Demand charges: the commercial bill line solar can attack

The line nobody explains

Open a commercial electricity bill and you will find a charge that has nothing to do with how much power you used. It is based on how hard you drew it - your highest rate of consumption during the billing period, usually measured over a half-hour window, and charged per kilowatt or kilovolt-ampere.

That is the demand charge. On sites with spiky loads it routinely rivals or exceeds the usage charge, and unlike usage it cannot be reduced by simply using less overall. A business can cut total consumption meaningfully and watch this line barely move.

How one half hour sets your bill

The mechanism is what makes it counterintuitive. If your site runs comfortably at 40 kW all month but spends one half-hour at 110 kW - compressors and air conditioning starting together on the first hot afternoon, say - your demand charge is calculated against that 110 kW peak, not the 40 kW you actually operated at.

Worse, many tariffs then apply a ratchet: that peak sets a floor for subsequent months, sometimes for up to a year. One badly-timed half hour can be paid for repeatedly.

This is why demand deserves attention before anything else on a commercial site. It is the only bill line where a single event has outsized, lasting consequences.

Where solar helps, and where it doesn't

Solar reduces demand charges when your peak coincides with generation. For a business peaking at 2pm in summer - refrigeration, air conditioning, daytime production - solar is directly shaving the moment that sets the charge, and the saving comes on top of the usage reduction.

Solar does not help when the peak sits outside generating hours, or on a cloudy day when the site peaks anyway. That is the honest limit: solar reduces demand charges probabilistically, not reliably, because it cannot guarantee output at the moment the meter is watching.

This is exactly why demand analysis belongs in the design, not after it. A system sized purely on consumption may leave most of the demand saving on the table.

Where a battery changes the picture

A battery does what solar alone cannot: it can discharge on command at the exact moment demand spikes, regardless of weather or time of day. Configured for peak shaving, it holds the site's measured demand below a set ceiling by covering the difference from storage.

That converts an unpredictable saving into a controlled one - and it is the single strongest commercial argument for storage, often stronger than the arbitrage case that drives residential batteries. The battery does not need to be large; it needs to be able to deliver enough power for long enough to cover the spike, which is a different sizing question from the residential one.

Combined with soft starters, staged plant start-ups and scheduling changes, the ceiling can often be brought down further before a single kilowatt-hour of storage is bought.

What we need to model it properly

Demand cannot be modelled from a bill total. It needs interval data - typically 15 or 30 minute readings, which your retailer can provide for your NMI - so we can see the actual shape of your load, identify when the peaks occur, whether they are seasonal, and how much of the spike is coincident plant rather than genuine base load.

From there the question becomes: what ceiling is achievable, what does holding it require, and what is the resulting annual saving against the cost. Sometimes the answer is that operational changes get most of the way for nothing. We would rather find that than sell storage that was never needed.

If your bill has a demand line, bring it to the conversation. Our commercial assessment asks for it specifically, and it is usually where the largest single saving on a commercial site is hiding.