To calculate solar battery backup time, divide usable energy delivered to the appliances by their average electrical load. Nominal battery capacity, allowable discharge, conversion losses and changing demand all affect runtime. A kilowatt inverter rating describes power capacity; it does not describe hours of stored energy.
Step 1: calculate nominal battery energy
For a battery rated in volts and amp-hours, nominal watt-hours are voltage × Ah. A hypothetical 12V 100Ah battery contains 1,200Wh, or 1.2kWh. For a battery already specified in kWh, use that rating and confirm the voltage, discharge-power limits and manufacturer-defined usable energy.
Series connections raise bank voltage while Ah remains the same; parallel connections raise Ah at the same voltage. Only use combinations authorised by the battery and inverter manufacturers. Mixed-age or mismatched batteries may cause unequal operation and premature failure.
Step 2: allow for usable discharge and conversion losses
An illustrative formula is nominal kWh × assumed usable fraction × assumed conversion efficiency. At 50% usable discharge and 85% efficiency, the example 1.2kWh battery delivers approximately 0.51kWh to the load. These assumptions are not universal limits: use the actual model’s permitted operating window and discharge data.
Battery age, temperature, discharge rate, inverter cutoff and standby consumption further influence performance. Lead-acid capacity can fall at higher discharge rates; rated Ah must therefore be interpreted with its test conditions. Our battery selection guide covers the main comparisons.
Step 3: divide by the appliance load
At a constant 200W load, 0.51kWh ÷ 0.2kW gives about 2.55 hours. A 400W load would give about 1.28 hours under the same simplified assumptions. Real loads cycle and change, so a measured load profile is better than adding every appliance’s label rating as though it runs continuously.
Solar battery backup for home: energy and power checks
Suppose an essential load averages 800W for four hours. It needs 3.2kWh at the appliances; at an assumed 90% conversion efficiency, about 3.56kWh of usable battery energy is required before reserves. The battery must also deliver sufficient continuous and peak power for those circuits.
Check refrigerator, pump and compressor starts separately. A bank with enough energy can still trip if its BMS, cables or inverter cannot support the demand. Use a compatible hybrid inverter and qualified installation for protection and wiring.
Recharge and daily operation matter
An off-grid system must recharge the bank while supplying daytime loads and accommodate low-sun periods. A grid-connected system may use different charging schedules, but those must stay within manufacturer limits. Read the off-grid sizing guide and energy-management guide to plan the whole operating cycle.
When comparing Osaka or AGS products, use the complete model and intended application. Model numbers and plate counts are not substitutes for capacity, charge requirements or cycling suitability.
Frequently asked questions
Does doubling Ah always double runtime?
Not exactly in real operation. The usable operating window, current distribution, discharge behaviour and equipment losses matter. The arithmetic is a starting estimate, not a battery-bank design guarantee.
Can solar panels extend runtime during an outage?
Only when the inverter and installed backup arrangement support simultaneous PV supply and charging in that condition. Confirm the particular model’s documented operation rather than assuming it from the word hybrid.
What is the most useful purchase figure?
Usable kWh at the intended operating conditions, supported discharge power, compatibility and warranted cycling/throughput terms. Compare complete installed cost rather than nominal Ah alone.
Related guides: plan the next step
- Solar Batteries in Pakistan: Capacity & Buying Guide
- Osaka Battery Price in Pakistan: Models & Buyer’s Guide
- AGS Battery Price in Pakistan: Models & Buyer’s Guide
- Hybrid Solar Inverters in Pakistan: Features & Buying Guide
- Off-Grid Solar Systems in Pakistan: Sizing & Cost Guide
- Smart Solar Energy Management in Pakistan: Practical Guide
Content reviewed: 12 October 2026. Calculation examples are illustrative. Solar Price Pak publishes independent information and does not supply or install equipment.