Introduction
Selecting a LiFePO4 battery capacity is only half of an energy storage design.
The next question is:
How will you recharge it?
A customer may purchase:
- 10kWh
- 20kWh
- 50kWh
- 100kWh
of battery storage without checking whether the available solar array, grid connection or inverter charger can restore that energy within the required time.
The result can be a technically functional system that charges far too slowly.
For example, installing 50kWh of battery storage does not automatically mean the customer has a 50kW charging system.
Battery capacity is measured in kWh.
Charging power is measured in kW.
Understanding the difference is essential for correctly designing LiFePO4 energy storage systems.
kW and kWh: The Most Important Difference
Consider a 51.2V 200Ah LiFePO4 battery.
Nominal energy:
51.2V × 200Ah = 10.24kWh
This tells us approximately how much energy the battery stores.
It does not tell us how quickly it can be charged.
If the battery receives approximately:
2kW charging power
a large part of the battery may require several hours to recharge.
If it receives:
5kW charging power
charging can be substantially faster.
Therefore:
kWh = energy capacity
kW = charging or discharging power
Both must be specified in an ESS project.
Start with the Energy That Actually Needs to Be Replaced
A battery is rarely charged from exactly 0% to 100%.
Suppose a 20kWh system begins charging at 20% SOC and stops at 90%.
SOC increase:
90% − 20% = 70%
Energy to be restored:
20kWh × 70% = 14kWh
This is the starting point for estimating charging time.
However, the charging source must supply somewhat more energy because:
- Inverter conversion is not 100% efficient
- Cables have losses
- Battery charging is not perfectly lossless
- Loads may operate during charging
Therefore, real input energy will be higher than the nominal energy added to the battery.
Basic Charging-Time Formula
A simple first estimate is:
Charging Time ≈ Energy Required ÷ Effective Charging Power
For example:
Energy required:
14kWh
Effective battery charging power:
3.5kW
Estimated charging time:
14 ÷ 3.5 = 4 hours
This provides a useful starting point.
Actual time may be longer because charging power can change during the charging process.
Example 1: 51.2V 100Ah Battery
Nominal energy:
51.2V × 100Ah = 5.12kWh
Suppose the battery is charged at 20A.
Approximate nominal charging power:
51.2V × 20A ≈ 1.02kW
If charging from 20% to 90%, approximately 70% of capacity must be restored:
5.12 × 70% = 3.58kWh
Ignoring losses and charge taper:
3.58 ÷ 1.02 ≈ 3.5 hours
This shows why charging current directly affects recovery time.
Example 2: 51.2V 314Ah Battery
A 51.2V 314Ah battery provides approximately:
51.2V × 314Ah = 16.08kWh
Suppose routine charging current is 60A.
Approximate nominal charging power:
51.2V × 60A ≈ 3.07kW
If SOC increases from 20% to 90%:
Required nominal energy:
16.08 × 70% ≈ 11.26kWh
Idealized charging time:
11.26 ÷ 3.07 ≈ 3.7 hours
In practice, allow additional time for losses and changing charging conditions.
Example 3: Two 51.2V 314Ah Batteries
Two batteries in parallel provide approximately:
16.08kWh × 2 = 32.16kWh
But installing two batteries does not automatically double inverter charger power.
If the inverter still charges at only 60A total, the bank will take approximately twice as long to recover compared with one battery.
This is a common expansion mistake.
Customers increase battery capacity but leave unchanged:
- Solar array
- Inverter charger
- Grid input
- Generator
The system then has excellent storage capacity but inadequate recharging capability.
The Battery BMS Is Only One Charging Limit
Suppose a battery has a 200A BMS.
A customer may assume:
“Then I should charge it at 200A.”
That conclusion is incorrect.
The BMS maximum current is an equipment limit, not necessarily the preferred daily operating current.
Actual charging power may be limited by:
- Cell recommended charging current
- BMS limit
- Battery cable
- Breaker
- Busbar
- Inverter charger
- MPPT controller
- PV power
- Grid connection
- Generator capacity
The lowest applicable limit determines actual system performance.
