Sizing an energy storage system for a PV installation comes down to three parameters that must be read together: usable capacity (kWh), which determines how much energy the battery holds; power (kW), which determines how fast it can deliver or absorb that energy; and battery voltage (low or high), which must match the inverter. Too much capacity paired with too little power, or a battery whose voltage does not match the inverter, are the most common reasons an install looks good on paper but fails to carry the load in practice.
What really drives battery sizing — kWh or kW?
The most common mistake at the sizing stage is looking only at capacity. The customer asks how many kWh, the installer gives a number, and the conversation ends there. But capacity and power answer two different questions. Capacity (kWh) tells you how much energy the battery can hold at all — how long it will power a home or facility. Power (kW) tells you how much of that energy can be drawn at any given moment — what can actually run at the same time.
Picture a workshop that fires up a compressor, an EV charger and the hall lighting on the same evening. A battery with large capacity but low output power has, in theory, hours of stored energy — but it will not release it fast enough for those loads to start simultaneously. The result is that, despite a large battery, the system falls back to the grid or reports an overload. This is why instantaneous power often matters more than capacity alone, especially with loads that have a high inrush current.
In practice, good sizing starts from the consumption profile: how much energy the site actually uses over a day (this sets the capacity) and what its peak instantaneous demand is (this sets the required battery and inverter power). Only with both figures in hand do you pick a specific product.
Low or high battery voltage — what does it change in the installation?
Energy storage systems fall roughly into low-voltage (typically around 48-51 V) and high-voltage (from around 100 V up to several hundred volts). This is not a detail for the electrician — it is a decision that determines which inverter the battery will work with at all, and how it behaves at higher power.
A low-voltage battery is simpler and often cheaper in smaller residential installs, but at the same power a higher current flows through it — meaning thicker cabling, higher losses and a practical scaling limit. A high-voltage battery runs at a lower current for the same power, which makes it better suited to higher power ratings and three-phase installations, but it requires an inverter designed for high voltage. The key rule: the battery voltage and the inverter voltage window must overlap. There is no eyeballing this — it either matches or the system will not start.
Voltage and the choice of manufacturer
In distribution practice this means the battery and inverter are best chosen as a pair from a manufacturer confirmed compatibility list. Some manufacturers design their own coherent inverter-battery ecosystems where compatibility is guaranteed. In the Photonica offer, batteries and inverters from brands such as Deye, SolaX, Huawei and SolarEdge come in exactly these matched sets — while Felicity and Sluxer broaden the choice on the battery side. The Photonica sales team helps select the right inverter-battery pair for a specific project.
How do you calculate the capacity the customer actually needs?
The starting point is daily energy consumption outside PV production hours — because a battery mostly makes sense when it stores the daytime surplus for the evening and night. If a household uses, say, 8-10 kWh in that window, a battery with usable capacity in that range will cover a typical evening without drawing from the grid. Sizing capacity beyond real night-time consumption rarely pays off — the battery then sits unused, and it is the most expensive part of the installation.
An important distinction: nominal capacity and usable capacity are not the same thing. Manufacturers state depth of discharge (DoD) — what percentage of nominal capacity can actually be used without shortening cell life. A battery with 10 kWh nominal capacity and 90% DoD delivers about 9 kWh usable. When comparing offers you therefore have to look at usable capacity, not just the biggest number on the datasheet.
The three sizing parameters — what they mean in practice
The summary below is a shortcut worth returning to on every sizing job. Under the table we come back to how these parameters play together.
| Parameter | Unit | What it drives | Common mistake |
|---|---|---|---|
| Usable capacity | kWh | How long the battery powers the site | Confusing it with nominal capacity (ignoring DoD) |
| Power | kW | How many devices run at once | Too little power with large capacity |
| Voltage | V (LV/HV) | Inverter compatibility and scaling | Battery outside the inverter voltage window |
This makes clear that the three parameters cannot be chosen in isolation. A battery sized only for capacity may fail to carry the power peak; one sized only for power may be over-specified and costly; and even the best capacity-and-power match is useless if the voltage does not fit the inverter. That is why sizing storage is always sizing a set, not a single product.
Why size the battery together with the inverter, not separately?
The inverter is the component that ties the installation together: it manages energy flow between the modules, the battery, the site and the grid. It decides which battery voltage it works with, what charge and discharge power it handles, and whether it provides backup power during a grid outage. Sizing the battery in isolation from the inverter is the most common source of problems — because even a good battery will not work if the inverter cannot see it or cannot handle its power.
For the installer and the trading company the practical takeaway is simple: the battery, inverter and modules are one system that has to install safely, run predictably and be serviceable for years. Start sizing from the customer consumption profile and finish on a confirmed inverter-battery pair — not the other way around.