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Because an inverter draws power from the battery bank, it is a critical component in RVs and off the grid solar systems. With a 1000 watt inverter you can run a lot of appliances, but how long can a 12V battery last on it? The answer depends on several factors.
A 12V 100ah battery with a 50% depth discharge will last 30 minutes on a fully loaded 1000 watt inverter. The same battery with a 300 watt load will run for about 3 hours on a 1000 watt inverter.
How Long 12V Batteries Last on 1000W Inverters
The following examples use a 100ah battery, as it is one of the most widely used. When people speak of 12V batteries, it usually refers to 100ah so that is what we will use here. If you are interested in buying, our recommendation is the 100ah LiFePO4 Battery by Btrpower as it ha a long life cycle and fast charge time.
The calculation is: total watt load / battery voltage / inverter efficiency = amps per hour
When you know the amps per hour, it is easy to calculate how long the battery will last.
Take a 100ah 12V battery and a 1000W inverter with an 85% efficiency rating. Just like solar cables, inverters lose some energy during conversion, so we have to factor that in.
If an 85% efficient inverter has an 800 watt load, a 100ah battery will last 65 to 70 minutes, or 1 hour and 10 minutes. Using the calculations above:
800 watts / 12 volts / .85 = 78 amp hours
An 800 watt load draws 78 amps an hour, so a 12V 100ah battery is good for an hour plus 10 minutes (1.1 hours) give or take.
Here is another example. Using the same battery but with a 250W load and a 90% efficient inverter.
250 watts / 12 volts / .90 = 23.1 amps
A 250W load draws 23 amps an hour, so round that off to 25 amps and the battery should last 3 to 4 hours.
Why do we say 3 to 4 hours? If we round off 23, that would be 25 amps, 25 x 4 = 100 amps, right?
The load determines how many amps an inverter draws, but as the voltage goes down, the inverter will pull more current to keep the voltage going, draining the battery faster. So the battery may not last as long as you think.
Battery capacity is measured in 5 amp, 20 hour cycles. That is, if a load draws 5 amps per hour the battery will last 20 hours. But 10 amps per hour is not strictly 10 hours, because the more amps are drawn, the faster the battery drains.
Which Battery Will You Use with the Inverter?
The type of battery you use with the inverter determines how long it can sustain the load. Some batteries have longer discharge rates than others, and some are more efficient working with inverters.
Lead acid batteries have a 50% discharge rate, so a 100ah battery only has 50ah usable for the inverter. With lithium ion batteries, you can use 70% to 90% of the capacity for the watt load.
Let us use the same example above. You have a 1000W inverter with an 85% efficiency rate and need to load 800 watts using a 100ah 12V battery.
Suppose it is a lead acid battery with a 50% depth of discharge.
800 watts / 12 volts / .85 = 78 amp hours
78 amps per hour is good for 1.1 hours. However, our battery only has a 50% usable capacity, so we have to divide 1.1 hours by 2, which is 35 minutes or so.
If you have a powerful lithium battery and a 90% efficient inverter, the results look like this:
800 watts / 12 volts / .90 = 74 amps
The load draws 74 amps an hour, but the bigger difference is the lithium battery. If it has a 90% discharge rate, you can run the battery until it is almost completely empty. In other words the load will last an hour or so.
Lithium vs. Lead Acid Battery
A lithium battery can last twice as long as a lead acid battery running the same load on the same inverter. Even if the inverters had the same efficiency rate, the lithium battery will last longer because it has a better discharge rate.
Of course lithium batteries cost more, sometimes twice as much as lead acid. But if you need to use 100ah, you will end up buying two lead acid batteries because each one is good for 50ah only. So the cost may even out in the end.
In the end it is up to you. If you find yourself recharging lead acid batteries frequently just to meet load needs, maybe it is time to upgrade to lithium . But if your battery bank meets requirements, there is no need to upgrade. For running large appliances however, lithium battery is the best option because of their longer life cycle.
What is the Maximum Load For a 1000W Inverter?
On paper you can load 1000 watts on a 1000W capacity inverter. But just as solar panels cannot always produce the power they are rated for, the same with inverters.
Because an inverter is not 100% efficient, it will not be able to run a full 1000 watt load. Inverters with a 90% efficiency rating can load up to 900W. Some very powerful systems like the BESTEK 1000W Pure Sine Wave Inverter may even be able to load 1000W, but that would be too much if done regularly.
There should always be reserve power available for inverters, solar panels, charge controllers and batteries. Voltages and currents have peaks and valleys, and a sudden surge could spike the wattage way higher than 1000 watts.
If the inverter is at its limit and there is a surge, it could damage the unit and every appliance connected to it. If the load is less than 1000 watts, there is enough reserve power to cope with the quick spike. A sudden watt surge can be disastrous especially if your solar panel does not have the right accessories.
Of course some inverters are better designed than others. If the manufacturer says it is all right to load a 1000W inverter, then take their word for it. As long as the inverter is covered by the warranty and everything you do is according to instructions, there should be no problems. It will also help to check customer feedback when buying an inverter.
Reminders For Running 12V Batteries with Inverters
The runtime always depends on the battery capacity and watt load. You can recharge batteries on electricity , but usable amps depend on the battery. As mentioned, lead acid batteries have a 50% DOD, but some AGM and gel batteries have better discharge rates.
Do not use the battery beyond the recommended depth of discharge rate. If the discharge rate is 50%, then recharge at 50% or higher. Using the battery below the rate could shorten the life cycle significantly.
Make sure the voltages match. Some 100ah batteries are available in 24V or 48V. Check if your inverter supports these batteries, as some inverters are designed to only work with specific voltages.
Do not overload the battery or inverter. Better yet, keep the load below the limit, in this case 1000 watts for the inverter. You can run more than 100 amps for the battery provided the draw does not exceed 100 amps an hour.
Pay attention to surge watts. Inverters have two important values, running and surge watt capacity. Motors and some appliances like refrigerators and AC systems have surge / starting watts that is 2 to 3 times higher than their running watt.
So a fridge labelled as 600 watts refers to the running wattage only. Its starting or surge watt requirement is probably 1200 to 1800 watts. Your inverter must be able to supply the surge watt and so does the battery bank. This applies not just to inverters but to solar generators as well.
Buy from a reputable manufacturer. Large inverters and batteries do not come cheap, even though prices have gone down. Get your money’s worth by purchasing from well known manufacturers who back up their products with customer support and good warranties.
Battery capacity limits for inverter. Some inverters can only handle a specific number of batteries. Refer to your inverter manual for the maximum number of batteries that you can connect to your inverter. Inverters have a charge current limit, and usually you should not connect more than 12 times the current maximum capacity.
In most solar panel systems, the inverter is only as good as the battery it is connected to. A 100ah 12V battery can last anywhere from half an hour to several hours depending on the draw. By planning ahead you will now exactly how many batteries are needed.