Demand Charges: Billed on the Peak, Not the Total
Billed on the highest interval rather than total consumption, which is why solar alone can cut energy cost while leaving the demand charge untouched.
Commercial electricity bills usually carry two different charges measured in two different units. Energy is billed for what was consumed. Demand is billed for the highest rate at which it was drawn.
Confusing the two is why some solar installations produce a smaller saving than expected.
Energy and Demand
Energy is a quantity accumulated over time. A site that draws steadily and one that draws in bursts can consume the same total.
Demand is a rate. It is measured over short intervals, and the highest interval in the billing period sets the charge.
The reason utilities bill this way is that infrastructure has to be sized for peak draw rather than for average consumption. A customer whose demand is concentrated imposes more capacity cost than one whose consumption is spread, even where the totals match.
| Energy charge | Demand charge | |
|---|---|---|
| Measures | Total consumed | Highest rate of draw |
| Set by | Everything you used | One interval |
| Reduced by | Using less overall | Not drawing hard at any one moment |
| Solar alone helps | Yes, directly | Only if it covers the peak interval |
| Storage helps | Indirectly | Directly, by discharging at the peak |
Why Solar Alone May Not Touch It
An array generates on the sun’s schedule. The demand peak occurs on the building’s schedule.
Where those coincide, solar reduces demand. An office whose peak is midday air conditioning on a clear day sees a genuine reduction.
Where they do not, it may not. A facility whose peak is an early-morning startup, or an evening shift, or a compressor cycling under heavy cloud, records that peak with the array contributing little. The energy bill falls and the demand charge is unchanged.
The consequence is that a saving estimate built from energy displacement alone can overstate the result on a bill with a significant demand component.
Cloud cover makes this sharper. A brief drop in output during the interval that happens to be the peak sets the charge for the period, and a site relying on solar to hold demand down has no control over when that occurs.
What Actually Reduces It
Storage. A battery discharging during a high-demand interval reduces what the site draws at that moment. Because the charge is set by the peak, shaving that interval changes it. This is the primary reason batteries appear on commercial sites, and it is a different justification from backup power.
The control system is the important part: it has to recognise a developing peak and respond within the interval. That is a real-time control problem rather than a scheduling one.
Load management. Not starting several large loads simultaneously, staggering startups, and shifting flexible processes away from the peak period. This costs little and is frequently overlooked in favour of equipment.
Process changes. Where a single item of plant sets the peak, changing how it is operated can be the cheapest intervention available.
Understanding Your Own Tariff
Tariff structures are set by the utility and the regulator, so they vary by utility and by state and generalisations are unreliable. Check the rules that apply to your own account rather than assuming a structure from elsewhere.
Four details determine how much effort demand reduction is worth:
The interval length over which demand is measured. Shorter intervals are more responsive to brief spikes.
Whether the charge varies by time of day, with an on-peak window carrying a higher rate.
Whether a ratchet applies, under which a peak sets a floor for subsequent months.
Whether demand is measured on total draw or on some other basis.
Interval data from the meter is usually available on request and it shows exactly when peaks occur, which is the information needed before deciding anything.
What People Get Wrong About This
The first mistake is estimating solar savings from energy displacement alone on an account with a demand component. The two parts of the bill respond differently.
The second is assuming a battery sized for backup will reduce demand charges. Peak shaving is a control function, and a system without the right control does not do it.
A third is not obtaining interval data. The peak’s timing is the whole question and the meter records it.
A fourth is ignoring load management because equipment seems like the answer. Staggering startups costs nothing.
And the last is overlooking a ratchet. Where one applies, a single unusual interval can affect months of bills, which changes the economics of avoiding it entirely.
Related
Time of use covers the other way tariffs vary with timing, and commercial solar covers the wider question this sits inside. Battery bank sizing covers sizing storage, though sizing for peak shaving is driven by the demand profile rather than by backup duration.
Common questions
What is a demand charge?
A charge based on the highest rate of power draw during a billing period, measured over short intervals, rather than on total energy consumed. Two customers using the same total energy pay different demand charges if one drew it steadily and the other in concentrated bursts.
Why does solar not always reduce it?
Because the charge is set by a single peak interval. If that peak occurs when the array is producing little, in early morning, in the evening, or under heavy cloud, the demand charge is unchanged even though the energy bill fell. Solar reduces energy; it does not by itself control the peak.
How does storage help?
A battery can discharge during a high-demand interval so the site draws less from the grid at that moment. Because the charge is set by the peak, shaving that single interval changes the charge. This is peak shaving, and it is the main reason storage appears on commercial sites.
Do residential customers pay demand charges?
Traditionally not, though some utilities have introduced residential demand rates and the practice varies by utility and by state. Whether a demand component applies to a particular account depends on the tariff, which is set by the utility and the regulator rather than nationally.
How long does a peak affect the bill?
Usually the billing period in which it occurred, but some tariffs include a ratchet under which a peak sets a floor for subsequent months. Where a ratchet applies, a single unusual interval can affect bills for a long time, which changes how much effort is worth spending on avoiding it.
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