Size your battery bank quickly for reliable nights. Include autonomy voltage and depth limits safely. Get clear battery counts and exportable reports today instantly.
| Scenario | Daily Wh | Autonomy | System V | DoD | Battery Unit | Suggested Bank |
|---|---|---|---|---|---|---|
| Lights + fans + router | 2,500 | 2 days | 24 V | 50% | 12 V, 200 Ah | 2S × 2P (4 batteries) |
| Small office backup | 4,200 | 1.5 days | 48 V | 80% | 12 V, 100 Ah | 4S × 2P (8 batteries) |
| Fridge + essentials | 3,300 | 3 days | 24 V | 60% | 12 V, 220 Ah | 2S × 4P (8 batteries) |
The calculator estimates the nominal stored energy needed so that the usable energy meets your autonomy target after limits and real‑world losses.
DoD, ηRT (round‑trip efficiency), Temp (temperature factor), and DC (wiring/controller factor) are entered as percentages.
Battery sizing starts with a defensible daily energy number. Convert every appliance to watt-hours by multiplying watts by run time, then add 10–20% for behavior drift. A 2500 Wh/day load over 2 autonomy days creates a 5000 Wh baseline, before any losses. If you reduce idle loads by 100 W for 8 hours, you save 800 Wh per day, which can remove an entire parallel string in small systems.
Autonomy is an insurance decision. One day often fits grid-tied backup; two to three days fits off-grid sites with cloudy spells. Each additional day scales energy linearly, so going from 2 to 3 days increases required storage by 50%. In monsoon or winter fog regions, autonomy paired with conservative derating reduces deep cycling, which protects lifetime performance and improves morning voltage stability.
Depth of discharge drives both usable energy and cycle stress. A 50% limit roughly doubles the nominal capacity needed versus a 100% limit. Many lead-acid designs target 50% to extend service life; many lithium designs target 80–90% for higher usable capacity. If your required nominal energy is 10 kWh at 50% DoD, increasing DoD to 80% can drop nominal energy to about 6.25 kWh, all else equal.
Round-trip efficiency captures conversion and storage losses. Add a DC factor to reflect cable length, connection quality, and controller behavior. Small systems can lose more than expected with undersized wiring, raising heat and voltage drop. Improving wiring from 90% to 96% reduces required nominal storage by about 6.25%, which is significant when batteries are purchased in whole units.
Cold reduces available capacity for many chemistries, and heat can reduce effective life. A 90% temperature factor is a practical planning value for mild climates; harsher climates may warrant 80–85%. Derating prevents the “looks fine on paper” issue where the bank meets targets only on warm afternoons. Pair seasonal derating with margin to keep discharge currents moderate during long nights.
Higher voltage reduces current for the same power, enabling smaller conductors and lower losses. The calculator converts required amp-hours at the selected bus voltage, then proposes series strings to reach that voltage and parallel strings to reach capacity. For example, a 24 V bus with 12 V batteries uses 2 in series; if the required capacity is 360 Ah using 200 Ah units, it becomes 2S × 2P, or four batteries total, plus margin-driven headroom.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.