Calculating UPS or solar battery backup time before load-shedding hits prevents the frustrating experience of running out of power unexpectedly. This calculator shows how long your battery bank will power your connected load based on battery capacity, voltage, type, and current draw.
How to use: Enter your battery bank's total Ah capacity and voltage (both printed on the battery label), select the battery type (lead-acid or LiFePO4 — this determines usable depth of discharge), and enter the total watts of appliances you'll run. Common loads: ceiling fan 75W, LED bulb 10W, laptop 65W, router 15W.
Calculate how long your UPS or solar battery will last during load shedding based on your battery bank and the load connected.
Actual backup time is typically 80-90% of calculated due to inverter efficiency losses. Keep loads minimal during outages to maximise backup duration.
Actual vs Calculated Backup Time
Expect 80–90% of the calculated time in practice — inverter efficiency losses (typically 10–15%) and battery capacity that's slightly less than rated (especially in older batteries) reduce real backup time. For a safety margin, plan your load-shedding coverage around 80% of the calculated figure. Battery backup time also decreases over battery age — lead-acid batteries lose 20–30% of capacity by year 3–4.
Frequently Asked Questions
Higher voltage systems (24V, 48V) use thinner cables and are more efficient for the same power output. A 24V 200Ah system stores the same energy as a 12V 200Ah system but is preferred for larger inverters (above 1000W). Most home UPS systems in Pakistan run at 12V or 24V; larger solar systems often use 48V.
Rated capacity printed on the label is the new battery capacity. After 2–3 years of use, lead-acid batteries typically retain 60–80% of rated capacity. If backup time has noticeably decreased from when you first installed the batteries, capacity degradation is the likely cause — not a calculation error.
Never mix old and new batteries in the same bank — this degrades all batteries to the weaker one's performance. If existing batteries are 3+ years old and backup time is insufficient, replace the entire bank rather than adding new batteries alongside old ones.