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As a simple rule, to calculate how long a 12v deep-cycle battery will last with an inverter multiply battery amp-hours (Ah) by 12 to find watt-hours, and divide by the load watts to find run time hours. Finally, multiply run time hours by 95% to account for inverter losses.
Page Contents
- Introduction to battery run-time with inverters
- Video – How long can a 12 volt battery last with an inverter?
- Pure sine wave vs modified sine wave inverter types – what’s the difference?
- How efficient are 12v inverters?
- Related Questions:
- Table – 12 volt 100Ah battery run times with various AC loads
- How long will a 12v car battery last with an inverter?
Introduction to battery run-time with inverters
It’s a common question to wonder just how long your 12 volt battery would last running an AC load through an inverter. To answer this question fully we need to define these 3 things:
- The type of battery being used
- The size of the battery in amp-hours (Ah)
- The AC load size in watts
Why is battery type important?
Different battery technologies determines how much of a battery’s capacity you can actually use. It sounds great having a 100Ah battery to power an inverter, but what if you could only use 15% of that without damaging it?
The image below shows that a car battery gradually loses it’s capacity if it’s discharged regularly more than 10 to 15%.
Automotive batteries are often call SLI (Starting, Lights and Ignition) and they are designed to do just that. They can provide up to 900 amps to crank a cold engine but don’t handle medium current draw for long periods of time very well.
Once a car battery has delivered that enormous starting power, the engine alternator immediately starts to charge it rapidly. Within minutes it’s fully charged again!
Car battery vs deep cycle
Within the lead-acid battery technology, there is another type designed to deliver sustained currents and is often used for energy storage in solar generation systems.
The deep-cycle, or leisure battery as it’s called, can be discharged to 80% of its capacity. However, 50% discharge is mostly recommended as the average 12 volt deep-cycle lead acid battery will last up to 30% longer.
This type of battery are used for all kinds of energy storage purposes and are common in RVs.
Are lithium batteries better than lead acid?
The most stable lithium battery variant is probably lithium iron phosphate (LiFePo4). I have a 30Ah model which can be regularly discharged comfortably to 95% of it’s capacity.
I also have a 100Ah lead-acid deep-cycle battery which I regularly discharge to 50%, which means 50Ah is available for use.
It’s interesting to note that a 50Ah LiFePo4 battery would give almost as much usable capacity as a 100Ah leisure battery!
Of course, it would cost almost twice as much but could last 4 to 5 times longer.
Video – How long can a 12 volt battery last with an inverter?
Pure sine wave vs modified sine wave inverter types – what’s the difference?
Pure sine wave inverters have better quality electronics and produce an almost perfect sine wave output, just like that supplied by utility companies.
A modified sine wave inverter has an almost square wave output. I have the 1000 watt model shown in the above image.
Table – Pure sine wave inverter vs. square wave inverter
Pure Sine Wave Inverter | Modified Sine Wave |
Perfect sine wave output | Practically square wave |
High quality electronics | Minimum electronic components |
Low level hum | Can be noisy, can hear definite buzz |
Does not interfere with other electronic gear | Can cause interference on TVs, cell phones |
Safe for all appliances | Some appliances may overheat |
90% to 95% efficient | 75% to 85% efficient - wastes power |
2 to 3 times more expensive than modified | Cheaper option |
How efficient are 12v inverters?
Let’s assume that most people would buy the pure sine wave inverter variety. They are much more efficient than their modified sine wave cousins and the price reflects that.
Considering the possible negative effect on appliances and possible interference with electronics, it is the best option. However, even the best quality electronics devies have losses.
The best inverters have an efficiency of between 93% and 97% and the price increases accordingly. This kind of efficiency only occurs if the inverter is reasonable loaded.
You can see form the graph in the image below that below 30% load most inverters lose efficiency rapidly, which means you’re wasting your power.
If sourcing the inverter and energy storage batteries with solar, then this can be a big deal.
The inverter size should be matched to the average load expected so that it’s always (or mostly) above 40% loaded.
Popular Inverters Data Sheets and Efficiency Curves PDF Download
SMA-STP-15000-17000TL-Data-Sheet
Satcon PowerGate Plus 100 kW UL
Related Questions:
Questions often refer to a 12 volt battery, but this covers a very broad spectrum of possibilities. 12V lead acid deep-cycle batteries can be from 50Ah to 200Ah capacity.
Obviously, the bigger Ah batteries will last longer than the smaller. A typical battery size for leisure batteries is 100Ah, so I’ll use this capacity to answer the following questions:
How long will a 12 volt battery last with a 1000 watt inverter?
A 12 volt 100Ah deep-cycle battery with regular depth of discharge 50% would run a fully-loaded 1000 watt inverter for 34 minutes. This calculation takes into account average pure sine wave inverter efficiency of 95%.
