If your electricity bill has lines you have never seen on a smaller site, kVA figures, availability charges, something about reactive power, you are on a half-hourly supply. These lines are not errors and they are not optional extras. They are how larger supplies are billed, and two of them are among the few places on an energy bill where you can genuinely save money without changing supplier at all.
The short version
- Half-hourly means your consumption is measured every 30 minutes and billed accordingly.
- Available capacity is reserved for you, and you pay for it whether you use it or not.
- Too much capacity wastes money every month. Too little triggers penalty charges.
- Reactive power charges are usually an equipment problem, not a tariff problem.
- Both are worth reviewing if your site has changed since the capacity was set.
On this page
What half-hourly actually means
Smaller sites are billed on a total: how many units you used between one reading and the next. A half-hourly supply is measured in 48 separate periods a day, and the data is sent automatically.
That matters for two reasons. First, your price can reflect when you use energy, not just how much, because the cost of supplying electricity varies enormously through the day. Second, the supplier knows your consumption shape in detail, which is why half-hourly quotes often need your actual data rather than an estimate.
Sites end up half-hourly either because their peak demand crosses a threshold or because the meter was installed that way. It is not something you usually opt into.
Available capacity, and the kVA number
This is the one most businesses are paying too much for, often for years.
Your site has an available capacity, measured in kVA. It is the maximum demand the network has agreed to make available to you at any moment. Think of it as the width of the pipe rather than the amount of water you use.
You pay a charge for that capacity every month, whether you use it or not. It is reserved on your behalf, so the network keeps it available.
Why this goes wrong: capacity is usually set when the supply is installed or when the building was last significantly changed, and then nobody revisits it. A unit that once ran heavy machinery, or a site that has moved to more efficient equipment, LED lighting and fewer processes, may be reserving far more than it will ever draw. You are paying monthly for headroom that no longer exists in reality.
Your actual maximum demand appears on your bills, usually as a monthly peak. Compare that to your available capacity across a full year, including your busiest months. If there is a persistent large gap, a capacity reduction is worth investigating with your network operator.
Two cautions. Reductions are not always reversible without cost, so leave sensible headroom for growth and for equipment that draws heavily on startup. And this is a change to your network agreement rather than your supply contract, so it is a separate process from switching supplier.
Excess capacity charges
The opposite problem, and the more alarming one when it appears.
If your demand exceeds your agreed capacity, you are charged for the excess, usually at a penalty rate that is considerably higher than the standard capacity charge. A few spikes can produce a bill line that is startling relative to the energy actually consumed.
It usually means one of three things: new equipment has been installed, something is starting up at the same time as something else, or the capacity was set too low to begin with. The fix is either increasing capacity or staggering what starts when, and the cheaper answer is often the second one.
Reactive power
The line that confuses people most, because it does not feel like energy at all.
Some equipment, particularly motors, pumps, lifts, compressors and older fluorescent lighting, draws current that does not perform useful work but still has to be carried by the network. That is reactive power, and above a threshold you are charged for it.
The key point is that this is an equipment and installation issue, not a tariff issue. No amount of switching supplier removes a reactive power charge. What removes it is correcting the underlying power factor, usually with correction equipment installed at the distribution board.
If reactive power is a meaningful line on your bill, it is worth getting an electrical engineer to look at it, because the saving repeats every month and the equipment is a one-off cost.
Pass-through charges
On larger supplies you will also see third party costs itemised rather than bundled into the unit rate: distribution, transmission and various levies.
These are set by others and passed through by your supplier. They are not negotiable, but how they are handled in your contract is:
| Contract type | What it means |
|---|---|
| Fully fixed | Third party costs are rolled into one unit rate. Predictable, and the supplier prices in the risk. |
| Pass-through | Those costs are shown separately and can move during your term. Potentially cheaper, less certain. |
Neither is automatically better, but a headline rate on one basis cannot be compared with the other. That is one of the most common ways half-hourly quotes are misread.
What to actually review
- Compare your peak demand to your available capacity across a full year. A persistent large gap is money.
- Check for excess capacity charges. If they appear, find out what changed.
- Look for reactive power charges and treat them as an engineering job rather than a procurement one.
- Establish whether your contract is fixed or pass-through, so you can compare quotes properly.
- Get your half-hourly data before seeking quotes. It produces better prices than an estimate.
- Review after any significant change to the site, in either direction.
Paying for capacity you do not use?
Send us a recent bill. We will compare your available capacity against what you actually draw, flag any excess or reactive power charges, and tell you whether there is a saving worth chasing. Some of it is a conversation with your network operator rather than a switch, and we will say so.