engineering checklist that summarizes the key inputstypically needed to prepare a first-pass OPEX estimate for a process plant.

What you need for any OPEX model

For every line item in an OPEX sheet, you generally need two inputs:

1. Quantity (e.g., kg of chemicals consumed, number of operators, kWh used)
2. Unit cost (e.g., $/kg, $/person-year, $/kWh)

OPEX (cost per period) = Quantity × Unit cost

Key deliverables (what, why, and who owns them)

1) Consumable costs

Goal: estimate the cost of materials consumed and waste/packaging/sampling tests associated with production.

What you need (quantity drivers)

• A sufficiently mature mass balance to quantify:
  • Raw materials (i.e.: feedstocks)
  • Consumables (i.e.: process chemicals)
  • Products produced (to estimate packaging requirements)
  • Waste streams (solid, liquid, or gas

What you need (unit costs)

• Raw material purchase cost (e.g., $/kg, $/ton)
• Packaging unit cost (e.g., $/box, $/bag, $/pallet)
• Waste disposal cost, based on the disposal approach, for example:
  • Solid waste: bins/bags + hauling offsite
  • Liquid waste: totes + transportation + treatment fees
• Analytical / QA testing costs, based on the QA standard you plan to meet (e.g., per batch sampling and testing)

Typical outputs (what the model calculates)

• Raw material costs
• Packaging costs
• Analytical sampling testing costs
• Waste disposal costs

Notes / boundaries

• “Raw materials” can be a broad umbrella and may include utility items such as water consumption and other utilities. However you can separate utilities elsewhere to a separate section if you wish.

Typical owner

• Process engineer (mass balance)

2) Energy costs

Goal: estimate energy consumption and apply local electricity rates.

2.1 Electricity

Quantity driver: electrical load for the process and building.

• You typically need an electrical load list that includes:
  • Process equipment loads
  • Auxiliary building loads (i.e.: lighting)
• Apply appropriate assumptions/factors for maximum/typical usage depending on the study stage.

Unit cost: local electricity cost (e.g., $/kWh) based on site/location.

Typical owner: Electrical engineer (load list)

2.2 Natural gas

Natural gas may cover:

• Process equipment fuel (e.g., boilers)
• Building heating (seasonal)

Quantity driver:

• From mass balance and/or equipment list for process needs
• For building heating: early-phase estimates often use conservative assumptions for a first-pass

Unit cost: local natural gas cost (e.g., $/GJ, $/MMBtu)

Typical owner: process, mechanical, and/or utilities engineer

3) Labor costs

Goal: estimate staffing levels and apply compensation rates.

What you need

• Operating philosophy (how the plant runs day-to-day)
• Staffing requirements (roles, shift coverage, headcount)
• Salary (i.e: wage rates)

Generally more automation would typically result in higher CAPEX, lower OPEX (less labor) and vice versa.

Typical roles included

• Plant leadership (e.g., superintendent)
• Supervisors
• Operators / technicians
• Maintenance roles (mechanical, electrical, instrumentation)

4) Maintenance

Goal: estimate ongoing upkeep costs for the facility and equipment.

What this typically includes

• Building maintenance
• Laboratory equipment maintenance (if applicable)
• Process Fixed equipment maintenance

Common estimating approach

• Maintenance cost as a percentage of total direct CAPEX

Best practice

• Validate assumptions with equipment vendors to ensure the percentage/allowance is reasonable.

Quick checklist to ensure you are on the right track

[ ] Mass balance mature enough for material + waste quantities

[ ] Raw material unit costs gathered

[ ] Packaging assumptions defined + unit costs gathered

[ ] Waste disposal method selected + unit costs gathered

[ ] QA/testing frequency defined + unit costs gathered

[ ] Electrical load list developed + electricity tariff identified

[ ] Natural gas users identified (if any) + consumption estimated + gas rate identified

[ ] Operating philosophy defined + staffing plan built

[ ] Salaries/wages gathered

[ ] Maintenance basis selected (e.g., % of direct CAPEX)