The easiest way to calculate 100Ah 12 volt deep-cycle run-time is to convert the amp-hours to watt-hours. Simply multiply available Ah by the battery voltage to find its watt-hours capacity:
Battery watt-hours = amp-hour capacity x battery volts
Battery watt-hours = 100Ah x 12V = 1200 watt-hours
Remember that the recommended regular discharge level for a lead-acid deep-cycle battery only 50%, so the actual available capacity is 600 watt-hours.
This means that the battery will last 600 Wh/1000W = 0.6 hours
Furthermore, if we apply the average inverter efficiency of 95%, then a closer estimate of a 100Ah 12 volt battery run-time would be:
Run-time = 0.6 hours x 95% = o.57 hours (34.2 minutes)
Calculator – How long will a 12v battery last with a 1000 watt inverter?
Enter the size of your 12 volt battery into the calculator below to find how long it will run a 1000 watt inverter load:
How long will a 12v battery last with a 1500 watt inverter?
A 12 volt 50Ah lithium iron phosphate (LiFP04) battery with regular depth of discharge (DoD) of 80% will run a fully-loaded 1500 watt inverter for 13 minutes. The calculation incorporates typical pure sine wave inverter efficiency of 95%.
I’ll do a calculation now for a 12 volt 50Ah lithium iron phosphate battery with recommended regular discharge rate of 80% running a fully loaded 1500 watt pure sine wave inverter with a 95% efficiency.
95% depth of discharge (DoD) is possible, but the lesser discharge rate extends the life considerably.
Battery watt-hours = amp-hour capacity x battery volts
Battery watt-hours = 50Ah x 12V = 600 watt-hours
Due to the discharge maximum of 80% only 480 watt-hours are available for use.
Run time = 480Wh/1000W = 0.48 hours or 28.8 minutes
Applying the 95% inverter efficiency the run-time = 0.45 hours or 13 minutes
Calculator – How long will a 12v lithium iron phosphate battery last with a 1500 watt inverter?
Simply enter the capacity of your lithium iron phosphate battery to calculate how long it will run a fully loaded 1500 pure sine wave inverter:
Table – 12 volt 100Ah battery run times with various AC loads
100Ah 12V deep-cycle lead-acid battery with AC loads and inverter - 50% discharge | ||
Home Appliance | Rated Load (watts) | Estimated Running time (hours) |
Air purifier | 25 watts | 24 hours |
Portable Air Conditioner | 70 (average per hour) | 8 |
Ceiling Fan | 60 | 10 |
Coffee Maker | 800 | 0.75 |
Deep Freezer | 25 | 24 |
Electric Shaver | 15 | 40 |
Fluorescent Lamp | 30 | 20 |
Fridge | 120 | 5 |
Freezer | 40 | 15 |
Laptop Computer | 100 | 6 |
Pedestal Fan | 60 | 10 |
Tablet Charger | 10 | 60 |
Vacuum Cleaner | 500 | 1.2 |
WiFi Router | 10 | 60 |
Water Filter and Cooler | 80 | 7.5 |
Treadmill | 300 | 2 |
How long will a 12v car battery last with an inverter?
Car batteries are not designed to be discharged over time and I don’t recommend that you use one except in exceptional circumstances and then only once.
Each time an automotive battery deep-cycles it loses a little capacity, until it’s almost useless, so the run time with an inverter will get progressively less with each cycle.
That said, you can use the calculator below to give you an estimate.
Calculator – How long will a 12 volt battery last with an inverter?
There are many different capacities of 12 volt lead-acid deep-cycle batteries and they can also be connected in parallel to provided even more amp-hours.
Enter the capacity of your battery (lead-acid only) and the size of your inverter in the calculator below to find how long it will run fully-loaded:
How many batteries to run a fridge?
Other Questions:
What does an inverter do?
Solar panels and associated battery storage systems generate/store DC electricity. One terminal is positive and the other is negative, which never changes.
Home appliances operate with AC electricity, whose polarity is changing all the time as the voltage follows a smooth sweeping shape called a sinusoidal waveform.
An inverter is a device which chops up the straight line of DC voltage, adjusts the size and then shapes the pulses electronically into a smooth AC waveform for use in the home.
Can an inverter power a house?
An inverter can easily power a house. Power inverters come in all sizes, common sizes being 5 to 20kW.
The average US home consumes about 30kWh per day and is equivalent to an average power draw of 1.25kW or 12500 watts.
What is the purpose of a power inverter?
A power inverter is simply another name for an inverter, or solar inverter. ‘Power’ is just an indication that it’s quite a high rating, rather than being used for small electronic devices, for example.
All inverters have the same purpose, which is to convert DC voltage from solar panels or batteries to the AC voltage needed to operate home appliances.
How big of a power inverter do I need?
As a general rule, power inverters should be sized the same as the power rating of the DC supply, usually a solar panel array.
This is because inverters are very efficient when fully loaded, but can waste a lot of power if very lightly loaded.
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