Battery makers will tell you a 150Ah inverter battery runs a 300-watt load for six hours. Run the same office through a long power cut and the lights usually go out well before that. The gap comes from what the standard formula leaves out. To calculate inverter battery backup time properly, multiply the battery’s capacity in amp-hours by its voltage, then by the inverter’s efficiency and by the share of the battery you can safely use, and divide by the load in watts. For a 150Ah, 12V battery feeding a 400W small office, that works out to roughly 3.2 hours rather than the 4.5 hours the simple version promises. This guide is written for small offices and shops in India, where inverter batteries are the default backup.
Battery Ah Times Volts Over Load, Then the Real-World Cuts
- The basic formula is backup hours = battery Ah × battery volts ÷ load in watts.
- Multiply by inverter efficiency (manufacturers quote 80% to 95%) and by usable depth of discharge to get a realistic figure.
- A 150Ah, 12V battery on a 400W load gives 4.5 hours on paper, about 4 hours after inverter losses and about 3.2 hours if you stop at 80% discharge.
- Inverter size is a separate calculation: VA = watts ÷ power factor, with 0.8 a common assumption.
- Heavy loads drain lead-acid batteries faster than their rated capacity suggests, so leave headroom.
The Basic Inverter Backup Time Formula
A battery stores energy, and energy in watt-hours is amp-hours multiplied by volts. A 150Ah battery at 12V holds 1,800 watt-hours. Divide that by the power your equipment draws and you have hours of runtime. Tata Green Batteries uses exactly this version in its backup-time guide: 150Ah × 12V ÷ 300W gives six hours.
Livguard adds one correction. Its version of the formula multiplies by inverter efficiency, noting that most inverters run at 90% to 95%. Its worked example, a 150Ah battery on a 290W load at 0.95 efficiency, comes to 5.9 hours. Tata Green’s page puts efficiency at around 80% to 90% but leaves it out of the arithmetic.
A fuller formula has four parts:
Backup time (hours) = Battery Ah × Battery V × Inverter efficiency × Usable depth of discharge ÷ Load (W)
What Each Number Means
- Battery Ah: the capacity printed on the battery. It is measured at a stated discharge rate, often over 10 or 20 hours. A battery drained faster than that delivers less.
- Battery V: 12V for a single battery. Two 12V batteries in series give 24V, which is what many larger home and office inverters use.
- Inverter efficiency: the share of battery energy that reaches your sockets after conversion from DC to AC. Use the figure in your inverter’s manual; if there is none, 0.9 is a middle-of-the-road assumption based on the ranges makers publish.
- Usable depth of discharge: how much of the battery you can drain without shortening its life. This is the factor most online calculators skip.
- Load (W): the total running wattage of everything on the inverter circuit, taken from rating labels or a plug-in power meter.
Why Depth of Discharge Changes the Answer
The simple formula assumes you can drain a battery to empty. With lead-acid batteries, the tubular and flat-plate types sold for most Indian inverters, that is the fastest way to shorten their life. Discover Battery, a deep-cycle battery maker, says in its technical note on discharge that 80% depth of discharge is the maximum safe level for its industrial lead-acid ranges, that flooded batteries should be held to 50% and that deeper, longer discharges cut cycle life.
Your inverter will usually cut off automatically at a low-voltage threshold before the battery is fully flat, so you may never hit zero. But if you size a system to empty the battery on every outage, you will be replacing it sooner. Plan with 80% as an upper limit and 50% if outages are long and frequent. Lithium iron phosphate batteries tolerate deeper discharge than lead-acid; use the depth of discharge in the maker’s datasheet.
The High-Load Penalty
Lead-acid capacity also shrinks as the current rises, an effect described by Peukert’s law. Discover’s note uses a 100Ah battery as its example: rated to supply 5 amps for 20 hours, it delivers well under 100Ah when drained at 100 amps. A 400W load on a 12V system pulls more than 33 amps, several times the 7.5 amps a 150Ah battery is rated at over 20 hours. Expect real runtime at heavy loads to fall below even the corrected formula. Moving to a 24V bank halves the current for the same load, which is one reason bigger systems use it.
