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TLM and GSP GCF adjustments
Understand the industry-wide changes to GSP Group Correction Factors and Transmission Loss Multipliers, and what they mean for your electricity invoices.
Understanding industry changes to your electricity billing
The way electricity usage is calculated and settled in Great Britain has changed. From your August 2026 invoices onwards, you may notice an adjustment to your billed consumption, which reflects recent changes across the energy industry to two long-standing parts of electricity billing, Transmission Loss Multipliers (TLMs) and the Grid Supply Point Group Correction Factor (GSP GCF). This adjustment is being made following updates to industry data used across the energy market. It is not specific to SmartestEnergy, but part of ensuring customer billing reflects the finalised industry values.
What is a Transmission Loss Multiplier (TLM)?
Electricity doesn't arrive at its destination in exactly the amount it left. As power travels across the national transmission network, the high-voltage lines carrying it over long distances and a small portion is lost as heat along the way. Transmission Loss Multipliers account for that loss, adjusting recorded consumption so it reflects the energy that had to be put into the system. TLMs are set by Elexon, the body responsible for electricity settlement in Great Britain, and applied consistently across the market. When losses on the network are higher, recorded consumption is adjusted up. When losses are lower, it comes down. And because final TLM values are confirmed after original bills go out, occasional reconciliation is a normal part of how electricity settlement works.
What is a GSP Group Correction Factor (GSP GCF)?
Electricity is measured twice on its journey, once at grid level as it enters each region, and again at individual customer meters. Even after transmission losses are accounted for, the two readings never match exactly because of metering differences and timing across millions of meters. The GSP Group Correction Factor is the industry's way of reconciling that remaining gap, so the total energy bought and sold across the market balances correctly. GSP GCFs are calculated by Elexon and apply to every supplier and every metered site. The factor can nudge the volume of energy allocated to your meter in either direction, and it varies with your location, your meter type and when you use energy.
Frequently asked questions
Why are TLM and GSP GCF adjustments being made?
Electricity bills are only ever as accurate as the industry data behind them. Alongside your meter readings, every invoice uses values set centrally for the whole market, and those values can be revised after billing.
For the period since April 2025, the finalised values came in at different levels, and invoices need to reflect the correct data.
Why are TLM and GSP GCF adjustments being made?
Electricity bills are only ever as accurate as the industry data behind them. Alongside your meter readings, every invoice uses values set centrally for the whole market, and those values can be revised after billing.
For the period since April 2025, the finalised values came in at different levels, and invoices need to reflect the correct data.
How do the adjustments affect your bill?
No two customers will see quite the same adjustment; it depends on where your sites are, how they're metered and when you used energy during the affected period.
Will my billed consumption go up or down?
It could be either. Both TLMs and GSP GCFs adjust the volume of energy allocated to your meter, and because most electricity charges are based on volume, the charges linked to that consumption move with it. The corrections work in both directions. Some customers will see an increase, while others will see a reduction, depending on location, meter type and consumption profile during the affected period.
The consumption data shown on your bills will not change. The adjustment is calculated using the corrected volumes, but your recorded consumption for the period stays as originally reported.
How are these costs passed through under my contract?
These costs are already part of your supply agreement. TLMs and GSP GCFs are what's known as pass-through components; when industry adjustments occur, the associated costs are passed through under the existing terms of your contract, and where the finalised values work in your favour, the reduction is passed back to you on exactly the same basis.
How is the adjustment calculated?
The finalised TLM and GSP GCF values are applied to your consumption during the affected period, and the result is compared against what was originally billed. The difference between the two is the adjustment on your invoice. Nothing about the calculation is specific to SmartestEnergy; the same finalised values apply across the market.
What this means for you
Understanding the shift to half-hourly settlement
Market-Wide Half-Hourly Settlement is the biggest overhaul of how electricity usage is measured and settled in over two decades, and it's the programme driving these adjustments. When the market moved to half-hourly settlement in September 2025, the values used to calculate bills, including TLMs and GSP GCFs, were updated as part of the transition. Read our MHHS explainer for the full picture of what's changed and what it means for large business customers.
Additional Resources
Both TLMs and GSP GCFs are administered by Elexon, and the details behind every value are published openly.
Frequently Asked Questions
Is this a new charge?
No. The adjustment corrects charges that were already part of your billing, applying the finalised industry values to the affected period. Nothing new is being added to your contract or your pricing.
Why wasn't this reflected on my original invoices?
Because the finalised values didn't exist yet. Settlement data is confirmed after invoices are issued, and for this period, the confirmed values came in at different levels than the ones available at the time. The adjustment closes that gap.
Will I be charged more or refunded?
It depends on your sites and how you used energy during the affected period. The finalised values can increase or decrease the consumption allocated to your meters, so some customers will see an additional charge and others a credit. Whichever direction it goes, it's calculated on the same basis.
Will the adjusted volume be reflected in the consumption data on my bills?
No, the GCF and TLM charges have been calculated based on your adjusted volume, but our consumption data for the relevant period will not be amended to reflect this.
How do I know the adjustment is right?
Every adjustment is calculated from your consumption during the affected period and the finalised values published by Elexon, the industry's settlement body. If you'd like to see how the figures apply to your account, your account manager can take you through the breakdown.
My legal notice mentions Line Loss Factors. What are they?
Line Loss Factors (LLFs) work like TLMs but at local level, accounting for energy lost on the regional networks that carry electricity the final distance to your sites.
Will this happen again?
Possibly, and it's worth being upfront about that. TLM values are still being finalised for the current period, so a further reconciliation may be needed before the process is complete. Industry work is also underway on longer-term arrangements designed to keep bills accurate from the start.
Who can I speak to about my invoice?
Your account manager is the best first contact for anything specific to your account. They can explain how the adjustment applies to your sites and provide the details behind the figures.
Speak to your account manager
Every account is different, and the quickest way to understand what these adjustments mean for yours is to speak to your account manager directly. They will walk you through the figures and explain how the finalised values apply to your sites.