Is there enough electrical capacity for your new HVAC plant?

What do "agreed capacity" and "maximum demand" mean?

Agreed capacity is the amount of electricity the network operator has agreed to supply to the site, measured in kVA. National Grid Electricity Distribution (NGED) calls it the Maximum Import Capacity, says it is agreed between operator and customer and stated in the connection agreement, and notes it may also appear in the electricity supply contract. Other names are in use: the IET refers to the Authorised Supply Capacity, which "should not be exceeded", and the Energy Networks Association (ENA) to the Declared Supply Capacity. The ENA's find my network operator page shows how to identify the operator, using the Distributor ID on the bill.

Maximum demand is a different thing: the highest amount of electricity the site actually uses, in the ENA's words. The IET adds that a designer calculates maximum demand and the supplier then determines the capacity. In a retrofit both numbers matter, because the question is what the building draws today and what it would draw with the new plant. Settle it before plant is specified; the commercial air conditioning installation page covers the wider sequence.

How do you find out what the site already uses?

From metered data rather than nameplates. The ENA says the two simplest ways to find maximum demand are to check the smart meter, if there is one, or to contact the energy supplier. The IET says BS 7671 Regulation 134.1.1 requires the supply rating to be verified before extra loads are added, and that power logging the supply for an extended period is a good way to do it. In practice that means finding the busiest half-hour in a full year and noting when it fell.

Capacity is quoted in kVA, which includes reactive power as well as the useful kW. NGED says that where the power factor is below unity the current has to rise to deliver the same real power, that motor and power-electronic loads have historically caused this on commercial sites, and that the operator must size the circuit for reactive power too. If a bill or logger shows only kW, ask for kVA or the power factor as well.

How do you add chillers, VRF and air handling units without double counting?

The IET defines maximum demand as the sum of the connected loads multiplied by a factor for diversity. Diversity is a judgement about how many loads run together. Cooling plant makes it awkward: chillers, VRF outdoor units and condenser fans all work hardest in the same hot spell, so a designer should be ready to justify any allowance. Winter has the mirror problem for heating: CIBSE Journal says a move from gas or oil heating to heat pumps is likely to bring a significant increase in electrical demand and may need early engagement with the network operator.

Line in the check (invented figures)	kVA
Agreed capacity	400
Highest measured demand in the last 12 months	310
Headroom today (400 minus 310)	90
New plant after justified diversity	120
Shortfall (120 minus 90)	30

The figures are made up to show the arithmetic. Two adjustments improve it: subtract the demand of any plant being removed if it was running at the peak, and compare the new load with the season it runs in as well as the annual peak. Circuits inside the building are a separate question, covered in the guide to electrical supply for air conditioning.

What can you do if the plant does not fit?

• Change how the site uses power. The ENA says modifying how a site already uses power can create capacity at certain times of day.
• Control the load. The ENA describes load management as controlling power "to ensure you do not go over your overall supply limit". Its page is about vehicle charging, but staging chillers and VRF outdoor units follows the same logic.
• Phase the work. Installing and commissioning in stages lets each stage fit; the guide to retrofitting air conditioning in an existing office covers working in an occupied building.
• Use a hybrid for heating. CIBSE Journal says that where electrical capacity is limited, a hybrid system pairing an undersized heat pump with a retained gas boiler may be more appropriate; see commercial heat pumps.
• Ask about a flexible connection. In May 2022 Ofgem decided that a standardised non-firm access option must be available for larger network users, with clear curtailment limits and end-dates.
• Apply for an increase. The next section covers the steps.

How do you ask for more capacity, and who pays?

1. Apply to the network operator with the existing and proposed capacity. NGED asks for these plus an application form for increases; SSEN says a commercial application needs the required kVA.
2. The operator assesses the request. NGED says it will advise whether the capacity is available or whether works are needed on the connection or network.
3. The operator issues an offer for the cost of the works, as Northern Powergrid puts it.
4. Accept and pay within the offer's validity period. SSEN's quotes are valid for 90 days, and once accepted and paid it must complete the work in 90 days.
5. The operator schedules and makes the connection change.

The customer pays the quoted cost. NGED says an increase may incur a charge under its published connection charging methodology for reinforcement works, and the ENA explains that a quote is split into non-contestable works at the substation and contestable cabling. Ofgem decided to reduce the overall connection charge for those connecting to the distribution network, implemented from 1 April 2023, so ask for the operator's current charging statement rather than relying on older figures.

The reverse also needs thought. If an efficiency project lowers demand, capacity can be reduced, but NGED says it cannot promise to make relinquished capacity available again later, and a reduced value is fixed for 12 months.

How long does it take?

Operators publish timescales mostly for new connections, so an increase on an existing connection may be quicker or slower. Northern Powergrid lists these figures, with days as it states them:

Connection type	Standard for a quotation	Average time to receive a quotation	Average time to connect after payment
Typical small business	25 days	27 days	24 weeks
Typical large business	35 days	40 days	50 weeks
Extra high voltage	65 days	68 days	2 years

For smaller changes SSEN says quotes usually take 5 to 15 working days depending on complexity, and that where land rights or council permits are needed, works typically start after an average of 6 to 8 weeks. Applications are also rising fast: Ofgem said in November 2025 that demand connection applications had risen sharply since November 2024, far above even the most ambitious forecasts. The practical lesson is to measure demand and ask the operator early, before plant is ordered. A reader who wants a contractor's view can send an enquiry through the site's form.

Sources

• Changing your electricity load and What causes losses, National Grid Electricity Distribution; checked 22 October 2026.
• Electrical Diversity and Load Curtailment webinar questions and answers, Institution of Engineering and Technology; checked 22 October 2026.
• Connecting electric vehicle fleets to the network and Find my network operator, Energy Networks Association; checked 22 October 2026.
• Increase or decrease your electricity supply, SSEN; checked 22 October 2026.
• Connection services and Add to your supply, Northern Powergrid; checked 22 October 2026.
• Ofgem: Access SCR decision and direction, 3 May 2022; Decision on DCP404, 15 December 2022; Demand connections update, 6 November 2025; checked 22 October 2026.
• Module 265: commercial heat pump strategies, CIBSE Journal, June 2026; checked 22 October 2026.

Common questions

Is kVA the same as kW?

No. kW is the useful power and kVA is the total the network has to carry, including reactive power. NGED says a power factor below unity needs more current for the same real power. For example, a 100 kW load at a power factor of 0.9 is about 111 kVA, which is simple arithmetic rather than a published figure.

Do I need to tell the network operator before adding a chiller or VRF system?

NGED's page on changing load applies where equipment that may affect load requirements is installed or removed, or where motors or welders that may adversely affect the voltage are connected. The IET adds that BS 7671 requires the supply rating to be verified before loads are added. The designer should confirm both before plant is ordered.

Can I reduce my agreed capacity after an efficiency upgrade?

Yes, NGED accepts requests to reduce load. It warns that it cannot promise to make relinquished capacity available again and that the reduced value is fixed for 12 months. Weigh a possible future expansion first.

Does replacing a gas boiler with a heat pump change the capacity question?

Usually, yes. CIBSE Journal says buildings moving from gas or oil heating to heat pumps are likely to see a significant rise in electrical demand and may need early engagement with the network operator. A hybrid with a retained boiler is one way to stay within limited capacity.

Related guides

• Air conditioning condensate pumps and drains in commercial buildings
• Electrical supply for commercial air conditioning: single or three phase?
• Air conditioning in listed buildings and conservation areas
• Commercial air conditioning noise: limits, assessments and complaints