If you've ever looked closely at a business electricity bill, you'll have noticed that the unit rate you agreed in your contract isn't the only thing you're paying. Buried underneath the commodity cost are a set of network charges — and two of the biggest are DUoS and TNUoS.
Most businesses never question them. Most brokers don't check them. And that's a problem, because these charges are frequently wrong.
What are DUoS and TNUoS charges?
DUoS — Distribution Use of System charges are what you pay for using the local electricity distribution network. Think of it as the cost of the wires between the national grid and your meter. Your local Distribution Network Operator (DNO) sets these charges, and they vary depending on your region, your voltage level, and the time of day you use electricity.
TNUoS — Transmission Network Use of System charges cover the cost of the high-voltage national transmission network. These are set by National Grid and are passed through to end users. TNUoS charges have a demand component (based on your peak consumption during certain winter half-hours called Triads) and a generation component.
Together, DUoS and TNUoS can account for 20-30% of your total electricity bill. On a business spending £50,000 a year on electricity, that's £10,000-£15,000 in network charges alone.
Why do errors happen?
These charges aren't simple flat rates. They depend on multiple factors that change regularly:
Wrong line loss factor. Line loss factors account for electricity lost during transmission and distribution. Every meter has an assigned line loss factor based on its location and voltage. If the wrong factor is applied, every unit you're billed for is slightly wrong — and it compounds across every billing period.
Incorrect meter profile class. Your meter is assigned a profile class (01-08) that determines how your DUoS charges are calculated. If your business has changed — different operating hours, higher or lower consumption — your profile class may no longer be correct. Being on the wrong class means you're paying DUoS rates that don't reflect your actual usage pattern.
Tariff band errors. DUoS charges vary by time of day — red, amber, and green bands. If your meter's time switch isn't set correctly, or the supplier applies the wrong regional tariff, your consumption gets allocated to the wrong bands. Red band rates are significantly higher than green, so misallocation can be expensive.
Triad charge miscalculation. TNUoS demand charges are based on your consumption during the three highest-demand half-hours on the national grid between November and February. These are called Triads. If your Triad consumption is estimated rather than actual, or if the wrong Triad data is used, your TNUoS demand charge will be wrong.
Outdated tariff tables. DUoS and TNUoS tariffs are updated annually (DUoS in April, TNUoS in April). If the supplier's system doesn't pick up the new tariff on the right date, you could be overcharged or undercharged depending on which direction rates moved.
How to audit DUoS charges
Start with the basics:
- Confirm your meter's profile class. Check your bill or ask your supplier. If your business hours or consumption have changed significantly since the meter was installed, request a review.
- Check the regional tariff. DUoS tariffs vary by DNO region. Make sure the tariff on your bill matches the correct region for your meter's postcode. You can find the current tariff tables on your DNO's website.
- Verify time band allocation. If you're on a half-hourly meter, check that consumption is being allocated to the correct red, amber, and green bands. Compare the banded consumption on your bill against your half-hourly data.
- Check the line loss factor. Your LLFC (Line Loss Factor Class) is assigned based on your meter's characteristics. The correct LLFC for your meter can be confirmed through the ECOES database or your DNO.
How to audit TNUoS charges
TNUoS is trickier because it involves Triad data:
- Confirm your Triad consumption. Your supplier should be able to provide the half-hourly consumption figures used for your Triad calculation. Compare these against your actual meter data for those specific half-hours.
- Check the TNUoS tariff zone. TNUoS demand tariffs vary by geographical zone. Verify that the correct zone has been applied.
- Review the reconciliation. TNUoS charges are initially based on estimated Triads and then reconciled against actual data. Check that the reconciliation has happened and that any credit has been applied.
What to do when you find an error
If the charges don't match, raise a formal query with the supplier. Include the specific charge, the expected value based on the correct tariff, and the billing periods affected.
Suppliers have an obligation to correct billing errors. If the overcharge has been running for multiple periods, you can claim back the difference — typically up to 12 months, though the Limitations Act allows claims up to six years.
Why most brokers don't check this
Honestly? Because it's hard. DUoS and TNUoS validation requires tariff data that changes annually, meter-specific data that varies by site, and a level of technical knowledge that goes beyond comparing a unit rate against a contract.
Manual checking is realistic for a handful of meters. At scale — across a portfolio of 50, 100, or 500 sites — it's impractical without automation.
That's where automated bill validation earns its keep. Software that holds the current DUoS and TNUoS tariff tables and checks every bill against the correct rates catches errors that manual review simply can't cover.
Next steps
If you're a broker looking to add network charge validation to your service, our guide to common energy billing errors in the UK covers the broader landscape of what goes wrong on bills. For a practical comparison of manual versus automated approaches, see automated bill validation vs manual checking.
Ready to see what your bills look like when every charge — including DUoS and TNUoS — is validated automatically? Try Edge free and run your first batch through.