SMEs often see unit price changes without a clear explanation and assume the whole invoice is wrong. In variable (indexed) contracts, price movement is normal-energy and gas prices are typically linked to a reference index (e.g. Belpex for electricity, or a gas index) and a review period set in the contract. The key question is whether the applied adjustment matches the contract: correct index, correct reference period, and correct formula. Validating this each month avoids both unjustified disputes and undetected overcharges.
Start with contract method, not invoice total
Before reviewing amount differences, confirm the contract's pricing model and update rhythm. Find where the indexation is defined: which index (e.g. Belpex, gas index), which reference period (e.g. monthly average), and which part of the unit price it applies to (energy component only, or also other elements). Many Belgian variable contracts index only the energy component; standing charges and network elements may change on a different schedule. Comparing a blended 'headline' unit price to last month hides what actually moved.
Monthly validation routine
- 1. Record prior and current period unit price components separately.
- 2. Validate that period references used for adjustment are coherent.
- 3. Check if standing charges changed independently of variable components.
- 4. Quantify the financial impact of each changed component.
- 5. Log unresolved differences with evidence-ready references.
Common failure patterns
The most frequent issue is comparing only one headline unit price to the previous invoice. Energy bills combine several layers (energy, capacity, distribution, taxes); a single blended number hides which component moved and whether the move is consistent with the index and period. Another failure is reviewing indexation without checking period alignment-e.g. the contract says 'index of month X' but the invoice uses a different reference month-which leads to disputes on the wrong basis or missed errors.
Typical indexation sources in Belgian contracts
Electricity variable contracts often refer to a wholesale or day-ahead index (e.g. Belpex or a derivative). The contract will specify the reference period-e.g. monthly average of the index for month X-and possibly a formula (e.g. index + spread). Gas contracts may use a gas commodity index or a basket. The exact index name and publication source (e.g. website, date of publication) should be in the contract or in the supplier's general terms. If it is not clear, ask the supplier in writing; you need the exact reference to validate each invoice.
Index values are public: you can look up the Belpex or gas index for the relevant period and replicate the expected unit price change. If your calculation does not match the invoice, you have a clear discrepancy to raise-with index name, period, and your calculation-instead of a generic "price seems high".
When indexation is wrong: how to dispute with evidence
When you have confirmed the contract method (index, period, formula) and the published index values for that period, compute the expected unit price (or the expected change). Compare to the invoice. If the difference is material, open a dispute with: contract excerpt showing the indexation clause, the index values you used (with source and date), your calculation, the invoice value, and the € impact. Ask for a corrected invoice or credit note and for an explanation of how they applied the indexation.
Suppliers sometimes use a different reference period (e.g. "average of month X" vs "value on date Y") or a different index variant than you assumed. Getting their exact method in writing helps you either confirm the invoice is wrong or update your control for next month. Do not accept "it's correct" without the calculation breakdown.
Fixed vs variable contracts: when indexation does not apply
In a fixed-price contract, the energy component (and sometimes other components) is locked for the contract duration. There is no indexation to check-the unit price should stay the same until the end of the fixed period or until an agreed revision date. If your contract is fixed and the invoice shows a different unit price than before, that is either an error (wrong application of the fixed rate) or a legitimate change (e.g. after a contract amendment or renewal). Check the contract dates and any amendments; if the price change is not documented there, dispute it with the contract excerpt and the previous invoice.
Some contracts are hybrid: fixed for a period, then switching to variable with indexation. In that case, know the switch date and from which invoice onward the indexation logic applies. Before that date, the fixed price applies; after, you use the indexation control routine.
Building a simple indexation log for the year
Keep a short log each month: contract name, index name and reference period (e.g. "Belpex Aug-24"), expected unit price or expected change (from your calculation), actual unit price on the invoice, and any delta (€ and %). Over twelve months you will see the full indexation pattern and spot any month where the supplier's application deviated from the contract. That log is also your evidence pack if you open a dispute: you can attach the relevant rows and your calculation for the disputed period.
The log need not be fancy-a spreadsheet with columns for month, index value (from public source), your expected price, invoice price, and notes is enough. The discipline of filling it each month is what turns indexation from a vague "prices went up" into a clear, auditable control.
What good control looks like
A good control process gives one clear answer each month: expected change (from contract + index + period), observed change (from invoice), and any unexplained delta. If there is a delta, you can open a dispute with precise references-index name, reference period, and contract clause-instead of vague complaints about 'the price going up'.