Battery Sizing Calculator for UPS
Buying a UPS battery without doing the sizing math first is how people end up with either wasted money on an oversized battery, or a battery that runs out in half the time they expected. Enter your load and how many hours of backup you want below, and this tool tells you exactly what battery capacity (Ah) you need.
Battery Sizing Calculator for UPS
Find out what battery capacity you need for your desired backup time.
Load example: 5 LED bulbs (50W) + 3 fans (210W) + router (10W) = 270W. Calculation assumes a standard 50% safe discharge depth for lead-acid/tubular batteries.
The Formula
Required Battery Size (Ah) = (Load in Watts × Backup Hours ÷ 50%) ÷ Voltage
This works backward from the same math used to calculate battery backup time. First, work out the total energy (watt-hours) you need for your desired backup period. Then, since a standard lead-acid or tubular battery can only be safely drained to 50% of its rated capacity, double that energy requirement to find out what the battery's rated capacity needs to be. Divide by voltage to get the final Ah number.
What Each Field Means
- Total Load (Watts) – Add up the wattage of everything you plan to run: lights, fans, a router, a fridge. This number is usually printed on each appliance or its box.
- Desired Backup Time (Hours) – How many hours you want your UPS to run during a power cut. Think about your area's typical load-shedding pattern when setting this.
- System Voltage (V) – The voltage of your UPS/inverter system: 12V for a single battery, 24V for two in series, 48V for four in series (common for larger solar setups).
The result gives you the minimum rated Ah capacity you need. If that number is bigger than any single battery available, it means you need multiple batteries wired in parallel to reach that total, rather than one oversized unit.
A Quick Example
Say you want 6 hours of backup for a 270W load on a 12V system. That's 270 × 6 = 1,620 Wh needed. Divide by 50% usable depth, and you need 3,240 Wh of rated capacity. Divide by 12V, and you need a 270Ah battery, or two 150Ah batteries wired in parallel to get close to that number. Compare real prices for batteries in that range in our Tubular Battery Price in Pakistan guide.
Already have a battery and want to check how long it will last instead of sizing a new one? Use our Battery Backup Time Calculator, which runs the same formula in reverse.