Worked Example: A Small Office on One 150Ah Battery
Take an office with four 20W LED tube lights, two 75W ceiling fans, two laptops on 65W chargers, a 12W Wi-Fi router and a 28W CCTV recorder. These are typical nameplate figures we chose for the example; read your own labels. The total is 400W. We ran every line in code.
| Method | Formula | Backup time |
|---|---|---|
| Simple (Tata Green style) | 150 × 12 ÷ 400 | 4.5 hours |
| With 90% inverter efficiency | 150 × 12 × 0.9 ÷ 400 | 4.05 hours |
| With efficiency and 80% depth of discharge | 150 × 12 × 0.9 × 0.8 ÷ 400 | 3.24 hours |
| With efficiency and 50% depth of discharge | 150 × 12 × 0.9 × 0.5 ÷ 400 | 2.03 hours |
| Two 150Ah batteries at 24V, 80% discharge | 150 × 24 × 0.9 × 0.8 ÷ 400 | 6.48 hours |
The spread is wide. The same battery could be sold as a 4.5-hour solution and deliver about two hours if you want it to last. The difference is what you promise your staff when the power goes at 3pm.
Working Backwards From the Hours You Need
Rearrange the formula to find the battery you need: Required Ah = Load (W) × Hours ÷ (Battery V × Efficiency × Depth of discharge). For four hours at 400W on a 12V system at 90% efficiency and 80% discharge, that is 400 × 4 ÷ (12 × 0.9 × 0.8), or about 185Ah. At a 50% discharge limit it rises to about 296Ah. In practice that points to two batteries rather than one larger 12V unit, wired for 24V if your inverter supports it.
How to Size the Inverter Itself
Battery capacity decides how long you run. Inverter rating decides what you can run at once. Inverters are rated in volt-amperes (VA), and Luminous gives the conversion in its capacity guide: VA equals total load in watts divided by power factor, using 0.8 as a typical value. A 400W load needs at least 500VA. Livguard’s guide mentions its own calculator pairing a 2,000VA inverter with a 150Ah battery, which shows that inverter and battery are sized separately.
| Running load | Minimum VA at 0.8 power factor | Backup on 150Ah, 12V (90% efficiency, 80% discharge) |
|---|---|---|
| 200W | 250VA | 6.48 hours |
| 300W | 375VA | 4.32 hours |
| 400W | 500VA | 3.24 hours |
| 600W | 750VA | 2.16 hours |
Leave margin above the minimum VA. Motors in fans, pumps and printers draw a surge when they start, and an inverter running near its limit runs hot.
Practical Ways to Get More Backup
- Trim the inverter circuit: put only lights, fans, routers and laptops on backup. Heaters, kettles, air conditioners and laser printers belong on mains only.
- Prefer laptops to desktops: a laptop on its own battery draws little from the inverter and keeps working if the inverter trips.
- Measure, do not guess: a plug-in power meter shows real draw, which is often below the label for chargers and routers.
- Keep batteries charged and maintained: a battery that never reaches full charge between cuts starts every outage short.
- Plan for ageing: capacity falls over a battery’s life, so a system that just meets your target when new will fall short later.
Power backup matters most for businesses that lose money every minute the screens go dark. Our guide to starting a gaming cafe in Delhi treats it as a core setup cost. Routers are among the smallest loads on the list, even new ones covered in our explainer on Wi-Fi 7. If you are buying a two-battery system on instalments, our guide on how to calculate EMI shows what the monthly cost works out to. More guides like this sit in our Technology section.
150Ah and 200Ah Backup Times and the 12V vs 24V Choice
How Long Will a 150Ah Battery Last on an Inverter?
It depends on the load. At 300W, the simple formula gives six hours. Allowing for 90% inverter efficiency and 80% discharge, expect about 4.3 hours.
How Long Will a 200Ah Battery Last at 500 Watts?
On paper, 200 × 12 ÷ 500 is 4.8 hours. With 90% efficiency and 80% discharge, plan for about 3.5 hours.
Does Inverter VA Rating Affect Backup Time?
Not directly. VA rating caps the load you can connect at once. Backup time depends on battery energy, efficiency and the actual load.
Why Is My Actual Backup Lower Than the Calculation?
Common causes are inverter losses, the low-voltage cut-off, high current reducing usable capacity, an ageing battery and a battery that was not fully charged when the outage began.
Is a 24V System Better Than 12V for Backup?
For the same load, 24V halves the current drawn from the batteries, which reduces losses. Two 150Ah batteries at 24V also double stored energy compared with one.
