Calculator inputs
Example data table
| Temp (°C) | Capacity factor | Power factor | Note |
|---|---|---|---|
| -10 | 0.7 | 0.6 | Cold start limitations |
| 0 | 0.82 | 0.75 | Reduced peak power |
| 10 | 0.9 | 0.86 | Moderate derating |
| 25 | 1 | 1 | Reference condition |
| 35 | 0.98 | 0.96 | Heat-limited sustained power |
Formula used
Derating factors
- Capacity factor = 1 − |T − Tref| × rcap (piecewise: different rates above/below).
- Power factor = 1 − |T − Tref| × rpwr (piecewise: different rates above/below).
- Below 0°C, an additional small penalty is applied, capped at 20%.
- Both factors are clamped to a minimum floor (min factor).
Energy, value, and NPV
- Nominal energy (kWh) = Ah × V ÷ 1000.
- Usable energy = nominal × DoD × efficiency × capacity factor.
- Annual value = usable energy × cycles/year × value per kWh.
- NPV discounts annual value and applies annual degradation.
How to use this calculator
- Select a chemistry preset that matches your battery type.
- Enter expected battery temperature and the reference temperature from specs.
- Fill in capacity, voltage, DoD, and efficiency to get usable kWh.
- Enter max charge/discharge currents to see derated kW limits.
- Add cycles and value per kWh to estimate annual impact and NPV.
- Use CSV/PDF exports to share results with stakeholders.
Temperature-to-factor mapping
At 25°C, the model sets both factors at 100%. Below the reference, a cold slope applies. For example, a 0.006 capacity rate means a 0.6% drop per °C. At 10°C with a 25°C reference, the 15°C gap implies about 9% capacity reduction before any sub‑zero penalty. Above the reference, a smaller warm slope is typical, reflecting moderate heat derating in sustained operation.
Usable kWh and duty-cycle impact
Nominal energy equals Ah×V÷1000. Usable kWh then multiplies by depth of discharge and round‑trip efficiency. With 200 Ah and 51.2 V, nominal energy is 10.24 kWh. At 80% DoD and 92% efficiency, reference usable energy is 7.54 kWh. If the capacity factor is 90%, temperature‑adjusted usable energy becomes 6.79 kWh per cycle.
Power derating and system sizing
Power limits are modeled by scaling kW with the power factor. A 200 A discharge limit at 51.2 V is 10.24 kW at reference. With an 86% power factor, the allowable discharge power becomes 8.81 kW. This gap is material for inverter selection, peak shaving, and fast‑charge targets, especially when cold starts coincide with high loads.
Annual value and loss framing
Annual energy throughput is usable kWh times cycles per year. At 300 cycles, 7.54 kWh yields about 2,262 kWh annually at reference. At 6.79 kWh, throughput is about 2,037 kWh. With a value of 0.18 per kWh, the modeled annual value difference is roughly 40.5, which helps quantify temperature risk in revenue or avoided‑cost cases.
NPV and thermal-control decision
The NPV view discounts annual value over the selected horizon and reduces it with an annual degradation rate. If temperature loss creates persistent cashflow drag, thermal control can be evaluated with a simple payback using annual loss. A 5°C improvement can recover several percent usable energy annually. Use this as a screening metric, then validate with manufacturer curves and site temperature profiles.
FAQs
1) What temperature should I enter?
Use the battery’s expected internal temperature during operation, not room air. For outdoor systems, consider sun exposure, enclosure heating, and discharge rate. If unsure, run multiple cases (cold morning, average, and peak heat) and compare outputs.
2) Why are there separate rates above and below the reference?
Cold conditions typically reduce available capacity and power faster than warm conditions. Separate slopes allow a steeper cold response and a gentler warm response, which better matches many datasheets for lithium and lead‑acid batteries.
3) Does the model replace manufacturer derating curves?
No. It is a planning approximation for early design and financial screening. If you have a published curve, adjust the rates and minimum factor to better match it, or use the curve directly for final engineering decisions.
4) How does the 0°C penalty work?
Below 0°C the calculator applies a small additional penalty to reflect reduced electrochemical kinetics and higher internal resistance. The extra penalty is capped, so the model stays conservative without collapsing outputs unrealistically at deep sub‑zero temperatures.
5) What does “value per kWh” represent?
It can represent electricity tariff savings, avoided generator fuel, demand‑charge reduction value, or market revenue. Use the same value for both reference and temperature cases so the difference isolates temperature derating rather than pricing assumptions.
6) When is thermal control economically justified?
If the annual value loss is large and persistent, compare it to thermal-control cost using the payback estimate. Also consider reliability, warranty conditions, and peak power needs. A long payback may still be acceptable for critical loads.