Solar battery sizing and troubleshooting guide

Learning Zone · calculation and fault finding

Solar battery sizing and troubleshooting guide

Short runtime does not always mean a failed battery. It may be undersized, undercharged, cold or connected through poor cabling. Measure before replacing.

Daily energy — Add watt-hours, not nameplate watts.
Autonomy — Choose the sunless days to cover.
Peak current — Inverters and motors can demand a lot instantly.
Diagnosis — The battery is often accused; it is not always guilty.

1. Build the daily energy budget

Multiply each load's power by actual operating hours. For compressors and pumps, use measured energy or a realistic duty cycle.

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Load Calculation Daily energy
LED lighting 10 W × 5 h 50 Wh
Laptop 60 W × 4 h 240 Wh
Refrigerator Average 45 W × 24 h 1,080 Wh
Water pump 500 W × 0.5 h 250 Wh

2. Calculate nominal and usable capacity

Nominal capacity = daily Wh × autonomy days ÷ (DoD × efficiency)

Example: 1,200 Wh/day and two days' autonomy. Compare AGM planned at 50% DoD and 85% efficiency with LiFePO₄ at 90% and 92%. These are assumptions, not universal ratings.

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Chemistry DoD Efficiency Required nominal energy Approx. capacity
AGM / lead 50% 85% 5,647 Wh ≈ 470 Ah @ 12 V
LiFePO₄ 90% 92% 2,899 Wh ≈ 226 Ah @ 12.8 V
This still does not show whether the battery can supply peak power. Energy and current require separate checks.

3. Check inverter current

Low-voltage systems can draw very high current. Estimate using AC power, loaded battery voltage and inverter efficiency.

DC current ≈ AC power ÷ (battery voltage × efficiency)

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AC power System Efficiency DC current
1,500 W 12 V 90% ≈ 139 A
1,500 W 24 V 90% ≈ 69 A
1,500 W 48 V 90% ≈ 35 A

Troubleshooting: from symptom to measurement

Do not replace a battery after one reading. Record state of charge, temperature, load and measurement point.

On a phone, swipe the table horizontally to see every column.

Symptom Possible causes First measurement Next step
Voltage collapses under load Discharged/aged battery, cable or joint At terminals and inverter simultaneously Calculate drop; known-load test
Never reaches full charge Low PV, wrong thresholds, temperature, load Solar energy and charging stage Review logs and profile
BMS trips on surge Over-current, weak cell, cold Peak current and BMS alarm Reduce surge or resize
Series units diverge Different age/capacity, imbalance Each unit at the same time Test separately
Heat or odour Loose joint, overcharge, failure Temperature without exposed contact Isolate and call a technician

Safe test sequence

  1. Record model, age, chemistry, diagram and recent changes.
  2. Inspect enclosure, corrosion, terminals and cable without opening the pack.
  3. Measure rested voltage after an adequate period and record temperature.
  4. Apply a known permissible load; measure at battery and inverter.
  5. Check current, energy balance and BMS/controller alarms.
  6. If required, run a controlled capacity test following the manufacturer.
Stop immediately for swelling, leakage, unusual smell, smoke, hissing, rising temperature or discoloured cable. Never open a lithium pack or short a battery as a test.

How to interpret measurements

On a phone, swipe the table horizontally to see every column.

Measurement Useful for Limitation
Resting voltage First lead-acid indication Needs rest; weak for lithium
Loaded voltage Internal resistance and wiring Depends on current and temperature
Cable drop Connections and conductor size Measure under load
Coulomb counter Ah/Wh balance Can drift
BMS log Cells, alarms and temperatures Access varies
Capacity test Actual delivered energy Takes time and procedure

Margins calculators often forget

  • future load growth;
  • ageing and capacity loss;
  • consecutive poor-weather days;
  • motor/compressor surge;
  • temperature derating;
  • whether the PV array can recharge the bank after deep discharge.

Use the calculation to filter batteries

Energy, current, temperature and cycle duty matter more than price per Ah alone.

Browse all batteries

Frequently asked questions

Is a 12.2 V battery failed?

Not enough information: chemistry, rest, temperature and load all matter.

Why does the inverter stop with a charged battery?

Cable drop, surge current, BMS trip or a weak cell are possible. Measure during the fault.

Can I size from daily kWh alone?

No. Also check peak current, recharge power and temperature.

How is capacity tested?

By controlled discharge within limits while measuring Wh to the defined endpoint.

Is more autonomy always better?

No. An oversized bank can remain chronically undercharged if the PV cannot refill it.

Related guides

Complete battery guide · MPPT sizing

Technical sources

Educational examples; manufacturer limits and system engineering prevail.