A 150 Ah battery does not give 150 hours at 1 amp, and a 150 Ah battery does not run a 1 kW load for 1.8 hours either. Backup time depends on several factors that the label does not show. This article works through a realistic example, one number at a time, so you can estimate your own backup time and spot where the surprises come from.
The basic formula
Backup time (hours) = Battery voltage x Battery Ah x usable fraction x inverter efficiency / Load in watts
Each term deserves a word:
- Battery voltage x Ah gives the stored energy in watt-hours (Wh). A 12 V, 150 Ah battery stores 1,800 Wh on paper.
- Usable fraction is how much of that you are allowed to take out. Lead-acid batteries last longer if you do not discharge them fully, and inverters shut down at a low-battery cut-off anyway. A figure around 0.7 to 0.8 is typical for planning with tubular batteries; follow the maker’s guidance.
- Inverter efficiency is typically in the 80 to 90 per cent range. Some energy is lost as heat in the inverter.
- Load in watts is the actual power drawn, not the VA rating of the inverter.
Worked example: a 12 V, 150 Ah battery with a 300 W load
- Stored energy: 12 V x 150 Ah = 1,800 Wh
- Usable energy at 0.8: 1,800 x 0.8 = 1,440 Wh
- After inverter efficiency of 0.85: 1,440 x 0.85 = 1,224 Wh delivered to the load
- Backup time: 1,224 Wh / 300 W = 4.08 hours
So the answer is about 4 hours, not the 6 hours that naive arithmetic (1,800 / 300) gives. The gap of about one third is normal.
Why the real number can be even lower: the Peukert effect
Battery Ah ratings are measured at a particular discharge time, commonly the 10-hour or 20-hour rate depending on the battery type and the manufacturer. If you discharge faster than that rate, the battery delivers fewer ampere-hours. This is the Peukert effect, and it matters most at high loads.
In our example, the load current is about 300 W / 0.85 / 12 V = 29.4 A. A 150 Ah battery rated at the 20-hour rate would be discharged at 150/20 = 7.5 A normally, so 29.4 A is about four times the rated current. The discharge table in the datasheet shows how many Ah the battery really gives at that rate. For a 4-hour discharge a battery will often deliver noticeably less than its nameplate Ah, so the 4.1 hours above should be treated as the best case. Take the Ah the datasheet quotes for the discharge time that matches your load (for example the 4-hour or 5-hour rate) and put that figure into the formula in place of the nameplate Ah. Rather than trust a rule of thumb, use the actual table for your battery.
Table: effect of load on backup time
For the same 12 V, 150 Ah battery with usable fraction 0.8 and inverter efficiency 0.85, ignoring Peukert (so true figures are lower at the heavy end):
| Load (W) | Delivered energy (Wh) | Backup time (approx.) |
|---|---|---|
| 150 | 1,224 | 8.2 hours |
| 300 | 1,224 | 4.1 hours |
| 500 | 1,224 | 2.4 hours |
| 800 | 1,224 | 1.5 hours |
The heavy-load rows are optimistic, because a faster discharge reduces the Ah actually available. Aim to size the bank by the longest and heaviest case you expect.
Other things that shorten backup
- Battery age. A battery at 80 per cent of original capacity gives 80 per cent of the time. Plan for it.
- Low charge at the start. If power cuts return before the battery refills, you begin from less than full.
- Temperature. Cold reduces capacity; heat shortens life.
- Cable and connection losses. Loose or thin cables waste energy at high current, especially on 12 V systems.
- Idle consumption. The inverter itself draws a few watts even with no load.
- Standby loads you forgot, such as chargers, routers and set-top boxes.
Working it backwards: how much battery for a target time
To find the Ah you need, rearrange the formula:
Ah = Load (W) x Hours / (Voltage x usable fraction x inverter efficiency)
For a 300 W load for 5 hours on 12 V, with 0.8 and 0.85: Ah = 300 x 5 / (12 x 0.8 x 0.85) = 1,500 / 8.16 = about 184 Ah. A 200 Ah battery would be the practical choice. Try different values in our battery backup calculator, or browse all tools on the calculators page.
Reading the inverter and battery together
If your goal is longer backup, you can add batteries in parallel (more Ah at the same voltage, if the inverter allows) or in series (higher voltage, only if the inverter is built for it). Do not connect batteries randomly; the inverter manual states the allowed bank. For help selecting the pair, see home inverter batteries and home inverters and UPS. For industrial banks, see industrial and standby batteries, where discharge tables and ageing factors are the norm.
A quick field check
You can test your estimate in practice. Charge the battery fully, switch off the mains, run your normal load and note the time until the inverter alarms or cuts off. Compare that with the calculation. If real time is far shorter, measure battery voltage at rest and under load, check connections for heat, and see whether the battery is old. A clamp meter on the battery cable gives the true load current, which is often higher than people expect once the fridge and pump run together.
What to send us for a quotation
Send the list of appliances with watts, the backup hours you want, the inverter or UPS model and voltage, and the space available. Submit through request a quote or contact us via contact us. Quotations are answered within 24 hours.
Frequently asked questions
How long will a 150 Ah battery run a 500 W load?
On a 12 V system with typical planning values (80 per cent usable, 85 per cent inverter efficiency) the estimate is around 2.4 hours. The real figure can be shorter because a high discharge rate reduces available Ah and an older battery has lower capacity. Check the datasheet discharge table.
What is the Peukert effect in simple words?
A lead-acid battery gives less total energy when it is discharged quickly than when it is discharged slowly. Its Ah rating applies at a certain discharge time, usually 10 or 20 hours. If your load drains it in 3 or 4 hours, expect fewer usable Ah than the label says.
Does adding a second battery double the backup time?
If the second battery is connected in parallel at the same voltage, the capacity in Ah doubles, so backup time roughly doubles for the same load. It must be an identical battery, and the inverter must be designed to accept that bank size. Check the inverter manual before connecting.
Why is my backup time much less than the calculation?
Common causes are an ageing battery, an incompletely charged battery, higher actual load than assumed, loose or undersized cables, and high discharge rate. Measure the real load with a clamp meter or the inverter display, test battery voltage under load, and check the battery's age and charging condition.
