smart solar and power helps Bangladeshi customers turn sunshine into reliable electricity. One question that pops up every time a system is sized is: how many hours will the battery keep the lights on when the grid disappears? This guide walks you through the exact math, the variables that matter, and the common pitfalls to avoid.
Why Backup Time Matters for Every Solar Owner
Knowing the backup duration lets you match the battery bank to your daily load, avoid unexpected blackouts, and protect expensive equipment. It also lets you compare lithium, lead‑acid, or tubular options on a level playing field.
Key Variables That Influence Backup Time
Battery Capacity (Ah or Wh)
Capacity is the amount of energy a battery can store. Manufacturers list it in amp‑hours (Ah) at a nominal voltage (usually 12 V, 24 V, or 48 V). To get watt‑hours (Wh), multiply Ah by the nominal voltage:
- Wh = Ah × V
Example: A 200 Ah, 12 V battery holds 2,400 Wh.
Inverter Load (W)
The inverter converts DC from the battery to AC for household appliances. Add up the power rating of every device you expect to run simultaneously. Use the name‑plate wattage or, for motors, the running wattage.
Depth of Discharge (DoD)
DoD is the percentage of the battery you are willing to use before recharging. Lead‑acid batteries typically stay above 50 % DoD, while lithium can safely go to 80‑90 %.
System Efficiency
Inverter and wiring losses reduce usable energy. A good inverter operates at 90‑95 % efficiency; add another 2‑3 % for wiring.
Step‑by‑Step Backup Time Formula
Follow these steps for a reliable estimate:
- Calculate total usable energy:
Usable Wh = Battery Wh × DoD × Inverter Efficiency - Sum the expected load in watts (W).
- Divide usable energy by load:
Backup Hours = Usable Wh ÷ Load W
All numbers must be in the same unit system (watts and watt‑hours).
Real‑World Example: Typical Bangladeshi Home
Assume a family uses 1,200 Wh per night (lights, fans, TV, refrigerator). They plan a 48 V lithium bank with four 100 Ah modules.
- Battery Wh = 48 V × 400 Ah = 19,200 Wh
- DoD = 85 % (lithium)
- Inverter Efficiency = 94 %
Usable Wh = 19,200 × 0.85 × 0.94 ≈ 15,340 Wh
Backup Hours = 15,340 ÷ 1,200 ≈ 12.8 hours. The system can comfortably run the night‑time load with a safety margin.
Adjusting for Seasonal Temperature Changes
Battery capacity drops in high temperature. A rule of thumb for lead‑acid is a 5 % loss for every 10 °C above 25 °C. Lithium chemistry is more tolerant but still loses about 2‑3 %.
In Dhaka summer (≈35 °C), a lead‑acid bank might deliver only 80 % of its rated capacity. Re‑calculate using the reduced Wh value.
Comparing Lithium vs. Lead‑Acid for Backup Time
Both chemistry types store energy, but lithium offers higher DoD, lower weight, and better efficiency. The math shows that a lithium bank of the same Ah rating will deliver roughly 30‑40 % more usable hours.
Safety and Wiring Considerations
- Size cables to handle the maximum current (I = Power ÷ Voltage) with a 20‑25 % safety margin.
- Install a proper fuse or circuit breaker at the battery terminal.
- Use a battery management system (BMS) for lithium packs to prevent over‑discharge.
Optimizing Your System for Longer Backup
Three practical ways to stretch hours without buying a bigger bank:
- Reduce load: Replace incandescent bulbs with LEDs, use energy‑efficient fans.
- Increase inverter efficiency: Choose a modern MPPT‑enabled inverter.
- Raise DoD safely: If you have lithium, set the BMS to 90 % DoD after confirming warranty terms.
Choosing the Right Battery from smart solar and power
smart solar and power supplies both lithium and tubular lead‑acid options. Their engineering team runs a free site survey, recommends the optimal Ah rating, and handles installation. Visit explore our high-quality solar products to see the full lineup.
Cost Implications and Return on Investment
While lithium costs 2‑3× more upfront, the higher usable energy and longer life (8‑10 years) often lower the levelized cost of storage. Calculate the payback period by dividing the extra upfront cost by the annual savings from reduced diesel or grid electricity.
Maintenance Tips to Keep Backup Time Consistent
- Check terminal tightness every 6 months.
- Balance charge lead‑acid banks monthly.
- Keep lithium BMS firmware updated.
- Store batteries in a cool, dry place when not in use.
Frequently Overlooked Factors
Solar charge controller sizing does not affect backup time directly, but an undersized controller can limit the depth of charge, reducing usable energy. Always match controller current to the array’s short‑circuit current plus a 25 % margin.
Putting It All Together: Quick Checklist
- List all critical loads (W).
- Choose battery chemistry and nominal voltage.
- Calculate total Ah and convert to Wh.
- Apply DoD and inverter efficiency.
- Divide usable Wh by load to get hours.
- Validate with temperature derating.
Follow this checklist for every new installation, and you’ll never be surprised by a short‑lived battery.
Get a Free Consultation
Ready to size your battery bank? get a free solar consultation from smart solar and power today. Our engineers will run the numbers, suggest the best product mix, and ensure a smooth handover.
Related reading: Smart Solar and Power Guide to Bifacial Solar Panels: Double‑Sided Power in Bangladesh
Frequently Asked Questions
How do I determine the correct battery capacity for my home?
Add up the nightly watt‑hour demand of all essential appliances, apply the desired depth of discharge and inverter efficiency, then choose a battery bank whose usable Wh meets or exceeds that figure.
Can I use a lead‑acid battery for backup in hot Bangladeshi summers?
Yes, but expect a 15‑20 % capacity loss at 35 °C. Size the bank accordingly or consider lithium, which retains more capacity in high heat.
What is the typical depth of discharge for lithium batteries?
Lithium packs safely operate at 80‑90 % DoD, giving you more usable energy per amp‑hour compared with lead‑acid, which is limited to about 50 %.
How does inverter efficiency affect backup time?
Higher inverter efficiency means less energy lost during conversion. A 95 % inverter provides more usable Wh than a 90 % unit, directly extending backup hours.
Do I need a battery management system for lithium batteries?
A BMS is essential for lithium packs; it monitors cell voltage, temperature, and prevents over‑charge or over‑discharge, protecting both performance and warranty.