For stationary ESS projects, moderate routine charging rates are often sufficient.
How Much Solar Power Is Needed?
Imagine a 20kWh battery bank requires 14kWh to recharge.
A customer may assume:
“Then I only need 14kWh of solar production.”
But solar generation must also supply daytime loads.
Suppose:
- Battery needs 14kWh
- Daytime loads consume 10kWh
The PV system must provide at least:
24kWh of useful energy
before considering system losses.
If daily solar production is insufficient, the battery may never fully recover during poor weather.
This can result in progressively lower SOC over several days.
PV Size and Battery Size Should Be Designed Together
An oversized battery is not always an advantage.
Consider two systems.
System A
Battery: 15kWh
Daily PV production: 25kWh
Daily consumption: 15kWh
There may be enough surplus solar power to recharge the battery.
System B
Battery: 40kWh
Daily PV production: 18kWh
Daily consumption: 15kWh
Only a small amount of solar energy remains for battery charging.
Even though System B has a much larger battery, it may spend most of its life at partial charge.
Therefore, battery capacity should not be selected independently from solar generation.
Peak PV Power Is Not Daily Energy
A customer may have a 10kW solar array.
That does not mean the system generates:
10kW × 24 hours = 240kWh/day
Solar output changes according to:
- Time of day
- Weather
- Panel orientation
- Temperature
- Shading
- MPPT efficiency
- Location
A 10kW array may only produce near its rated power for a limited period.
For battery charging calculations, daily energy yield is often more useful than PV nameplate power alone.
Inverter Charging Limits Can Become the Bottleneck
Suppose a project has:
- 30kW PV array
- 50kWh battery bank
but the inverter allows only:
10kW battery charging
Even if the solar array produces 25kW, the battery cannot necessarily absorb all of it.
The remaining solar power must:
- Supply active loads
- Be exported to the grid
- Be curtailed
depending on the system design.
This is why commercial ESS projects should check the inverter’s:
- Maximum PV input
- Maximum battery charge power
- Maximum battery charge current
- Maximum combined AC + PV charging power
These values may all be different.
Multiple Inverters Require a System-Level Calculation
Large battery banks may use multiple inverter-chargers.
Suppose three inverters can each charge the battery at 5kW.
Potential total charging power:
5kW × 3 = 15kW
Before enabling all three chargers simultaneously, verify that the battery bank can accept the combined power.
Check:
- Total BMS charging limit
- Number of batteries
- Busbar rating
- Main breaker
- Cable capacity
- Communication architecture
Never evaluate one inverter in isolation when several units share the same battery bank.
Loads Reduce Net Charging Power
Suppose solar power is:
12kW
while building loads consume:
7kW
Only approximately:
5kW
remains available for battery charging.
If a water pump starts and loads increase to 10kW, battery charging may temporarily fall to only 2kW.
Therefore, customers often see charging power changing during the day.
This does not necessarily indicate an unstable battery.
The inverter is simply distributing available energy between:
- Loads
- Battery
- Grid
Why a Battery Can Charge Quickly in the Morning but Slowly Later
Several factors can reduce charging power later in the cycle.
Battery Approaches High SOC
The charging strategy may reduce current near the upper SOC range.
Cell Voltage Difference Increases
The BMS may limit charge current if one cell approaches its upper voltage threshold.
PV Power Falls
Afternoon sunlight decreases.
Loads Increase
Household or commercial energy consumption rises.
Battery Temperature Changes
The BMS or inverter may apply thermal limits.
Therefore, charging time should not always be calculated assuming maximum charging power from start to finish.
Designing for a Required Recovery Time
For commercial projects, a better approach is to begin with the customer’s requirement.
For example:
“The 50kWh battery must recharge from 20% to 90% within 5 hours.”
