What Size Inverter You Need (Calculations + Battery)

Choosing an inverter can be tricky when working out all the various factors involved in this calculation. You want to be sure you have enough power to run all the different appliances when the power goes out and makes sure they run for as long as you need them to.

The size of the inverter required will be determined by the total wattage of the appliances you need to operate and the time they need to run. You also need to add a bit more on to compensate for the startup current and have a wattage ‘cushion.’

You would need to look at the following when sizing an inverter:

  • What is an inverter
  • How to calculate the total wattage
  • Surge current vs. typical current
  • Fuse and cable sizes
  • Inverter Vs. Generator

If you’re genuinely sick and tired of dealing with power outages, it’s time to find out what size inverter you need!

Solar inverters on a wall

What Is An Inverter And How They Work

An inverter is a device that converts DC power to AC power to replicate and produce electric current to power electrical appliances and devices. It is usually a rectangular-shaped steel box containing batteries, charging input, and AC output plugs.

It works by using a battery or series of batteries inside the casing that have been charged and then utilizes that DC electricity to provide power to AC appliances.

Inverters have a power rating in watts (W), which determines how much power they can supply, and the batteries have an amp-hour rating, which measures how much current (measured in Amps) they can supply for how long before they deplete.

What Are The Different Sizes Of Inverters

Inverters are made with different power capacities, depending on the size of the system you want to run.

For this discussion, we are looking at a domestic inverter that you can use to run a few devices if the power fails, and these are relatively small.

When looking at an inverter to run your entire home from a solar PV System, these are much bigger, but in essence, the principles behind the calculation are the same. Still, these calculations will be done by the PV system installers before they specify the system you need.

See also: What Is A Solar Inverter? (Explained With Examples)

What Types Of Inverters Do You Get?

There are two basic types of inverter, the modified sine wave inverter and the true sine wave inverter.

Now, don’t get put off by these lofty terms; they are simply a measure of the current quality the inverter will output without getting too technical; the better the current quality, the less likely the risk of damage to sensitive equipment.

  1. The modified sine wave inverter is the most common and least expensive and will provide a current good enough to run most appliances. These are also the most compact types of inverters available.
  2. True sine wave inverters will run an AC device and provide consistent, high-quality output. While these are more expensive, they are better suited for sensitive equipment like laptops, professional audio systems, tool chargers, and medical devices.

Best Selling Solar Inverters

Look at this using water as an analogy.

If you put palatable (drinkable) water through your water faucet and it has a lot of tiny dirt particles, the water quality would not be as good as water coming through your faucets that is already clean and without particles.

While either will be sufficient to use without risk, the water with more particles has a slightly higher risk.

So the cleaner and more consistent the quality of the wave output, the better and safer the devices will run, so if you can afford it, get a true sine wave inverter.

How To Calculate The Total Wattage Required

To understand what size inverter you need, you need to know a few fundamental values. The first one is the total wattage of the devices you use the inverter to run.

Every device, from your laptop to your cellphone charger and fridge, has a power rating in watts; of course, some are higher than others. To ascertain the size of the inverter you need, you first need to know precisely how much power your devices require.

To calculate the power rating of each device, you can look on the back and find the label that will give you the wattage of the device, or you can check the voltage (V) and current rating ( measured in Amps {A}) and multiply them to get the power.

For example, your mobile charger uses 110V and 2A , using this equation Watts= Voltage X Amps = 110 X 2 = 220W.

Now, you need to list all the devices you need the inverter to run and come up with a total wattage. Use the example below as a guide.

Internet Router50W
Appliance Devices and Wattage Needed

Using these numbers, the total is 1650W, but that is not the end. It would be great if this were easy and simple, but there are additional considerations when looking at the wattage.

The Wattage Safety Margin

Add a wattage safety margin – it’s better to add about 10%-20% onto the wattage to ensure that the inverter you get can handle the total load. So let’s add 20% to the total = 1650 x 20% = 330W = 1980W total.

Now, you have a better and more accurate idea of the size of the inverter you would need. You need an inverter to deliver a 1980W continuous output for this example.

That’s just the first part.

Now that you know the wattage, you need to determine how long you need them to run for, which needs a different calculation.

How To Calculate The Amp-Hours Required For The Battery

To finalize the specs on the inverter, you need batteries. The type of battery you will need and how many are based on how long you need the inverter to run them.

So for this example, let’s presume you need the devices to run for eight hours, but realistically, these devices won’t be running all the time as fridges run intermittently, so let’s assume that all the devices will run for 50% of the time only – so four hours.

