Battery capacity in kilowatt-hours answers only one question:
How long can the bank supply energy?
It does not automatically answer:
Can the bank deliver enough current for the inverter right now?
That distinction is the core of reliable LiFePO4 sizing.
This flagship example uses the Pytes E-BOX 48100R as a concrete rack-battery reference. Pytes publishes 51.2V nominal voltage, 100Ah capacity, 5.12kWh nominal energy, 100A maximum charge/discharge current, and 102A peak current for 15 seconds in its manual family.
1. Four constraints, not one
For a modular rack-battery system, calculate these separately:
- energy requirement — nominal kWh needed for the desired autonomy
- continuous-current requirement — sustained inverter DC current
- surge-current requirement — short-duration current during motor/inverter surge
- configuration requirement — manufacturer limits for parallel quantity, busbar, communications, protection, and cabling
The final minimum pack count is:
Then verify that the manufacturer permits that configuration.
2. Worked example A — 6kWh/day, two days of autonomy
Assume:
- daily AC energy = 6,000Wh
- autonomy = 2 days
- planned usable battery fraction = 80%
- inverter efficiency = 92%
- rack module = 5.12kWh nominal
Required nominal bank energy:
Pack count from energy:
So the energy constraint requires four packs.
Four modules provide 20.48kWh nominal. Under the stated 80% usable and 92% inverter-efficiency assumptions, modeled usable AC energy is:
At 6kWh/day, that is about 2.51 modeled days before considering other derating or reserve decisions.
3. Continuous-power check
Now test a 6,000W inverter load at a conservative 47.5V battery voltage, the low end of the published Pytes operating range.
One module’s published maximum continuous charge/discharge current is 100A, so one pack is insufficient for this example.
Two packs provide a simple summed rating of 200A before considering current-sharing and installation details.
Therefore:
- packs required for energy = 4
- packs required for continuous current = 2
Energy is still the controlling constraint.
4. Surge check
Assume the inverter/load combination can demand 9,000W for several seconds.
At the same 47.5V and 92% conversion-efficiency assumption:
The reference battery publishes 102A peak for 15 seconds.
Simple pack-count screening:
So:
- energy = 4 packs
- continuous = 2 packs
- surge = 3 packs
Final planning minimum:
5. Why current does not divide perfectly by arithmetic alone
It is tempting to assume four parallel 100A batteries always produce exactly 25% of the load current each.
Real sharing depends on:
- cable resistance and length
- connection topology
- terminal/contact resistance
- state of charge
- battery temperature
- BMS limits and control behavior
- communications with the inverter
- manufacturer-required busbar arrangement
Use the manufacturer’s parallel-installation instructions. Pytes specifically documents parallel groups and notes busbar use for higher-current projects.
6. Worked example B — small inverter, long autonomy
Now assume:
- daily AC energy = 1.5kWh
- autonomy = 5 days
- 80% planned usable fraction
- 92% inverter efficiency
- 1.2kW maximum AC load
Energy requirement:
That requires:
Continuous current at 1.2kW and 47.5V:
One module can satisfy the power requirement, but the energy requirement forces two packs.
This is the opposite of a high-power, short-runtime system where current can become the controlling constraint.
7. C-rate is useful, but the BMS limit is more actionable
C-rate describes current relative to capacity:
For a 100Ah module:
- 50A = 0.5C
- 100A = 1C
But for a finished rack battery, use the manufacturer’s actual continuous and peak current limits rather than assuming a generic cell-level C-rate applies to the complete pack.
8. Cycle life must keep its test conditions attached
Do not use a universal table claiming that every LiFePO4 cell will deliver a fixed number of cycles at a given state-of-charge window.
Pytes publishes ≥6000 cycles at 0.5C charge / 0.5C discharge at 25°C for the reference manual. That number belongs with those test conditions.
Different:
- cells
- temperatures
- charge voltages
- discharge limits
- C-rates
- storage states
- BMS behavior
produce different aging results.
IEEE 1679.1-2025 is useful for understanding characterization, aging, failure modes, safety, and evaluation of lithium-based stationary batteries. The standard explicitly says sizing techniques are outside its scope, except where they affect evaluation. It should therefore not be presented as the authority for the sizing formula used above.
9. Final battery-bank worksheet
Before selecting pack count, record:
| Input | Why it matters |
|---|---|
| Daily Wh | Energy requirement |
| Autonomy days | Storage duration |
| Planned usable fraction | Reserve / operating window assumption |
| Inverter efficiency | Battery-side energy/current |
| Lowest battery voltage | Conservative current calculation |
| Inverter continuous watts | Sustained current |
| Inverter surge watts + duration | Peak current |
| Pack nominal kWh | Energy per module |
| Pack max continuous current | Power limit |
| Pack peak current + duration | Surge limit |
| Manufacturer parallel rules | Configuration limit |
Then calculate energy, continuous current, and surge independently.
Use the Battery Runtime Calculator to explore alternative loads and bank sizes, but keep manufacturer current limits as a separate design check.
Explore this decision in an appliance-based plan
Use the Build My Off-Grid System calculator to adjust your own appliances and compare daily solar balance, battery reserve and optional reserve recovery. The preset values are illustrative, not tested equipment specifications or local weather data. The tool is a planning screen, not an electrical compliance check.