When planning a solar system, the battery decision starts with what the system will be used for. Covering daytime consumption, using energy in the evening and running certain loads during an outage are different goals. The same equipment and capacity may not suit every goal.
So the first question should not be “how many kWh of battery should I buy?” but “which loads do I want to supply, at what times and for how long?” Once that is clear, the inputs for the capacity calculation also become clear.
How is energy used in a system without a battery?
In a grid-connected system without a battery, generation and consumption times are assessed together. Storage can make it possible to use part of the generated energy at a later time; on the other hand, it changes the system’s cost and operating structure. The value of storage depends on the purpose of use and the energy flow. U.S. Department of Energy — Solar energy and storage (opens in a new tab)
For example, a workshop working during the day and a business with high evening consumption may need different assessments even if their annual consumption is equal. Rather than comparing only annual bill totals, the working hours during the day should also be recorded.
Does a battery always mean power during an outage?
No. The ability to operate during an outage depends on the system being designed for that purpose. The battery, inverter, the arrangement for disconnecting from the grid and the circuits to be backed up are assessed together. Some systems supply only selected critical loads. Adding a battery does not by itself guarantee that the whole building will keep running. Australian Government energy.gov.au — Batteries and backup power (opens in a new tab)
Many standard grid-connected solar systems stop generating when the grid goes down. A suitable inverter and system arrangement are the essential parts of a backup function. A quotation should clearly state “which outputs work during an outage” and “how the changeover happens”. U.S. Department of Energy — Solar and resilience basics (opens in a new tab)
Why are kW and kWh assessed separately?
A battery’s energy capacity is expressed in kWh, and the power it can deliver at any moment in kW. A large energy capacity does not mean that loads of any size can run at the same time. Usable energy may also differ from nominal capacity. When comparing products, check under which conditions these values are given. Australian Government energy.gov.au — Battery capacity and power (opens in a new tab)
For example, supplying a constant 2 kW load for 4 hours requires 8 kWh on the load side. If we assume a usable depth of discharge of 0.80 and a battery-to-AC-load efficiency of 0.90:
Nominal energy = 8 / (0.80 × 0.90) ≈ 11.11 kWh
This is only an energy calculation under the stated assumptions. Temperature, ageing, initial state of charge, auxiliary consumption and the product’s power limits are assessed separately. If the manufacturer already states usable capacity, the same depth of discharge should not be deducted a second time.
How is a critical load list prepared?
For a preliminary assessment, a table can list the device name, operating power, whether it runs simultaneously with others and the target backup duration. Backing up communications equipment, essential lighting and control systems is a different scenario from backing up an entire production line.
Starting behaviour also matters for motor loads. Even if the energy calculation looks sufficient, the inverter’s instantaneous power capability may not be enough to start the load. For this reason, a sample kWh result should not be turned directly into a product order list.
Off-grid use is examined separately
In off-grid systems, annual yield alone is not enough. Low-generation periods, daily load distribution and the probability of the battery running empty are also important. PVGIS’s off-grid calculation approach compares generation and consumption over time and gives results such as unmet energy. European Commission JRC — Off-grid PV systems (opens in a new tab)
When comparing options with and without a battery, use the same consumption data, the same panel layout and clearly stated goals. Prepare your roof information and load list, and you can discuss your facility’s electrical needs with us.