Calculator Inputs
Example Data Table
| Load | Watts | Qty | Hours/Day | Daily Wh |
|---|---|---|---|---|
| LED Lights | 10 | 6 | 6 | 360 |
| DC Fridge | 120 | 1 | 10 | 1200 |
| Laptop | 60 | 1 | 5 | 300 |
| Router | 15 | 1 | 24 | 360 |
| Water Pump | 250 | 1 | 0.8 | 200 |
| Total Daily Energy | 2420 Wh/day | |||
A 48 V system with 2 autonomy days, 80% depth of discharge, and 12 V 200 Ah batteries would typically need four batteries in series.
Formula Used
1) Daily load energy
Daily Load (Wh/day) = Σ (Watts × Quantity × Hours per Day)
2) Required nominal battery storage
Required Storage (Wh) = [Daily Load × Autonomy Days × Safety Factor] ÷ [Depth of Discharge × Inverter Efficiency × Battery Efficiency × Temperature Derating]
3) Required bank amp-hours
Required Capacity (Ah) = Required Storage (Wh) ÷ System Voltage (V)
4) Series batteries
Series Count = Ceiling(System Voltage ÷ Battery Unit Voltage)
5) Parallel strings
Parallel Count = Ceiling(Required Capacity Ah ÷ Battery Unit Ah)
6) Total batteries
Total Batteries = Series Count × Parallel Count
7) Suggested solar array
Suggested Array (W) = Daily Load (Wh/day) ÷ [Peak Sun Hours × Solar Derating]
Safety Factor = 1 + Safety Margin ÷ 100. Efficiencies and depth of discharge must be entered as percentages, then converted into fractions inside the calculation.
How to Use This Calculator
- Choose the DC system voltage for your off-grid setup.
- Enter the number of autonomy days you want during poor solar weather.
- Select a battery chemistry preset or manually enter your preferred discharge and efficiency values.
- Set the battery unit voltage and unit amp-hour capacity for the battery model you plan to buy.
- Enter your location’s peak sun hours and a realistic solar derating factor.
- Add every appliance in the daily load table with watts, quantity, hours, and surge factor.
- Press Calculate Battery Bank to view the result summary above the form.
- Use the CSV or PDF buttons to export the load list and sizing summary.
FAQs
1. What does autonomy mean in battery sizing?
Autonomy is the number of days your battery bank should power loads without enough solar charging. Higher autonomy improves resilience, but it increases battery size, weight, and cost.
2. Why is depth of discharge important?
Depth of discharge controls how much stored energy you plan to use. A lower allowed discharge protects battery life, while a higher value reduces required bank size but may shorten service life.
3. Why include inverter efficiency?
Most AC loads pass through an inverter, and that conversion wastes some energy. Including inverter efficiency prevents undersizing the bank when real delivered energy is lower than nominal stored energy.
4. What does temperature derating do?
Cold conditions often reduce available battery capacity. Temperature derating applies a conservative factor so your bank still meets demand when the environment lowers usable energy.
5. Should I size for surge watts too?
Yes. Motors, pumps, and compressors can draw much more power during startup. Surge demand mainly affects inverter selection, but it also helps you understand whether the battery bank can support short bursts safely.
6. How accurate is the suggested solar array size?
It is a practical planning estimate. Real designs should also consider panel orientation, seasonal sun variation, controller limits, cable losses, shading, and whether you want same-day recharge after deep discharge.
7. Why are batteries arranged in series and parallel?
Series connection increases voltage to match the system bus. Parallel connection increases amp-hour capacity. Off-grid banks usually need both to meet voltage and storage targets at the same time.
8. Can I use this for lithium and lead-acid batteries?
Yes. The preset helps with typical discharge and efficiency assumptions. Still, you should confirm exact limits from the battery datasheet, BMS settings, and manufacturer charging recommendations before final purchase.