Battery Bank Input Form
Use the form below to size an off-grid battery bank with autonomy, storage losses, charge limits, and future load growth included.
Example Data Table
This sample appliance audit shows one practical way to estimate daily energy before sizing the battery bank.
| Appliance | Watts | Hours/Day | Quantity | Daily Wh |
|---|---|---|---|---|
| LED lighting | 15 | 6 | 8 | 720 |
| Ceiling fans | 60 | 10 | 3 | 1,800 |
| Refrigerator | 150 | 8 | 1 | 1,200 |
| Laptop computers | 65 | 6 | 2 | 780 |
| Water pump | 750 | 1 | 1 | 750 |
| Router and networking | 25 | 24 | 1 | 600 |
| Total | 5,850 | |||
Formula Used
Adjusted Daily Load = Daily Load × (1 + Safety Margin) × (1 + Future Expansion)
Design Daily Load = Adjusted Daily Load × (1 + Seasonal Adjustment)
Required Nominal Wh = (Design Daily Load × Autonomy Days × (1 + Design Reserve)) ÷ (Inverter Efficiency × Battery Efficiency × Usable DoD × Temperature Derating)
Required Ah = Required Nominal Wh ÷ System Voltage
Series Batteries = Ceiling(System Voltage ÷ Battery Unit Voltage)
Parallel Strings = Ceiling(Required Ah ÷ Battery Unit Ah)
Total Batteries = Series Batteries × Parallel Strings
Delivered AC Wh = Installed Nominal Wh × Usable DoD × Battery Efficiency × Temperature Derating × Inverter Efficiency
Runtime Hours = Delivered AC Wh ÷ Connected Load
How to Use This Calculator
- Enter your total daily energy usage in watt-hours.
- Add the highest continuous load and the largest startup surge.
- Choose how many backup days you want without solar input.
- Select battery chemistry, then confirm DoD and efficiency assumptions.
- Set voltage, battery unit size, temperature derating, and reserve margin.
- Enter peak sun hours and controller efficiency for recharge guidance.
- Click the calculate button to view battery count, strings, runtime, and charging recommendations above the form.
FAQs
1. What autonomy days should I choose?
Use one to two days for mild conditions and dependable solar input. Use three to five days for remote sites, cloudy seasons, or mission-critical loads.
2. Why does depth of discharge matter so much?
Depth of discharge controls how much of the battery you can safely use. Lower usable depth means you need more total storage to deliver the same backup energy.
3. Why is temperature derating included?
Cold and extreme operating conditions reduce usable battery performance. Derating keeps the estimate practical instead of overly optimistic for winter or exposed installations.
4. Can I mix different battery sizes or chemistries?
Mixing battery types, ages, or capacities in one bank is usually a bad idea. Uneven charging and discharging can shorten life and reduce usable capacity.
5. Why are series and parallel strings both shown?
Series batteries build voltage. Parallel strings build amp-hours. You need both values to purchase the correct number of batteries and wire the bank properly.
6. Is the suggested PV recharge power exact?
No. It is a planning estimate based on your load, sun hours, and efficiencies. Final array sizing should also consider weather patterns, cable loss, and charging strategy.
7. Should I include future expansion now?
Yes, if you expect more appliances later. A modest expansion margin can prevent expensive battery reconfiguration or early replacement when demand grows.
8. Is lithium always the best choice?
Not always. Lithium often delivers better usable capacity and efficiency, but cost, climate, serviceability, safety preferences, and budget still affect the best choice.