How to Calculate How Long an Inverter Will Last

Whether you are on the grid or off, the inverter plays an important in any solar system. It converts solar energy (DC) in the battery into AC so home appliances can use it. But how long can you expect an inverter to last? Some math is needed but it is a simple process actually.

Divide the inverter watts by battery voltage to get the amps, then divide the amps by the inverter efficiency rating. Divide the result by the amps and you get the inverter runtime.

How to Calculate How Long a Battery Will Last on an Inverter

An inverter draws its power from the battery so the battery capacity and power load determines how long the inverter will last. Regardless of the size, the calculation steps are always the same.

1. Watt load / volts = amps
2. Amps / inverter efficiency percentage = amps
3. Amps / available battery amps = inverter runtime

Using this calculation, a 24V inverter with a 100ah battery and 93% efficiency can run a 500W load for 2.3 hours.

You have a 24V inverter with a 150ah deep cycle battery. The inverter is 93% efficient. You want to run a 700 watt load, so how long can the inverter run this?

700 watts / 24 volts = 29.1 amps
29.1 amps / .93 = 31.2 amps
75ah / 31.2 = 2.4

The inverter can run a 700 watt load for 2.4 hours. Notice that we divided 31.2 amps with 75ah, not 150ah. That is because a deep cycle battery has a 50% discharge rate (DOD) so only 75ah is usable.

If you have a new AGM or gel battery the DOD can reach 70%. For lithium batteries you can fully discharge it without causing damage. The formula above still works though, just replace the usable amps with whatever applies for your battery. If you want an AGM battery there is the Mighty Max 12V 100ah. This is going to work with all 12V inverters and last a long time.

Inverter Efficiency Rating

The efficiency rating determines how much electricity can be converted from DC into AC. Most inverters have an 85% efficiency rating but newer models are at 93% or higher.

There are two main types of inverters , pure sine and modified sine wave. Pure sine wave inverters have a higher efficiency rating and ideal for high powered loads.

You can use a modified sine inverter for simple electronics and low power loads. For higher power watt loads and modern appliances, a pure sine wave inverter should be used.

Note that pure sine and modified sine wave inverters have different specs and efficiency ratings. Most pre sine wave inverters average around 93%, but some are in the low 90s while others are up to 98%.

How Many Batteries are Needed For My Inverter?

So if the inverter powers appliances using the battery, how many do you need? Well it depends on how long you want to run the load.

Suppose you have an off grid system and want to run a 1000 watt load for 4 hours. Your inverter has a 1500 watt capacity so it can handle the load, but what about the battery bank?

An inverter needs four 100ah 24V batteries to run a 1000 watt load for four hours. This runtime assumes that the batteries have a 50% DO and that you will be running the full load for four hours.

Here is how we came up with these numbers:

100ah x 24V= 2400 watts

But only half of this can be used in a deep cycle battery, so:

2400 watts / 2 = 1200 watts

Factor the inverter efficiency rating and the available capacity will be around 1000 watts.

1000 watts is enough to run your load for an hour. To run it in four hours, you need four x 100ah 24V batteries.

If you prefer to use amps instead of watts, the formula is:

Total amps drawn per hour x operating hours + 100% = battery size

If the total power load is 30 amps an hour and you need to run it for 4 hours, multiply 30 x 4 = 120 amps, plus 100% equals 240. You need four 240ah deep cycle batteries to supply the power for 4 hours.

How Much Battery Reserve is Needed?

If you have five 24V 200W solar panels, the battery should be 50ah 24V at least. Its capacity should be at least equal to the maximum daily output of the solar array. With a fully charged battery you can use it in lieu of the solar array during a rainy day.

This is the minimum requirement. If you expect 2 to 3 days of rain and want to use your inverter, the battery capacity has to be at least 3000 watts. And that is only to cover the day, not night. If you want to use the battery bank as a backup power, calculate how much capacity you will need.

If you are on a grid tied system you can run an inverter and solar system without batteries. You have access to the main grid which assumes the role of the battery. Any excess solar power produced goes into the grid. You can use these anytime you want and receive credits for it.

If you are off the grid or in an RV, you need a battery bank or a generator (or both) You can run an inverter on solar panels, but only as long as there is sunlight. After that you need to use the battery, generator or another power source. This will not be an issue if your RV spends most of its time in the campground because there is power available.

What Inverter Size Do I Need?

The rule of thumb is the inverter capacity should be at least 25% larger than the appliance watt load. The higher the appliance load, the more reserve power you should have.

Total watt load + 25% = inverter size. If you are going to run a 400 watt load that would be 400 watts + 25% = 500 watts.

Of course you can use a larger inverter capacity, but only if you expect to need more power in the immediate future. Buying an extra large inverter would be a waste of time and money, so look for one that closely matches your needs.

Most inverters are 24V, so make sure your batteries have matching voltage. If you want to run a solar panel system, use either 24V panels or connect 12V panels in a series to increase the voltage. Make sure that all the components are compatible with each other before connecting any appliance.

Inverter Frequently Asked Questions

How Do I Know if My Inverter is Working?

Your inverter has an indicator light (usually green) to signify that it is running. If you connect a load to the inverter and it runs, then the inverter is drawing power from the battery and works.

How Do I Know if the Battery is Full?

All solar batteries have indicators or icons that will notify you when it is full. It might emit a sound, while others will simply refuse to accept any more power when it is full, so you can leave it connected to the power source without causing damage.

Why is My Inverter Not Working?

The first thing you should check is the wiring. Even a slightly pulled wire can halt its operation. Replace any loose or damaged wires immediately. If the wires are all right, disconnect the inverter from the system and wait five minutes then reconnect it.

If the inverter is connected to a solar system, check the rest of the wirings. Make certain the wires are grounded to avoid any potential accidents.

Should I Turn Off My Inverter?

Most inverters can run 24/7 without a problem. If you run your appliances from it, you should not turn the system off. Otherwise you will have to reload everything when you turn the inverter on again. The only time you should shut off the system s if you will not be using it for long periods (for example, you will go on vacation).

Does an Inverter Draw Power When Turned Off?

Inverters should not draw power when they are turned off. However, most appliances have a standby mode that does use power. So even if you do not use the appliances, this mode will consume small amounts of power and drain the battery. For more info, check this guide.

Where Should I Place My Inverter?

Set the inverter in a shaded area away from direct sunlight. The inverter, charge controller, batteries and solar panels should be as close as possible. The closer they are, the shorter the wires used, which reduces energy loss.

Can an Inverter Catch Fire?

It is unlikely with modern inverters, but could happen with older models. An overloaded inverter may not explode or burst into flames, but it will damage whatever appliances are connected to it. This is why you need to use the right inverter size.

How Do I Stop My Inverter From Making Noise?

Some inverters produce a slight humming noise, but it is very minimal and difficult to hear. Others have an option that allows you to disable the sound, which should be in the product manual.

Conclusion

A lot of your questions about inverters are answered in its product manual. Because inverters come in various sizes, the specs vary so do not compare one system with another. The manual should also tell you what type of battery works best.