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An energy storage system’s power in kW indicates the load it can support at a given moment. Its energy capacity in kWh describes the stored energy. Estimating how long a system can support a business requires both figures, a load profile and the conditions under which the manufacturer specifies those figures. A rating of 100 kWh does not, by itself, mean the system can supply a 100 kW load.
Below we will analyze the sequence of the preliminary calculation: from the list of consumers to the verification of the offer. The examples are illustrative and help prepare input data for design. If you're just getting started, first check out the explanation of ESS design and operation: the battery, converter and controls perform different tasks.
kW and kWh: what is the difference
A kilowatt is a unit of power, and a kilowatt-hour is a unit of energy. A constant load of 10 kW for two hours consumes 20 kWh. The same amount of energy may be required by a 20 kW load in one hour, but the output power requirements of the storage will be different.
| Indicator | Unit | Practical question |
|---|---|---|
| Power | kW | Which consumers can work simultaneously? |
| Energy reserve | kWh | How much energy can be used in the agreed mode? |
| Duration | h | For how long will this reserve be enough for a given load? |
For a constant load, the basic relationship is simple: energy = power × time. For a variable load, each interval is counted separately, then the results are added. The values should refer to the same measurement point, such as the energy delivered to loads at the AC output.
Minutes are converted to hours before multiplication: 15 minutes equals 0.25 hours. Therefore, 8 kW for this interval corresponds to 2 kWh.
Solar plant capacity, the storage converter’s output and the business load cannot be treated as equal. Solar generation varies over time, while storage operation depends on the battery, power electronics and control settings.
When the useful energy is already determined specifically for consumers, the time under a steady load is estimated by dividing: time = available energy ÷ load power. An illustrative 60 kWh provides 30 kW for two hours or 15 kW for four. Such a comparison is valid only for the accepted power limits and losses taken into account. It shows why the answer "how many hours is enough" always needs a description of the load.
Start with a profile of the required loads
First determine what must keep operating: all production lines or only specified processes. For the warehouse, it can be lighting, communication and separate mechanisms, for production, a different list is needed. Record the actual electrical power and operating time of each consumer. Motor nameplate data and a monthly electricity bill do not replace measurements of actual operation.
Identify equipment that starts simultaneously, runs in cycles or can have its start delayed. This allows you to see two different needs: maximum simultaneous power and energy over the entire period. For a preliminary assessment, hourly or more detailed accounting data, supplemented with information on short starts, are useful.
Prepare a short input schedule for the designer. Label every value as a measurement, a business requirement or an assumption still requiring verification.
| Input | What to provide | What to check |
|---|---|---|
| Required processes | Equipment list, priority and permitted interruption | Which loads belong to the backup group |
| Consumption profile | Interval power or energy, dates, interval duration and time zone | Metering boundary, gaps and differences between operating schedules |
| Short starting events | Current, duration and starting sequence from measurements or documentation | Events hidden by an averaged profile |
| Backup period | Required duration, initial charge and agreed load-shedding priorities | Whether the assumed charge is achievable before each event |
| Recharging | Source, available hours and power after ongoing consumption | Supply limits, source compatibility and the next cycle |
- 1. Select processesList of consumers, their priority and permissible interruption.
- 2. PlotPower of each interval, duration of operation and simultaneity.
- 3. Check the systemEnergy reserve, power output, start and recharge.
If backup power is part of a new solar power plant project, these data should be agreed at the design stage of a solar power plant. Thus, the accumulation calculation will be related to real generation, connection and future consumption.
Nominal capacity and available energy
The specification may indicate the nominal energy of the battery or the useful energy of the system. Check whether the reading is on the DC side of the battery or the AC output, and what losses are already taken into account. Otherwise, two similar numbers will describe different possibilities.
State of charge, or SOC, shows the current charge level. Depth of discharge, or DoD, characterizes the fraction of capacity used. For a specific reserve period, it is important to know the initial charge and the lower permissible limit. Starting at 90% and ending at 20% leaves a working window of 70 percentage points if both limits are set from the same base.
Added to the loads are the system's auxiliary consumption, such as control and cooling. They are taken into account once: either they are already included in the declared useful energy, or they are subtracted separately. Likewise, there is no need to re-apply the discharge depth limit to a value that the manufacturer has already defined as useful in this window.
Check the calculation limit. Round-trip efficiency accounts for both charging and discharging. To estimate energy delivered from an already charged battery, use the losses for the relevant discharge path. These are different values.
