How to Size a Battery Storage System for Your Business

Sizing a battery storage system for a business means determining its power (kW) and capacity (kWh) from two layers of data: the site's consumption and production profile, then the economic parameters that determine profitability. Sizing based on technical criteria alone does not guarantee a return on investment.

What technical data is needed before sizing a battery?

Technical sizing of a BESS (Battery Energy Storage System) relies on three categories of data. Without them, any cost estimate remains a rough approximation.

The site's load curve from the network operator

The load curve records the power drawn by the site, typically at 10 or 30-minute intervals, over at least 12 months. In France it is obtained from the network operator (Enedis) via the smart meter or industrial telemetry, or through the operator's client portal. It reveals power peaks, their frequency, and the share of consumption that could be shifted or smoothed by a battery.

The size of the existing or planned solar installation (kWp)

The size of the solar installation, existing or planned, expressed in kWp (kilowatt-peak), determines the volume of energy available to store rather than export to the grid or curtail. A site without PV can still size a battery for peak shaving or time-of-use shifting alone, but pairing PV with storage changes the power and capacity equation.

Location, tilt and orientation to model solar production

For a PV project not yet built, the geographic location is used to model expected solar production hour by hour across the year, based on local irradiance data. Panel tilt and orientation significantly change annual output: an optimally tilted south-facing array maximizes cumulative production, while an east-west orientation spreads production across a wider daily window, which can reduce the battery capacity needed to cover peak production hours. For an installation already in service, actual production data is used directly instead.

How is optimal technical battery sizing calculated?

Cross-referencing the load curve and the hourly solar production profile identifies the windows where production exceeds consumption (surplus to store) and those where consumption exceeds production or the subscribed grid capacity (need for discharge). Technical sizing aims to cover the maximum share of these gaps without oversizing the asset.

• Power (kW): sized to the amplitude of the peaks to shave or the charge/discharge rate needed for peak shaving
• Capacity (kWh): sized to the volume of energy to store between a PV surplus and a later need
• Depth of discharge and SoH (State of Health): for a second-life BMS, a cell-by-cell diagnostic refines the real usable capacity, which differs from nominal capacity
• Expected cycling: number of cycles per day and per year, which determines lifespan and the contractual performance guarantee

This technical calculation gives an order of magnitude for power and capacity. It says nothing, however, about the investment's profitability.

Why is technical sizing alone not enough?

A battery that is perfectly sized from a technical standpoint can be oversized from an economic standpoint, or conversely undersized relative to an unexploited revenue potential. Arbitrating the return on investment requires precise tariff data.

• Energy supply price: the variable cost paid to the electricity supplier, in €/MWh, depending on contract type (fixed, indexed, dynamic)
• Electricity excise duty: a per-MWh tax whose level is revised periodically
• Network tariff: France's TURPE (grid usage tariff), revised every August 1 by the energy regulator. On August 1, 2026, it rises 3.04% for distribution and 3.34% for transmission, strengthening the case for power smoothing and time-of-use shifting ([CRE, ruling of May 21, 2026](https://www.cre.fr/actualites/toute-lactualite/la-cre-publie-levolution-annuelle-des-tarifs-dutilisation-des-reseaux-publics-delectricite-au-1er-aout-2026.html))
• Subscribed capacity: the contractual power level with the grid operator; exceeding it triggers penalties, and lowering it through peak shaving can generate a direct saving
• Export price, where applicable, for sites that feed part of their PV production back to the grid

Crossing these financial parameters with technical sizing is what produces an actual return-on-investment calculation, and therefore a battery sized not at the technical maximum but at the point where each additional kWh of capacity remains profitable.

How does time-of-use pricing affect battery size?

When the electricity contract includes time-of-use pricing (off-peak and peak hours), it becomes relevant to assess whether charging the battery during off-peak hours to discharge it during peak hours is more cost-effective than buying electricity directly at the peak rate. The gap between off-peak and peak rates has widened with the 2025-2026 tariff revisions, reinforcing the case for time-of-use shifting on sites with heavy peak-hour consumption.

On the wholesale market itself, the gap between the cheapest and most expensive hour of a given day can exceed 200 €/MWh depending on market conditions — a price signal that directly benefits sites able to shift consumption in time.

Can a battery be monetized beyond self-consumption?

Depending on the consumption profile and available power, a battery storage system can also participate in market mechanisms, in addition to its primary on-site use.

1. Grid flexibility: participation in demand-response mechanisms such as NEBEF in France or the balancing mechanism operated by the transmission system operator, which compensates the ability to reduce or shift consumption on request
2. Market arbitrage: buying and reselling stored energy based on hourly price spreads on the spot market, for profiles compatible with this type of exposure

These secondary uses do not replace the primary sizing calculation, but they can improve overall return on investment and, in some cases, justify capacity slightly above the strict self-consumption need.

What is the right method to size a battery correctly?

• Obtain the site's load curve from the network operator over at least 12 months
• Gather solar production data (kWp, location, tilt, orientation), or model expected production if the PV project is not yet built
• Cross-reference the load curve and production data to identify surpluses and discharge needs, deriving a technical power and capacity range
• Collect tariff data: supply price, excise duty, current network tariff, subscribed capacity, and any export price
• Model ROI including time-of-use shifting if the contract is time-of-use priced
• Assess additional monetization potential through grid flexibility or market arbitrage
• Set the final sizing at the technical-economic equilibrium point, not at the technical maximum

Battwoo offers a sizing study that covers all of these steps: load curve analysis, solar production modeling, economic arbitrage, and the KPIs needed for an investment decision. According to Ember's October 2025 assessment, the levelized cost of storage for long-duration utility-scale battery systems in global markets outside China and the US stood at $65/MWh (Ember, October 2025), a benchmark useful for evaluating a project's cost competitiveness.

Frequently asked questions about battery storage sizing

Do I need a solar project to size a battery?

No. A battery can be sized from the load curve alone, for peak shaving or time-of-use shifting, without an associated photovoltaic installation. Pairing it with PV does, however, change the optimal power and capacity.

How long does it take to obtain a site's load curve?

The timeline depends on the channel used: an online operator portal can return recent history quickly for a smart meter, while a formal request for a detailed multi-year history can take several weeks.

Does a second-life battery perform the same as a new one?

The usable capacity of a second-life cell depends on its SoH (State of Health), diagnosed cell by cell through the BMS. For stationary use, which is less demanding on instantaneous power than automotive use, this residual capacity remains usable over a guaranteed contractual duration.

Should sizing account for future network tariff increases?

Yes. In France, the TURPE network tariff is revised every August 1 and directly affects the economics of subscribed-capacity shaving and time-of-use shifting. An ROI model should account for its likely trajectory, not only its level at the time of the study.

Can a battery be resized after installation?

Some architectures allow additional capacity modules to be added later if the consumption profile evolves. This scalability should be planned for during the initial study, based on the site's expected growth or transformation.