Generator & UPS Runtime Calculator
Battery backup runtime, battery sizing for a target backup duration, and generator fuel running cost
Frequently Asked Questions
Backup power planning in Pakistan — UPS/inverter runtime, battery bank sizing, and generator fuel cost — involves three genuinely separate calculations that get conflated in casual conversation but actually depend on different inputs and answer different questions about a backup power setup.
Three different questions, three different calculations
UPS/inverter runtime answers "how long will my existing battery setup last at a given load" — it depends on battery capacity (Ah), voltage, how many batteries are connected, the depth of discharge (DoD) you're willing to draw down to before the battery is considered depleted, and inverter efficiency (some energy is always lost in DC-to-AC conversion). Battery sizing answers the reverse question — "how much battery capacity do I need to run a specific load for a specific number of hours" — starting from a desired runtime and working backward to a required Ah capacity. Generator fuel cost is a completely separate calculation about ongoing running expense rather than energy storage at all — fuel consumption depends on generator load (kW), fuel type (diesel typically runs more efficiently per kW than petrol for generators), and current fuel price, giving a cost-per-hour figure for actually running the generator rather than anything about batteries.
A worked example
Two households with identical battery banks can get very different backup runtime if one draws a much heavier load (running an AC or motor) than the other (just lights and fans) — runtime isn't a fixed property of a battery setup alone, it's a function of battery capacity and how much load is actually being drawn from it, which is why the same UPS system can last hours for a light load and a fraction of that for a heavy one.
How this connects to your other utility planning
If you're deciding between battery backup and a generator (or sizing both together for a hybrid setup), the running-cost side of a generator connects directly to the Fuel Cost Calculator's underlying fuel-price logic, while battery-based backup that's charged from grid electricity connects to your actual Electricity Bill Calculator tariff for true charging cost. For a fully off-grid or hybrid solar-plus-battery setup, the Solar System Size Calculator handles the separate question of how much solar generation capacity is needed.
Common mistakes
The most common mistake is sizing a battery bank using 100% of its rated capacity rather than the recommended depth of discharge — most battery chemistries have a maximum recommended DoD well below 100% (discharging deeper than recommended shortens battery lifespan significantly), so a sizing calculation that ignores DoD will overestimate actual usable runtime and undersize the real-world battery bank needed.