What you will see
- Backup ends sooner than the duration discussed at installation.
- The same battery lasts longer on some nights than others.
- Runtime falls after adding an appliance or changing business operations.
- Backup stops while a charge percentage is still displayed.
Why it happens
Energy capacity and power demand are different quantities. A useful starting calculation is: backup hours ≈ usable energy available to the appliances in kWh ÷ their average demand in kW. This assumes no recharge during the period and loads within the system’s power limits.
The usable energy for a particular outage may be less than the battery’s headline capacity. The starting charge and permitted discharge endpoint matter. Some displayed charge remains unavailable because the system must stop at its configured protective limit.
Conversion losses and battery condition also affect the result. Energy entering storage is not all returned as useful appliance energy, and capacity can change with use and age. The estimate should account for these effects rather than treating nameplate capacity as guaranteed delivered energy.
Also distinguish running out of usable energy from a shutdown caused by a fault or overload. Ask whether the estimate assumed daytime solar support; a night-time outage needs a different comparison.
Backup hours, roughly
Why usable energy is less than the headline
- 1Starting charge below full when the outage begins.
- 2Protective limit: some displayed charge is held back.
- 3Losses in conversion, and capacity change with age.
A shutdown from overload or a fault is not the same as running out of energy.
This guide is about backup that has slowly or suddenly become shorter than it used to be. If the battery will not charge at all, see solar battery not charging or draining too fast, which covers charging problems in detail.
What "battery backup time reduced" really means
Backup time is simple arithmetic: usable energy in the battery divided by the load you run from it. So a shorter backup can only come from three places:
- The load went up. The same battery feeds more appliances.
- Usable energy went down without the battery changing. Settings, an incomplete charge or a BMS limiting current in heat shrink the part you get to use.
- The battery holds less: aged cells, a failed module, or one weak cell cutting the whole bank short.
Example arithmetic, with round illustrative numbers only: say a battery gives 10 kWh of usable energy and your night load averages 1 kW. That is roughly 10 hours. If the load rises to 1.5 kW, backup falls to about 6.5 hours with nothing wrong with the battery. If instead the inverter's cut-off is raised so you only use 7 kWh, backup falls to 7 hours at the original load. Your figures will be different; the point is that each cause shows up differently in the app.
Backup time reduced
More load
New appliances
Ageing
Lower capacity
Settings
Higher cut-off
Quick-check table: battery backup less than before
| Symptom | Likely cause | Safe owner check | Call an engineer if... |
|---|---|---|---|
| Backup shorter since a new AC, motor, fridge or heater was added | Higher night load | Compare night load (kW) in the app now vs a few months ago | Load is unchanged but backup still fell |
| Battery shows "low" while the app still says 20-30% | Cut-off SOC or discharge-depth setting raised | Note the SOC at which backup stops | You did not change any setting and it changed by itself |
| Battery starts the evening at 60-80%, not 100% | Not fully charging (winter, dust, loads using the solar) | Check SOC at sunset for a week | It never reaches 100% even on clear days with light daytime load |
| Backup shorter only in June-July afternoons and evenings | Heat: BMS limiting discharge or charge | Read battery temperature in the app | Temperature alarms appear or the room is very hot |
| SOC drops suddenly, e.g. 40% to 10% in minutes | Calibration or cell imbalance | Screenshot the SOC graph | Every time; this needs cell-level readings |
| Steady, slow decline over years at the same load | Normal ageing (battery capacity loss) | Compare kWh delivered per night with the rating | Decline is fast, or well inside the warranty terms |
| One battery module in a bank shows a fault or lower SOC | Module or communication fault | Note the module number and alarm text | Always |
| Battery is warm, swollen, smells or makes noise | Possible cell damage | Do not touch. See the safety box. | Immediately |
Safety first
Safety box. Lithium batteries store a lot of energy. Do not open the battery case, remove covers, or disconnect battery or DC cables. If a battery is hot, swollen, leaking, hissing or smells of burning, switch the system off in the manufacturer's sequence only if you can do so from a safe distance, keep everyone away, and call a qualified engineer. Never use water on a battery fire. See lithium battery swelling for what to do.
Safe owner checks, step by step
These checks use only your eyes, the inverter screen and the monitoring app.
- Write down what changed. When did backup first feel shorter? Was an AC, water motor, heater or second fridge added?
- Read the night load in the app between sunset and sunrise. Note the average kW and compare with older screenshots if you have them.
- Read the SOC at sunset for a week. A battery that starts the night at 70% gives about 70% of the backup.
- Read the SOC when backup stops. If the inverter goes to grid at, say, 30%, that 30% is not available to you. That level is a setting; see SBU, SUB and utility-first modes.
- Read the battery temperature on a hot afternoon. See where to install your inverter and battery.
- Note any alarm (BMS, communication, imbalance, temperature) exactly as written.
- Estimate what the battery delivers: the kWh discharged from 100% to the point backup stops. Our replace or expand guide has a simple capacity check.
- Take photos of the screen, app graphs and battery label (model and serial number), then contact your installer.
Do not change battery type, charge current or cut-off settings yourself. Wrong values can shorten battery life or stop the BMS from protecting the cells.
Battery not giving backup at all
If the battery is not giving backup at all, rather than just less of it, the cause is usually different: the battery breaker is off, the BMS has gone into sleep or protection, the inverter cannot talk to the battery, or the backup output itself is not working. See lithium battery not turning on, battery and inverter communication and inverter backup not working.
