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Authorised Supply Capacity

What is Authorised Supply Capacity (ASC)?

Half-hourly metered UK electricity supplies have a critical number recorded in their connection agreement called Authorised Supply Capacity (ASC). It is the maximum kVA of electrical demand the Distribution Network Operator has agreed to reserve for that supply point, sometimes called Available Capacity or Maximum Import Capacity (MIC) in older documentation. Whichever name your bill uses, the figure does the same job. It defines what you pay each month for capacity (the capacity charge), what triggers an excess capacity charge if your maximum demand crosses it, and the upper bound on power you can draw without network reinforcement. For HH sites, the ASC is one of the actively manageable bill components and a frequent source of cost optimisation when it has drifted out of sync with actual peak demand.

ASC sits inside the wider mechanics of a business electricity bill. It’s the agreed ceiling at which your supply is allowed to draw, set when the connection was installed.

ASC is sometimes called Available Capacity (the older name in some regions) or Maximum Import Capacity (MIC). They mean the same thing. The figure matters because it drives a fixed monthly capacity charge, defines the threshold beyond which the much higher excess charge kicks in, and determines the limit beyond which the DNO would need to reinforce the network if you wanted to draw more power.

What ASC actually represents

ASC is the maximum kVA of electrical demand the DNO has reserved for your supply.

• Expressed in kVA.
• Set at connection, agreed between the customer and the DNO.
• Recorded in the supply agreement and visible on bills.
• Charged monthly as a capacity charge regardless of actual consumption.
• Threshold beyond which excess capacity charges apply.

The DNO commits network capacity to your supply equal to your ASC. You pay for that commitment whether you use it or not.

How ASC is set

ASC is established at three points.

• At new connection. The customer requests a defined capacity. The DNO assesses whether the local network can supply it (and, if not, what reinforcement is needed). The agreed figure becomes the ASC.
• At connection upgrade. An existing supply requesting more capacity goes through a similar assessment.
• At customer-initiated change. A customer can request a decrease in ASC if the actual peak demand is consistently below the current level.

For new connections that exceed what the local network can supply without reinforcement, the customer pays a capacity contribution toward the reinforcement cost. For routine supplies within network capacity, ASC is set administratively.

Why ASC matters for your bill

ASC drives two bill components on HH-metered supplies.

• Capacity charge. Monthly fixed charge based on ASC times the network capacity rate. Typically £0.80 to £1.50 per kVA per month depending on region.
• Excess capacity charge. If measured maximum demand (MD) exceeds ASC in any half-hour, an excess charge applies. Typically two to three times the standard capacity rate, multiplied by the excess kVA.

For a 500 kVA ASC at £1.20 per kVA per month, the standard capacity charge is £600 per month. A 50 kVA excess triggers a further £180 per month at typical excess rates.

Excess capacity charges

Excess charges apply when the half-hourly maximum demand exceeds the ASC.

• Triggered by any single half-hour above the threshold during the billing period.
• Applied as the maximum demand minus the ASC, multiplied by the excess capacity rate.
• Persistent across billing periods until the supply’s peak demand drops back below the threshold.
• Can be material when wholesale prices are high or when the supply is genuinely undersized.

The unforgiving nature of the calculation (a single half-hour can produce months of charges) makes managing peak demand important on HH sites near their capacity threshold.

How to size ASC correctly

The right ASC is the lowest level that comfortably accommodates the site’s real maximum demand with sensible headroom.

1. Pull 12 months of half-hourly data from the supplier portal.
2. Identify the highest half-hourly maximum demand across the period.
3. Add 10-15 per cent headroom to account for natural variation and growth.
4. Compare to the current ASC.
5. If actual peak plus headroom is below the current ASC by a material margin, the ASC can be reduced.
6. If peak demand is approaching or above the current ASC, an increase or load management may be needed.

Changing your ASC

ASC changes go through the DNO via the supplier.

• Decreases. Usually administrative. Take 4 to 8 weeks to apply.
• Increases. May require network reinforcement and a capacity contribution. Lead times of 3 to 12 months are common for material increases.
• The supplier coordinates the request between customer and DNO.
• A direct conversation with the DNO is sometimes more efficient for complex changes.

For larger UK businesses, regular ASC reviews (every 1-2 years) catch sites that have drifted out of sync with their actual peak demand.

kVA vs kW and the power factor question

ASC is in kVA (apparent power), not kW (real power). The difference matters.

• A site running at 200 kW with power factor 0.8 has a kVA demand of 250.
• An ASC of 250 kVA accommodates that demand. An ASC of 200 kVA does not.
• Improving power factor (via correction equipment) reduces the kVA needed for the same kW of useful work, potentially allowing ASC reduction.

See kVA and reactive power for the technical detail.

ASC, MIC, Available Capacity. Different names same thing

Different UK regions use different terms for the same concept.

• Authorised Supply Capacity (ASC). Most common current term.
• Available Capacity (AC). Older term, still used by some DNOs.
• Maximum Import Capacity (MIC). Used in some regions and on some supply formats.
• Agreed Supply Capacity. Variant phrasing used by some suppliers.

They all refer to the same thing. The figure on your bill should match the figure in your supply agreement regardless of which term is used.

ASC and small supplies

For UK supplies below 100 kW maximum demand (NHH category).

• ASC is set but rarely visible to the customer.
• Capacity charges are bundled into the standing charge.
• Excess charges do not apply in the same way.

The concept matters most for HH-metered supplies above the 100 kW threshold. Below that, the customer is shielded from the bill-line impact even though the underlying capacity allocation exists.

Practical implications for businesses

For HH-metered UK business customers, ASC is one of the most actively managed components of the bill.

• Annual ASC review. Pull HH data, compare peak demand to ASC, and request adjustments where the gap is material.
• Right-sizing for new connections. Setting the right ASC at install avoids both undersizing (excess charges) and oversizing (paying for capacity you do not need).
• Excess charge response. If excess charges appear on bills, investigate the half-hour that triggered them and consider load management or an ASC increase.

For a deeper walkthrough on the underlying capacity concept, see the live Clearsight kVA guide. Related entries. kVA, half-hourly meter, reactive power, DNO, DUoS, non-half-hourly, bill validation.

The ASC reduction process step by step

For HH-metered UK sites where the current ASC sits materially above actual peak demand, an ASC reduction can produce direct monthly savings. The process.

1. Pull 12 to 24 months of half-hourly meter data from the supplier portal.
2. Identify the highest half-hourly maximum demand value across the period.
3. Add 10-15 per cent headroom for natural variation and unforeseen events.
4. Compare the resulting target ASC to the current ASC.
5. If the gap is material (say, 20 per cent or more), prepare the reduction request.
6. Submit the request to the DNO via the supplier with supporting HH data.
7. DNO processes the request (typically 4 to 8 weeks).
8. The new ASC takes effect from the agreed date.
9. The next monthly bill reflects the new capacity charge.

For a 200 kVA reduction on a contract at £1.20 per kVA per month, the annual saving is £2,880. Reductions on multi-site portfolios compound across all sites.

Common ASC pitfalls

• Overcapacity inertia. Sites that were sized generously at install often carry that capacity for years even after operations have reduced. Active review unlocks savings.
• Undersizing then excess charges. Setting ASC too tight produces excess capacity charges when peaks happen, which usually cost more than the savings from being undersized.
• Forgetting seasonal peaks. Reductions based on summer data ignore winter peaks (or vice versa). Use a full year of HH data for the analysis.
• Ignoring power factor. A site with poor power factor has higher kVA demand for the same useful work. Improving power factor (see reactive power) can enable an ASC reduction.

Frequently asked questions