Commercial Battery Storage: Sizing a BESS for Peak Shaving, Backup and Solar Shifting

A commercial battery quotation usually leads with one number: so many kilowatt hours, in a container or a row of cabinets, at a price. It is the wrong place to start. A battery energy storage system, or BESS, is bought to do a job, and the job decides whether the important number is kilowatt hours, kilowatts, or how fast the two can be traded against each other. A system sized for backup can be badly wrong for peak shaving, and a system sized for solar shifting can be a poor generator partner.

This article sets out the jobs a commercial battery does, how each one changes the sizing, what sits around the battery cells, the safety standards that matter, and what to ask a supplier before the number on the quotation means anything.

The Four Jobs

Backup and resilience. The battery carries critical loads through a grid outage, either until the grid returns or until a generator starts and takes over. It spends most of its life full and waiting.

Peak or demand-charge shaving. Where the tariff carries a maximum demand charge, typically set by the highest demand recorded in the billing period, the battery discharges during the site's short peaks so that the meter never sees them. It cycles briefly and often.

Solar self-consumption and time shifting. Surplus solar generated at midday is stored and used in the evening or overnight instead of being curtailed or exported for little. The battery cycles once a day, deeply.

Diesel run-hour reduction. On off-grid and weak-grid sites, which describes a great many commercial sites in Africa, the battery lets a generator run fewer hours at an efficient load and then shut down, instead of running all night at a light load it was never sized for. Lightly loaded diesel sets waste fuel and suffer wet stacking, and every running hour brings the next service closer. Our comparison of solar and diesel in Nigeria covers what that generator time actually costs.

Many systems do more than one of these. That is sensible, but the jobs compete for the same stored energy, and the control strategy has to say which one wins. A battery that has just been cycled flat for solar shifting has nothing left for an outage, so a system that also provides backup needs a reserved state of charge that the other jobs are not allowed to touch.

The four jobs a commercial battery does, and the number each one is really sized on. Many systems do more than one, and the jobs compete for the same stored energy. Source: MIMAH engineering practice.

Power, Energy and Duration

Every battery system has two ratings, and confusing them is the most common sizing error.

Power, in kW, is how fast energy can be delivered. It is set mainly by the power conversion system, the bidirectional inverter that sits between the battery and the site, and by the current the battery itself can sustain.

Energy, in kWh, is how much can be delivered before the battery is empty. It is set by the cells.

Duration is energy divided by power. A system of 200 kWh behind a 100 kW converter has a duration of two hours at full output.

C-rate is the same relationship upside down: power divided by energy. The 100 kW, 200 kWh system runs at 0.5C at full output. Cells are rated for a maximum continuous C-rate, and running them hard shortens their life, so a short, high-power job needs cells rated for it.

Each job stresses a different rating.

Backup is sized on both. Suppose the critical load is 40 kW and it has to be carried for four hours until the grid returns or a generator is confirmed running. That is 40 × 4 = 160 kWh of usable energy, and a converter that can carry 40 kW continuously plus the starting surge of any motors on the critical board. The C-rate is gentle: 40 ÷ 160 = 0.25C.

Peak shaving is power-heavy and short. Suppose interval data shows the site peaking at 400 kW for up to 90 minutes against a target cap of 300 kW. The battery must supply 100 kW for 1.5 hours, which is 150 kWh of usable energy at about 0.67C. Real peaks are not rectangles, so the sizing has to come from 15 or 30 minute interval data over a full year, not from the monthly bill. On a tariff where the demand charge is set by the single highest interval of the month, one missed peak can erase most of that month's saving, which makes the control system's forecasting as important as the battery.

Solar shifting is energy-heavy. The battery is sized on the evening and overnight energy it has to supply, and it only works if the array produces enough surplus on a poor day to fill it. A large battery on an array with no surplus is an expensive box. Duration tends to be longer and C-rate lower.

Generator partnering is sized around the generator. The aim is for the set to run near its efficient load while it carries the site and charges the battery, then stop. The battery's power rating has to carry the site alone when the generator is off, and its energy decides how long the generator stays off.

All of those energy figures are usable energy. Nameplate capacity is larger, and how much larger depends on chemistry and on the depth of discharge the warranty permits. The arithmetic for converting between the two is worked through in our guide to battery kWh calculation.

The two ratings of every battery system and the two ratios that link them, with the worked examples from the article. Source: MIMAH engineering practice.

Chemistry: Why LFP Dominates Stationary Commercial Storage

For stationary commercial systems, lithium iron phosphate (LFP) is now the default. It tolerates heat better than the nickel-based lithium chemistries, it is more resistant to thermal runaway, it cycles deeply every day for years, and it has no cobalt. It is less energy-dense than nickel-based chemistries, which matters in a car and matters very little in a container on a concrete pad.

Lead-acid still has a place in small, lightly cycled backup systems, and tubular banks remain common behind small commercial inverters. For anything cycling daily at commercial scale, the life and usable depth of LFP usually win. Our comparison of lithium and tubular batteries goes through the trade-offs in detail.