Is it battery capacity loss or something else?
Every lithium battery loses some capacity with use and time. Research on LiFePO4 cells, the chemistry in most home solar batteries, found that temperature has the largest effect on calendar ageing, with high state of charge second; cells kept full and hot aged fastest. In practice, a battery that sits at 100% in a hot room through a Punjab summer will age faster than one kept cool and not always full. Our guide to lithium battery lifespan and care covers the full list of habits that shorten battery life.
Real capacity loss usually shows as a gradual fall over months or years with the same night load, a battery that reaches 100% unusually quickly, kWh delivered well below the usable rating, or a falling state of health (SOH) in the BMS data.
A sudden drop, after a firmware update, a storm or service work, points elsewhere: check battery percentage wrong, lithium battery cell imbalance and inverter self consumption.
What an engineer will check
- Cell voltages and temperatures through the BMS software, to find a weak cell or module.
- Settings: battery type, charge and discharge limits, cut-off SOC and grid-charge settings.
- Communication between battery and inverter, and battery compatibility.
- SOC calibration, which on some batteries needs a controlled full charge.
- Battery-side terminations and breakers, for heat marks or loose connections.
- Firmware and the real load profile, to see whether the battery is simply undersized for how the house is used now.
The answer is usually one of: change settings, rebalance or recalibrate, replace a faulty module under the manufacturer's warranty, add capacity, or reduce night load. Real ageing does not reverse, so it is worth ruling out the fixable causes first.
How to prevent backup loss
- Keep the battery cool and shaded, with free airflow. Heat is the biggest enemy.
- Avoid running it flat every night, and do not leave it at 100% unused for weeks; ask your installer about a storage setting.
- Size for how you live now. See how many kWh of battery you need and batteries for a full night of backup.
- Keep screenshots from when the system is new, as a baseline.
- Expand with matched modules only; mixing old and new modules or brands can cause imbalance. See our lithium battery price page.
How Solar Works Pakistan handles backup complaints
For systems we install, our own engineering and technical staff handle after-sales complaints. WhatsApp 0336 666 4321 with a photo of the inverter screen or app, the inverter model and the battery model. Once a complaint is registered, we target sending service personnel within 48 hours to begin diagnosis; the time to fix depends on the fault, parts and any manufacturer involvement. Our 1-year service warranty applies under the applicable project terms, and battery defects are handled under the manufacturer's warranty per product. See how after-sales service works.
We can also inspect batteries on systems installed by other companies, as a chargeable service, across Punjab.
For other problems, our solar troubleshooting guide lists every symptom on one page.
Frequently asked questions
Why is my battery backup time reduced?
Usually because night loads have grown, the inverter's cut-off or discharge settings limit how much of the battery you can use, the battery is not reaching full charge, or heat is making the BMS limit current. If none of these apply and backup has fallen slowly over years, the battery is probably ageing.
Why is my battery not giving backup like before?
Compare your night load and the SOC at sunset with how they used to be. If both are the same and backup is still shorter, ask an engineer to read the cell voltages and state of health through the BMS.
Is battery capacity loss normal?
Yes, all lithium batteries lose some capacity over time. Heat and being held at full charge speed it up. A fast or sudden drop is not normal ageing and should be checked.
Why is my battery backup less than before in summer?
Higher night loads (ACs and fans) are the usual reason. Heat can also make the BMS reduce current, and a hot battery ages faster. Keep the battery in a cool, shaded, ventilated place.
How do I check my lithium battery capacity at home?
Charge to 100% on a sunny day, run a steady load in the evening, and note the kWh discharged until the backup stops. Compare that with the battery's usable rating. Treat it as a rough guide; an engineer confirms with BMS data.
Is reduced backup covered by warranty?
It depends on the manufacturer's terms, which often refer to capacity after a certain period or energy throughput and to operating conditions. Your warranty; check those conditions with your installer.
Sources
- University of Waterloo (UWSpace), "Calendar Aging and Lifetimes of LiFePO4 Batteries and Considerations for Repurposing" (effect of temperature and state of charge on capacity fade)
- Battery University, BU-808 "How to Prolong Lithium-based Batteries" (heat, depth of discharge and high charge as ageing factors)
What you can check yourself
- Record the battery percentage when the outage begins and when backup ends.
- List appliances that actually ran, including pumps, refrigeration, ACs and office equipment.
- Note when larger loads started and how long they operated.
- Save load and battery-energy graphs, plus any shutdown warning.
- Compare with the load list and assumptions recorded at handover.
- Use the whole outage period for comparison. A quiet moment near the end does not describe the load the battery supplied earlier.
What not to do
- Do not lower discharge protection to extend runtime or deliberately run the battery to shutdown for an informal capacity test.
- Do not assume a shorter outage runtime alone proves a warranty defect.
When to call us
- Arrange a review if runtime falls sharply under comparable conditions, the system stops with an alarm, or performance repeatedly misses the documented estimate. The load record and delivered energy should be assessed together.
Backup reviews in Faisalabad and Lahore
Long summer outages in Faisalabad and Lahore, with ACs and pumps running, are where estimates are most often missed. Our team compares your load record with the energy the battery actually delivered.
Send us
The inverter and battery models, estimated and observed backup duration, outage timeline, appliance list, screenshots, and your city and area.
Technical references
Reviewed 25 September 2026. Manufacturer documents are cited for general principles; your exact model’s manual always applies.