A charged battery is not enough to give you backup

On 11 October 2024, a storm left more than two million customers without power in Greater São Paulo, and some of them went days without supply ^[1]. On 28 April 2025, a blackout hit the entire Iberian Peninsula, and the Spanish government declared a national emergency the same day ^[2]. For anyone who lived through either event, the question before the next investment is very direct: when the grid fails again, what will keep running the things that cannot stop?

The most common answer on the market is solar panels combined with a battery. But it does not always do that job. An installation can go dark with a fully charged battery, and not because of a fault: most systems are designed to shut down together with the grid, for safety, and only work without it if they were prepared to do so. Understanding that difference before buying avoids an expensive conversation on the day of the first outage.

Brazil: savings and backup have to be proven separately

The typical offer for a Brazilian business connected at medium voltage makes two promises: discharge the battery during peak hours to cut the electricity bill and, as an extra benefit, provide backup. They are two different promises, and each needs its own proof.

The first depends on the tariff. ANEEL, the Brazilian regulator, distinguishes two options: in the blue tariff, both energy and demand prices vary by time of day; in the green tariff, only the energy price varies, and demand has a single rate ^[3]. So reducing consumption at peak and reducing billed demand are not the same thing. Under the green tariff, for example, shifting consumption out of the peak does not reduce demand if the month's maximum happens at another time. Savings have to be calculated with the customer's tariff and load curve, not with a catalogue figure.

The second depends on a reserve policy that almost never appears in the offer: how much energy will be used for savings and how much will be kept for an outage. Promising savings with all the available energy and, at the same time, autonomy with that same energy is promising the same kilowatt hour twice. The financial model has to respect that reserve, and the reserve has to still exist once the battery has aged. Autonomy in year eight is assessed with a degradation model, not with day one capacity.

Why the system shuts down even with a full battery

Storing energy and forming the voltage are different jobs. The battery does the first. The inverter does the second, and only when it is prepared to do it without the grid.

Most installed inverters are of the kind that follows the grid, known in the market as grid following: they measure the voltage waveform that already exists and inject current in step with it ^[5]. When the grid disappears, they have no reference to follow and, on top of that, they are required to disconnect. Grid connection standards set, by default, up to two seconds for the inverter to stop energising an unintentional island, to protect people working on the line ^[4]. It is a safety requirement, not a promise of transfer to backup. A battery connected through a grid following inverter goes dark with it.

This is where a label deserves suspicion. The opposite of grid following is grid forming, but in catalogues that name covers two very different things. The first is forming the voltage as an island, separated from the public grid, to supply only the installation's own loads. Hybrid inverters with batteries and backup equipment have done this for decades. The second is forming the voltage in parallel with the grid, helping to hold up the system frequency. That is the new part, discussed for utility scale plants ^[5]. A shop, a clinic or a farm needs the first. So the useful question is not "is it grid forming?" but "can it separate from the grid and supply these loads on its own?".

Forming the grid is not the same as starting from zero

Sustaining an island that is already energised and energising it from a completely dead circuit are different capabilities. The second is called black start ^[7]. Reference specifications for grid forming inverters treat the two separately, and they are worth requiring separately too ^[6].

Two details decide whether the system can start. The first is where the controls get their power from. The battery has electronics that monitor and enable it, the BMS. If the BMS, the inverter control and the switching devices are powered from the grid, when the grid fails the system may be left with a full battery and no way to use it. Some manufacturers, to allow a start from zero, require the BMS to be powered in direct current from the battery modules themselves ^[8]. That is information from the manual, not an assumption.

The second detail is whether the loads can start. A compressor draws far more current when starting than when running, and an inverter's current limit is much lower than that of a generator set ^[6]. If several compressors start at once, the inverter reaches its limit.

That is why the test has to cover both situations: losing the grid with the system running, and starting with the installation de-energised. One demonstration does not prove the other.

The electrical pause and the process stop

"Automatic transfer" does not say how long the loads are left without power, and manufacturers publish times that measure different things. An online double conversion UPS switches to battery without interruption, because it was already feeding the load from its own inverter ^[9]. An open transition transfer switch, with contacts that open and close, takes in the order of a hundred milliseconds for the switching alone, without counting fault detection or source preparation ^[10]. An emergency generator set in the most demanding category has up to ten seconds to supply the load, including the whole starting sequence ^[11].

None of these figures is good or bad in itself. A cold room can go minutes without power, while a control system or a server can reboot after an interruption of a fraction of a second. And getting voltage back is not the same as getting production back: a cold room controller may impose a minimum off time and a minimum interval between compressor starts ^[12]. How long the load can tolerate is a question for the customer. How long the system takes is a question for the manufacturer. The usual mistake is to ask neither and promise "zero interruption" because the catalogue says the equipment forms the grid.

Separation from the grid is also part of the purchase

Disconnecting from the public grid is not only a design decision: in the markets we follow, that separation is regulated. In Brazil, each distribution company has its own standard. Neoenergia's, for example, requires physical disconnection to operate as an island and a minimum of 180 seconds of normal voltage and frequency before reconnecting the generation ^[13]. That delay applies to reconnecting to the grid and does not require leaving without power the internal loads that the backup is supplying. In Colombia, Enel requires customers' emergency systems not to energise its network, with an interlocked transfer ^[14]. In Ecuador, CNEL EP's distributed generation manual requires that the public line not be energised, anti-islanding protection in the inverter, backup loads on a separate panel and synchronisation when the grid returns ^[15]. In Spain, the rules come from the self-consumption regulations and from each area's distribution company. In all four cases, the approved scheme has to be part of the project file.

What must be in writing before financing

Commissioning happens after installation. That is why it makes sense to tie the last instalment of the financing to an approved commissioning certificate, with corrective measures set out in the contract if the system does not deliver what was promised. Fixing it afterwards, with the works finished, can mean replacing the inverter, adding the transfer switch and rebuilding the switchboard.

A charged battery is a useful condition, but backup is a function that has to be specified and tested. Before financing the project, it is necessary to require consistency between the promised savings, the available reserve, the autonomy assessed with a degradation model and the functional tests. All of it fits into one question in the project file: when the grid goes down, who forms the voltage? If the answer is "no one", what is being financed is not backup.

Douglas Rosales Baptista · Kyklios

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