How to Calculate Battery Backup Time (Free Calculator)

Most people buy a battery based on Ah rating alone, then get surprised when the real backup time is nowhere close to what the dealer promised. The good news is you don’t need to guess. Use the calculator below, or work it out by hand using the same formula it runs on.

The Formula

Backup Hours = (Voltage × Ah × Number of Batteries × 50%) ÷ Load in Watts

That looks complicated, but each piece is simple once you know what it means.

What Each Field Actually Means

• Battery Voltage (V) – Most home setups in Pakistan use 12V (single battery), 24V (two batteries in series), or 48V (four batteries, common in bigger solar systems). Check your inverter's spec sheet if you're not sure which one you have.
• Battery Capacity (Ah) – This is the number printed on your battery, like 150Ah or 200Ah. It tells you how much charge one battery can theoretically hold.
• Number of Batteries – How many batteries you have wired together in parallel at the same voltage. Two 150Ah batteries in parallel act like one 300Ah battery, this field lets you calculate for a full battery bank, not just a single unit.
• Total Load (Watts) – Everything you plan to run added together: lights, fans, a router, a fridge. Add up the wattage of each appliance (usually printed on the appliance itself or its box) to get this number.

This calculator assumes a standard 50% safe discharge depth, the safe usable limit for a normal tubular or lead-acid battery, which covers most home setups in Pakistan. If you're using lithium batteries, which allow deeper discharge, your real backup time will be noticeably higher than what this tool shows.

Why This Matters More in Pakistan

Pakistan's weather plays a bigger role in this calculation than most people realize. In peak summer, a battery that's a year or two old may already be holding less charge than its rated capacity, since heat speeds up the natural aging process inside every lead-acid battery. If your battery is older, it's worth using a slightly lower Ah number than what's printed on the label, closer to 80-90% of its original rating, to get a more realistic result.

Load also changes with the season. A fridge running harder in summer heat and a fan running non-stop can push your total wattage well above what the same household uses in winter, which is exactly why backup time can feel shorter in July than it does in January, even with the same Pakistan battery setup.

Using the Calculator

Fill in your battery's voltage and Ah rating, enter how many batteries you have, and enter your total load. The tool will instantly show your usable energy in watt-hours and your real backup time in hours and minutes.

If you want to compare battery brands and prices before buying, see our full Battery Price in Pakistan guide, or read How Long Will My Battery Last for a worked example using this exact method to calculate battery backup time by hand.