What Sits Around the Cells

A BESS is a system, and most of its failures happen outside the cells.

Battery management system (BMS). Monitors cell voltages and temperatures, balances cells, and disconnects the battery when a limit is breached. The quality of the BMS decides whether a cell fault stays a cell fault.

Power conversion system (PCS). The bidirectional inverter that charges and discharges the battery. It sets the kW rating, and in hybrid systems it may also form the grid for the site when the utility is down, which is a different capability from simply following it.

Energy management system (EMS). The controller that decides when to charge and discharge. Peak shaving needs a fast, accurate reading of site demand at the point of connection; solar shifting needs the PV output and the load; generator partnering needs control of the generator start and stop. The EMS is where the four jobs are prioritised, and it is the component suppliers describe least.

Thermal management. LFP life depends heavily on cell temperature. In the heat of Sudan, northern Nigeria or Upper Egypt, air conditioning or liquid cooling is part of the system, and its power draw is part of the losses. Ask whether the round-trip efficiency quoted includes it.

Enclosure. Containerised or cabinet systems need an ingress rating suited to dust and driving rain, a location with safe separation from buildings and escape routes, and access for maintenance and firefighting.

Fire and gas protection. Detection, suppression where specified, and ventilation or explosion control to deal with the flammable gases a cell in thermal runaway can release.

The integration of these parts is covered from the system side in our guide to hybrid solar systems.

The Safety Standards Layer

Four documents come up in almost every serious specification. They do different jobs, and a certificate for one is not a substitute for another.

IEC 62619 sets safety requirements and tests for secondary lithium cells and batteries in industrial applications, with stationary energy storage, telecom and UPS among the named uses. It covers foreseeable misuse such as short circuit, overcharge and thermal abuse, and includes requirements for the battery management system. The current edition is IEC 62619:2022, which replaced the 2017 first edition.

UL 9540 is the North American safety standard for energy storage systems and equipment. It evaluates the complete system as an integrated product, so a UL 9540 listing applies to the combination of battery, converter and controls that was tested, not to the cells alone.

UL 9540A is a test method rather than a pass or fail standard. It characterises how thermal runaway propagates, at cell, module, unit and installation level, and the resulting report is the evidence fire engineers use to set separation distances and protection.

NFPA 855, the standard for the installation of stationary energy storage systems, covers where and how a system is installed: separation, fire protection, and emergency planning.

UL and NFPA documents are North American, but they are widely referenced by insurers and in international project specifications, including on sites where no local code yet addresses battery storage. Where local codes or the authority having jurisdiction set requirements, those govern. The practical point is to ask for the test report behind each claim, issued by an accredited laboratory, for the exact product being offered.

Four documents that come up in almost every serious BESS specification. They do different jobs, and a certificate for one is not a substitute for another. Source: IEC, UL Standards and Engagement, UL Solutions and NFPA published scopes.

Degradation and What the Warranty Really Says

Every lithium battery loses capacity from two causes: cycling, and simply ageing with time. Heat and high state of charge accelerate both. A system that delivers its full rated energy in year one will deliver less in year eight, and the sizing has to decide whether to oversize at the start or plan to add capacity later.

Commercial battery warranties normally define end of warranty as whichever comes first of a number of years and a total energy throughput, stated in MWh. They guarantee a minimum remaining capacity at that point, and they attach conditions: an ambient or cell temperature range, a maximum C-rate, a state of charge window, a cycle limit per day, and often continuous monitoring connectivity so that the manufacturer can see how the battery has been used. A peak-shaving system cycling twice a day can exhaust a throughput allowance long before the calendar term, and a hot, poorly cooled container can void the terms entirely.

Read those conditions against the job the battery will do, not against the headline years. The same discipline applies to the rest of the installation, and our guide to solar PV warranties covers how the layers fit together.

What to Ask a Supplier

Usable energy, not nameplate. At beginning of life, at stated temperature and discharge rate, and at the depth of discharge the warranty permits.

Continuous and peak power. For the converter and for the battery separately, and for how long the peak can be held.

Round-trip efficiency at the meter. Including converter losses and the auxiliary load of cooling and controls.

Certificates and test reports. IEC 62619 for the battery, a UL 9540 listing if one is claimed, and the UL 9540A report, all for the exact model offered.

Warranty throughput and retention. Years, MWh throughput, remaining capacity guaranteed, and every operating condition attached.

Thermal design for your site. The ambient temperature the system was designed for, and what happens to power and warranty above it.

Control modes. Which of the four jobs the EMS can run, how it prioritises them, and how the backup reserve is protected.

Local service. Who replaces a failed module or converter board, from which country, and how quickly.

MIMAH's renewable energy team designs and installs off-grid and hybrid systems with battery storage, sized from measured load and interval data. If you have a BESS quotation and want it checked against the job it has to do, get in touch.