How to Estimate Solar Battery Backup Time
Battery backup time depends on usable battery energy, average appliance load and system losses. Learn the correct way to estimate it before buying a solar backup system.
Battery backup time depends on usable battery energy, average appliance load and system losses. Learn the correct way to estimate it before buying a solar backup system.
“How many hours will this battery last?” is one of the most important questions in solar buying. It is also one of the questions most likely to receive an incomplete answer.
Battery backup time cannot be determined from the battery label alone. You need three pieces of information: usable battery energy, average appliance load and system efficiency.
Step 1: Convert the battery capacity into kilowatt-hours
Some batteries are already rated in kilowatt-hours. Others are described using voltage and amp-hours.
To estimate nominal energy:
Battery energy in watt-hours = voltage × amp-hours
Divide the answer by 1,000 to convert watt-hours to kilowatt-hours.
A 48-volt, 100-amp-hour battery has a nominal capacity of approximately 4.8 kilowatt-hours. This does not mean all 4.8 kilowatt-hours should be used.
Step 2: Apply the usable depth of discharge
Battery chemistry and settings determine how much of the nominal capacity is usable. Lithium batteries often allow a higher usable fraction than lead-acid batteries, but the exact value should come from the manufacturer and system configuration.
If a 4.8-kilowatt-hour battery is operated at 90% usable depth, the usable energy before other losses is approximately 4.32 kilowatt-hours.
Step 3: Allow for inverter and system losses
Energy is lost in the inverter, battery, cables and conversion process. A preliminary estimate may apply an overall efficiency factor, but final expectations should be based on the equipment specifications and operating conditions.
If 4.32 kilowatt-hours is multiplied by an assumed 92% inverter efficiency, the estimated delivered AC energy becomes approximately 3.97 kilowatt-hours.
Step 4: Determine the average load during backup
Do not use the inverter size as the load. List the appliances that will actually operate and estimate how frequently they run.
A fridge may cycle on and off. Lights may not all remain on. A television may be used for only part of the outage. The average backup load is therefore often lower than the maximum possible simultaneous load.
Suppose the average load is 500 watts, or 0.5 kilowatts.
Step 5: Divide delivered energy by average load
Estimated backup time = delivered battery energy ÷ average load
Using the example above:
3.97 kWh ÷ 0.5 kW = approximately 7.9 hours
This is a planning estimate, not a guarantee. Actual time can change because of battery temperature, age, appliance cycling, inverter consumption, unexpected loads and system settings.
Why backup-time promises often disappoint
A supplier may calculate backup time using only lights and a television, while the customer later adds a fridge, pump, microwave or kettle. The battery then appears to be underperforming even though the real problem is a changed or poorly defined load.
Another common error is using the battery’s nominal capacity without accounting for usable depth and conversion losses.
How to get a more reliable estimate
- List each appliance that must operate during an outage.
- Record its wattage and likely operating hours.
- Separate essential loads from optional loads.
- Consider cycling appliances such as fridges and pumps.
- Confirm the battery’s usable capacity and discharge-current limit.
- Ask the supplier to state the average load used in the backup calculation.
Calculate a preliminary system size
Use the SheSpeaksSolar calculator to estimate inverter class, battery capacity, array size and panel quantity. For purchasing decisions, follow it with a professional load assessment and system design.
Elizabeth Nyambura
Elizabeth is a renewable-energy advisor, project manager, solar consultant and trainer with more than 10 years of experience across solar, energy management, water-energy systems and project development.