STATCOM Mode: What Your BESS Does at Night

A battery storage plant procured for peak shaving or renewable firming has a duty cycle measured in hours, not days. It charges in the off-peak window, discharges into the peak, and for the remaining part of the day the cells sit at rest while the power conversion system idles.

That converter does not have to idle. In reactive-support mode — commonly called STATCOM mode — it can deliver or absorb reactive power at the point of interconnection without moving energy through the battery at all. Our utility configurations carry ±10 MVAr at night on that basis: reactive support with no discharge.

This capability is almost always already installed, and almost never priced into the business case.

Reactive power is a converter function, not a battery function

The distinction is the whole point and it is worth being precise about.

Real power is energy per unit time — it comes out of the cells and it depletes them. Reactive power is the component of apparent power that oscillates between the source and the network's inductive and capacitive elements; averaged over a cycle it transfers no net energy. A grid-tied inverter produces it by controlling the phase relationship between its output voltage and current, which is a switching decision, not an energy withdrawal.

So a converter can source or sink reactive power with the battery at rest. What it consumes is a small amount of real power to cover its own switching and thermal losses, and headroom in its apparent-power rating.

That headroom is the constraint. A converter's rating is in MVA, and real and reactive power share it:

S² = P² + Q²

At full real-power output there is little left for reactive. At zero real power — which is where a storage asset spends most of its night — nearly the whole MVA rating is available as reactive capability.

Where the capability comes from

Take a utility configuration built on our containers with three 4.0 MVA conversion blocks, 12 MVA installed. At a 10 MW real-power output the remaining reactive capability is:

√(12² − 10²)  =  √44  =  6.63 MVAr  while delivering full rated real power

That figure is available continuously, four-quadrant, at any active power level. And when real power drops toward zero overnight, the constraint relaxes toward the converter's full rating — which is where the ±10 MVAr night capability comes from. The battery contributes nothing to that number except the requirement to stay available.

Scale it up and the same relationship holds. A 50 MW plant on thirteen 4.0 MVA blocks carries 52 MVA installed, leaving:

√(52² − 50²)  =  √204  =  14.28 MVAr  at full real output

without curtailing a single megawatt of active power.

Why this is usually left on the table

Three reasons, in rough order of how often we encounter them.

The business case was written for energy. Peak-shaving and arbitrage models count kWh moved. Reactive support moves no kWh, so it does not appear in the model at all — not as a small number, as no number.

The interconnection agreement does not ask for it. Many Indian connection agreements specify a power factor range at the point of interconnection and stop there. If the requirement is satisfied passively, nobody looks at what else the converter could do.

Nobody asked the converter vendor. Four-quadrant reactive capability at zero real power is a control-mode question. It is usually supported and it is usually not commissioned, because commissioning it requires a test that nobody scheduled.

The result is a plant with a capability sitting idle for roughly two-thirds of every day.

What it is actually worth

The honest answer is that it depends entirely on the connection point, and we are not going to put a rupee figure on it in a blog post.

What we can say is where the value shows up. Reactive support at the point of interconnection affects local voltage regulation, which in a weak-grid or long-feeder location is the binding constraint on how much renewable generation can be evacuated. Where a network operator is procuring voltage support, or where a connection is constrained by voltage rather than thermal limits, a co-located storage asset that can hold voltage overnight is solving a problem the operator is already paying someone to solve.

For a captive or C&I installation the mechanism is different: power factor penalties and demand charges are levied by the distribution utility, and a converter that can correct power factor continuously displaces a separate capacitor bank or a separate STATCOM.

In both cases the relevant comparison is not against the storage business case. It is against the cost of the dedicated equipment that would otherwise do the job.

The specification questions that decide it

If you want this capability to be real rather than theoretical, four things have to be true, and all four are specification decisions made before order.

Converter rating with deliberate headroom. Reactive capability at full real output is whatever the MVA rating leaves over. Sizing the conversion system exactly to the real-power requirement gives you nothing at full output.

Four-quadrant operation, confirmed at SKU. Absorbing as well as sourcing reactive power, at any active power level including zero, is a converter capability that must be confirmed against the selected model — not assumed from a family datasheet.

A plant controller regulating at the right point. Reactive support is measured at the point of interconnection, so the plant controller must close its loop on interconnection instrumentation rather than on converter terminals.

Auxiliary supply and availability. Delivering reactive power overnight means the plant is energised overnight, and the auxiliary load is real. It is a parasitic draw that belongs in the model.

These are the same control-layer questions that grid-forming specifications ask, and they are increasingly arriving in the same documents — we covered that shift in grid-forming is now a tender requirement. The architecture underneath is described on our technology page, and the application-by-application breakdown is on applications.

The short version

A four-hour storage asset is a converter that happens to have a battery attached, and for most of the day the battery is the part that is resting. Reactive support uses the half that is not.

It is not a reason to buy storage on its own. It is a reason to size the conversion system with headroom, confirm four-quadrant operation at SKU selection, and ask the network operator what voltage support is worth at your connection point before the interconnection agreement is signed — because after signature it is a variation.

Talk to our team for a configuration sized with reactive headroom against a specific connection point.