We know that the power required is 2000W, but this is an AC rating, so we need to convert AC watts to DC amp-hours, as this is how DC batteries are rated.

You divide the total watts by the DC to convert AC watts to DC amp-hours. You can get 12V / 24V / 36V, but let’s use 12V as this is the most common.

We have 1980W / 12V = 165 amp-hours to give you the power requirement per hour for the devices listed.

Remember that these will run at 50% or 4 hours, so we multiply the dc amp-hours by 4 to get the total DC power needed = 165X4 = 660 DC amps per hour.

Factor In The Loss Calculation

No electrical system is 100% efficient, and there will be losses incurred, so when determining what size inverter you need, you also need to factor in the losses to ensure you have enough power.

The loss factor is usually accepted at 5%, so we multiply the total Dc amp hours by 5% to give you the final DC amp-hour figure.

660 DC amp-hours X 5% = 33 Amp hours + 660 DC amp hours = 693 DC amp hours total.

So now you know your batteries must deliver almost 700 DC Amp-hours per usage hour.

Battery 12 Volts

What Batteries Would You Need?

Deep cycle batteries come in either 12V or 6V options, and depending on the type of system and power needed, you could use either size effectively.

But, for this discussion, we will look at both.

Your system requires 700 DC amp-hours, and if you have a 12V battery rated at 100 DC amp-hours, you would need seven batteries to power your system, which would be connected in parallel.

If you are looking at 6V, you would first need to connect two batteries in series to get to 12V and then connect the pairs of batteries in parallel to reach the dc amp-hour requirements.

So if your DC batteries are rated at 200 DC amp hours, you would still need seven batteries as they need first to achieve the 12V current level, and you could have four sets of 6V pairs connected in parallel to power this system.

How To Charge Your Batteries

Your batteries will be charged from the electrical supply when restored, which will be done from a charger similar to the ones found in home-sized PV systems.

You can consult your supplier regarding which charger you need for your inverter system.

What Is Surge Vs. Typical Power?

To explain this, the surge is the maximum power your inverter can supply for a short time; this is the 100m sprint for inverters. You’d need to pick an inverter whose continuous rating can handle the surge of the appliances, or you risk burning out the inverter.

So a fridge running at 1000W would have a surge rating of 2000w, so you may need to consider an inverter of 2000W or more.

Typical power is the continuous rating or the inverter’s power to supply continuously – so this is the marathon side of the inverter. Once the appliances are running, the inverter must supply this total power to keep them operating for the required time.

OK, now that we have the technical elements done and you know how to calculate the wattage and DC amp-hours, you have a good idea of the inverter size you would need.

But there are a few more aspects involved here, and one of those is the cable rating and the fuse size.

Why The Fuse And Cables Are Critical

Don’t let the idea that because you are using DC batteries, there are not substantial levels of current involved here, because there are. To be safe, you need to look at the cable you will use to connect the inverter to the battery.

For inverters rated up to 3500W, the cable size should be 1/0 AWG, sufficient to handle the startup and continuous current required.

Another consideration is the inline fuse, as this will protect both sides of the system in the event of a shortage in the system. To ascertain the fuse you need, divide the AC wattage by the DC Voltage.

In this example, we have 1980W / 12 V = 165 Amps. You can use a 200A fuse for this system and be safe. It’s always advisable to have a slightly bigger fuse than one that sits at the amp level to accommodate any additional current.

With a fuse rated at the exact spec, any slightly more current will blow it, and you may find yourself going through fuses quicker than you’d like.

Should I Choose An Inverter Or A Generator?

The answer to this depends on the load levels you want to run and how much noise you (and your neighbors) are prepared to put up with.

  • A small inverter is suitable for running appliances with a total load of 1000W, while bigger loads might require either a larger inverter or a generator.
  • Aside from the inverter itself, your highest cost will be good-quality deep-cycle batteries, and the more you need, the more it will cost you.
  • Generators are cheaper and easier to maintain, with spares costing less, but they are noisy and not that easy to move around. Plus, they require proper installation to connect to your home electrical system.
  • Inverters are quiet and can store electrical energy, while generators only generate power when running.
  • Inverters don’t pollute the air and can be charged from a solar PV system if you have one. They deliver better quality current than generators, which is good for sensitive electronic equipment.
  • Inverters provide immediate backup and have greater operational efficiency and lower running costs.

To determine which would be better, you would need to consider your load needs and balance the cost of each option against the medium and long-term usage based on the stability of the electrical supply in your area.

If you are looking to get an inverter, you have all the tools you need right now to get going and take the first steps to understand what size inverter you would need.


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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