A battery storage system is usually described by two numbers: how much power it can deliver, and for how long. Neither tells you about a third number that quietly affects every unit of energy you buy and sell through it.
That third number is auxiliary consumption — the electricity the system uses to keep itself running.
What “auxiliary” actually covers
Inside every battery container or cabinet there is a set of support systems that must stay on whether the battery is working or not:
- Thermal management — air conditioning or a liquid-cooling loop that holds the cells within their safe temperature range. This is almost always the largest load.
- Battery management system (BMS) — the electronics that monitor every cell’s voltage and temperature. We explain it in BMS explained.
- Energy management and controls, communications and remote monitoring.
- Fire detection and suppression systems, which must be powered continuously.
- Lighting, ventilation and small loads in the enclosure and switchgear.
The power conversion system also loses energy when it converts between AC and DC, but that is usually counted as a conversion loss rather than auxiliary load — our guide to the power conversion system covers it.
How big it can be
Suppliers rarely lead with this number, and there is no single universal figure — it depends on design, climate and how the battery is used. But industry estimates show it is not trivial.
William Lauwers of the engineering consultancy Enertis Applus+ has estimated that the auxiliary load can consume over 15 MWh of electricity every year for every MWh of BESS capacity. He also puts the thermal management system at roughly 10–15 per cent of enclosure volume and over 5–8 per cent of system cost.
To put 15 MWh in context: a 1 MWh battery that does one full cycle a day discharges at most about 365 MWh a year. On that simple arithmetic, auxiliary power of 15 MWh is equivalent to around 4 per cent of everything it delivers — before counting any conversion losses. In an Indian summer, with ambient temperatures well above 40°C at many sites, the cooling share of that can be higher still. That is why thermal management designed for Indian conditions matters commercially, not just for safety.
How an Indian tender treats it
Indian tender documents make the accounting explicit, and they are worth reading as a model for any contract.
SECI’s December 2025 tender for 125 MW / 500 MWh of standalone storage in Odisha, supported by viability gap funding, sets out three things:
- A guaranteed round-trip efficiency. The developer must guarantee a minimum AC-to-AC round-trip efficiency of 85 per cent, measured on a monthly basis as total energy discharged divided by total energy charged.
- Separate metering of auxiliary power. The developer either takes a separate metered connection for auxiliary load or draws it from the interconnection point through its own meter — either way, auxiliary consumption is measured separately and billed by the distribution company.
- A penalty ladder for underperformance. Falling short of 85 per cent triggers liquidated damages on the excess conversion losses, priced against the state’s average power purchase cost (APPC), and beating it earns a small incentive.
The tender also illustrates the charging arithmetic: to get 500 MWh out at an 85 per cent round-trip efficiency, the buyer must supply about 588.24 MWh in.
The lesson for any buyer: the same battery can report very different efficiency figures depending on whether auxiliary power is inside or outside the measurement. A quote claiming 90 per cent round-trip efficiency with auxiliary load excluded is not better than one claiming 86 per cent with it included — it may well be worse.
Why it matters more behind the meter
For a commercial or industrial site, auxiliary consumption shows up directly on your electricity bill.
- It runs around the clock. A battery waiting at full charge through a hot afternoon for the evening peak is still cooling itself.
- It can land in expensive hours. Cooling works hardest at midday and in the afternoon, which in many states overlaps with time-of-day tariff windows.
- It eats into small systems more. Fixed loads such as controls and fire detection are a larger share of a small cabinet’s throughput than of a large container’s.
None of this makes storage uneconomic. It means the savings in any proposal should be calculated net of auxiliary consumption, and that a round-trip efficiency figure should always be read together with its measurement point — the topic of our guide to round-trip efficiency and degradation. It also feeds directly into the levelised cost of storage: auxiliary power is part of the lifetime cost on top of the fraction.
Tender definitions of round-trip efficiency, metering and penalties vary between procurers and change with each notification. Verify the current terms in the actual tender document before bidding.
What this means for you
When you compare quotes — whether for a cabinet or a containerised system such as our ADESS 5000 — ask every supplier for the same four things:
- Measured auxiliary consumption in kWh per day, at a stated ambient temperature and duty cycle — ideally from an operating site in a comparable climate.
- The measurement point for round-trip efficiency: AC or DC, and whether auxiliary power is inside or outside it.
- Standby consumption when the system is idle at full charge.
- The thermal design basis — the maximum ambient temperature the cooling is sized for.
If you want to see how auxiliary consumption changes the numbers on your own tariff and duty cycle, talk to our team.