Illustrative three-hour example
Consider a facility with three specified groups of loads. Lighting and workstations consume 12 kW for three hours. An 8 kW pump runs for a total of one and a half hours. Automation and communication consume 2 kW throughout the period. All values are illustrative, not measurements of an ONG project.
- Lighting and workstations: 12 × 3 = 36 kWh.
- Pump: 8 × 1.5 = 12 kWh.
- Automation and communication: 2 × 3 = 6 kWh.
- Total consumers need: 54 kWh.
For the training calculation, we will assume a nominal battery energy of 100 kWh, an initial charge of 90%, a lower limit of 20% and an efficiency of the discharge path of 94%. Additional auxiliary consumption on the AC side is assumed to be 0.6 kW for three hours. These assumptions are not specific to any Sungrow model.
For simplicity, we consider the energy reserve to be proportional to the SOC. The actual dependence is specified by the characteristics of the battery and its control data.
Available to consumers: 100 × (0.90 − 0.20) × 0.94 − 0.6 × 3 = 64 kWh
Compared to the need of 54 kWh, 10 kWh remain. This is only the result of the selected scenario. It does not take into account the aging of the battery, a different initial charge or a change in pump mode, so it is not a guaranteed reserve for the entire operating life.
Now let's change only the initial charge from 90% to 60%. With the same lower limit and other assumptions, the available energy for consumers in three hours will be 100 × (0.60 − 0.20) × 0.94 − 1.8 = 35.8 kWh. 18.2 kWh will be missing from the need of 54 kWh. This means that it will not be possible to ensure the entire given schedule: a different initial charge, a different configuration or an agreed change to the operating plan is required.
According to the same assumptions, the arithmetic demand for nominal energy is equal to (54 + 1.8) ÷ (0.70 × 0.94), that is, approximately 84.8 kWh. It is too early to choose equipment based on this number alone: we still need to check the power, operating conditions and a separate agreed reserve for changes.
Why stored energy alone is not enough for starting loads
In our example, the simultaneous power of three groups is 22 kW, and together with the assumed auxiliary consumption equals 22.6 kW. The converter must provide this mode under actual conditions. At the same time, starting the pump may require a short-term current that is not visible in the kWh calculation.
For motors, the starting method, current, overload duration and permissible voltage sag are checked. The values of active power in kW and total power in kVA are not interchangeable. Power factor, phasing and load unevenness data are required for compatibility testing.
Also find out if the stated power is available at the desired temperature and charge level. An increase in the number of batteries does not mean an automatic increase in the capabilities of the power converter. The limitations of the entire configuration remain decisive, not the largest number in its description.
Working during power outages requires a suitable mode and switching scheme. A look at hybrid solution for a business will help move from the arithmetic to the discussion of a compatible system. Stored energy alone does not establish that the system can provide backup power.
When the system has time to charge again
After determining the reserve, check the interval for recharging. It is important not only to have enough battery before the first outage, but also to prepare it for the next one. The available charging power depends on the source, connection limit, parallel consumption of the enterprise and equipment capabilities.
For example, returning 60 kWh to the battery with a constant available power of 20 kW at its input would require three hours in an idealized calculation without losses. Actual schedule may be longer due to charging losses and limitations. If the specified 20 kW is measured at the input of the system from the network, the result must be recalculated taking into account the corresponding losses.
Do not assume the full rated capacity of a solar power plant is available for charging. Initially, part of the generation may go to current consumers. For the solar source, a time schedule of the remaining energy is required, and for the generator, compatibility and permissible operating mode are additionally checked.
What to ask for in an equipment proposal
Compare configurations under the same conditions. In the Sungrow PowerStack 255CS-2H and Sungrow PowerStack 510CS-4H cards, you can view the parameters of the corresponding models. The duration in a product name does not replace a calculation for your load profile or guarantee the same backup time at every facility.
- Loads: agreed consumers, simultaneous power, starts and required time.
- Energy limits: nominal or useful value, measurement side, initial charge and lower limit.
- Discharge conditions: available power, temperature, permissible overloads and auxiliary consumption.
- Recharge: source, available time and charging capacity.
- Future needs: agreed consideration of degradation, new loads and warranty conditions.
Ask to show which data is measured, which is taken from documentation, and which is still guesswork. Such a list makes offers comparable and shows what needs to be clarified before ordering. For a preliminary consultation, prepare a load schedule, a list of equipment and the desired duration of work.
Also record the date and mode of measurements. Weekend data does not describe a busy production shift, and the summer schedule does not always reflect winter demand. For each significantly different mode, it is worth checking a separate scenario, and not hiding the difference in one average value.