Required stored energy:
50 × 70% = 35kWh
Ideal minimum average battery charging power:
35 ÷ 5 = 7kW
After allowing for system losses and operating conditions, the charging source should be sized above this ideal minimum.
This requirement can then be checked against:
- Battery charging limit
- Inverter charger
- PV system
- Grid supply
- Generator
This is much better than selecting equipment independently.
Example: 50kWh Commercial ESS
Consider:
- Battery storage: 50kWh
- Starting SOC: 20%
- Target SOC: 90%
- Charging window: 4 hours
Required battery energy:
50 × 70% = 35kWh
Ideal average charging power:
35 ÷ 4 = 8.75kW
If the facility simultaneously consumes 15kW, the energy source needs to support both the facility and battery.
If charging from solar, PV generation during the charging window must cover:
Facility load + battery charging
This simple calculation can prevent serious system undersizing.
What Happens If Charging Power Is Too Small?
An undersized charging system may still operate.
However, customers may experience:
- Battery never reaching target SOC
- Insufficient nighttime energy
- Frequent generator starts
- Grid charging continuing for many hours
- Poor solar utilization
- Repeated low-SOC operation
- Reduced backup readiness
These are often described as “battery capacity problems” even though the battery itself is functioning normally.
The real problem is insufficient energy recovery.
Is Faster Charging Always Better?
No.
Larger chargers can:
- Increase equipment cost
- Require larger cables
- Require larger breakers
- Increase grid demand
- Require larger generators
- Increase thermal load
If a battery has 12 hours available to recharge, there may be little reason to design a system capable of completing charging in one hour.
Charging power should match the operational requirement.
The goal is not maximum possible charging speed.
The goal is sufficient charging speed with good system efficiency and reliability.
Information Required for Charging-System Design
For B2B LiFePO4 energy storage projects, distributors and EPC contractors should provide:
- Battery voltage
- Required battery capacity
- Number of modules
- Starting SOC
- Target SOC
- Required charging time
- PV array size
- Average daytime load
- Peak load
- Inverter model
- Grid availability
- Generator availability
From this information, the supplier can assess whether the proposed system has enough charging capability.
A Simple Project Calculation
Assume:
Battery capacity: 32kWh
Starting SOC: 25%
Target SOC: 90%
Required SOC increase: 65%
Energy to restore: 32 × 65% = 20.8kWh
Required charging window: 5 hours
Ideal average battery charging power: 20.8 ÷ 5 = 4.16kW
If daytime loads simultaneously consume 6kW, the system needs substantially more than 4.16kW from the charging source.
This type of calculation should be completed before finalizing the battery and inverter quotation.
For Distributors: Sell a Charging Solution, Not Only a Battery
A customer asking for a 50kWh battery may not actually understand the complete system.
Instead of immediately quoting battery price, ask:
- What inverter will be used?
- How many solar panels are installed?
- What is the PV capacity?
- How many hours of sunlight are available?
- What is the daily energy consumption?
- Is grid power available?
- How quickly must the battery recover?
- Is a generator used?
These questions help distributors avoid later complaints such as:
“Your battery charges too slowly.”
In many cases, the charging speed was determined by the surrounding system from the beginning.
Conclusion
A LiFePO4 energy storage system should be designed around both:
Storage capacity and Energy recovery capability.
A large battery bank is only useful if there is enough charging power to restore the energy that is consumed.
When calculating charging requirements, consider:
- Battery capacity
- SOC operating range
- Required charging time
- Inverter limits
- BMS limits
- PV production
- Grid power
- Generator power
- Simultaneous loads
- Conversion losses
For residential projects, this prevents slow overnight recovery.
For commercial and off-grid projects, it prevents an even bigger problem: installing expensive battery capacity that cannot be fully utilized.
HIZN manufactures LiFePO4 energy storage batteries including rack-mounted, wall-mounted and floor-standing solutions for residential, commercial, telecom and off-grid applications.
Send us your required battery capacity, inverter model, PV size, daily consumption and target charging time. We can help calculate a more practical charging configuration for your project.