What Size Solar Batteries Do I Need? | A Complete Guide to Sizing Solar Batteries

In the most optimal conditions, the size of the solar battery you need is going to be equal to your energy consumption. So, if you use 10 kWh of electricity per day, you’re going to also need a 10 kWh battery.

That is, of course, the oversimplified version. Many other factors that come into play when sizing your home’s battery bank needs. This includes the battery’s depth of discharge (DoD), your budget, how many days of backup power you need, and other

To streamline your process, I’ll walk you through these considerations and teach you how you can compute what size battery you need for your solar panels. Let’s start.

Factors you should consider when sizing your home’s solar battery:

Your home’s average daily energy usage

There are 2 ways to calculate this:

• Using your electricity bill: Divide your monthly, quarterly, or yearly bills by their corresponding number of days. For example, if you’re using your yearly bill, divide by 365. If using your monthly bill, divide by 30.
• Using a solar monitoring system: Some solar systems will have this, generally through an app on your phone where you can monitor either your solar energy produced, solar energy consumed, or both.

For your reference, the average Australian home consumes 8-12 kWh of electricity per day, depending on the size of the household. The more people who live in your home, the more electricity you will consume.

The size and output of your solar system

Among other things, such as direction, angle, shade, efficiency, and others, the size of your current solar system determines the solar energy it’s going to generate.

This is important to consider since solar batteries can only store the energy that your modules generate. The larger your system, the larger the battery bank you’re going to need.

Recommended: How many solar panels do you need to power your home?

Your prospect battery’s depth of discharge (DoD)

Depth of discharge (DoD) is the depth at which the battery bank can be discharged without shortening its lifespan. For example, let’s say you’re looking at a 10 kWh battery with a DoD of 80%. That means you can safely use 8 of the 10 kWh without harming your battery.

This is important to consider because, well, batteries are expensive, and you need backup solar power. Therefore, you’re going to need your solar batteries to last as long as possible while having ample energy for your home.

How many days of backup energy do you want?

Before you answer that, why do you want to have a battery for your solar panels anyway?

Is it because you’re living off-grid? Is it because you want to offset the expensive peak power rates from your energy company? Is it so you’re ready in case of a blackout?

No matter what your answer to that question is, it all boils down to how long you want to have solar energy.

If it’s because you’re living off-grid or that you live in a place where blackouts are frequent, you’d likely need a solar battery bank that’s large enough to store a couple of days’ worth of solar power.

If it’s just so you don’t rely on the grid during peak daylight hours, you should be good with a day’s worth of backup.

Your budget

The cost of installing solar batteries in Australia can range anywhere between 1000 and 2000 AUD per kWh.

• 1000 AUD if you’re installing a battery alongside your new system
• 2000 AUD if you’re adding a solar system battery to an existing setup.

Regardless, how much money you can spend is perhaps the most concrete limit on the size of your solar battery.

Recommended: Are solar batteries worth your money?

How to calculate the solar battery size your home needs

This is the culmination of all the factors I just described above. So, if you haven’t read them yet, I highly suggest you do. Otherwise, you can simply use our calculator to do the computations.

If you want to do some manual math, though, here’s how to do so:

Step 1: Compute your adjusted energy requirements based on your DoD

As previously stated, your solar battery’s DoD determines how much-stored energy you can use before you shorten your battery’s life.

Therefore, for the longevity of your solar system battery, I recommend using that depth in your computations instead of just relying on the battery’s listed size or usable capacity.

That being said, use this equation to compute your adjusted energy requirements:

• Equation:
  • Daily energy usage + [Daily energy usage x (100% – DoD)] = Adjusted energy requirements
• Example computation:
  • 10 kWh + [10 kWh x (100% – 80%)]
  • 10 kWh + [10 kWh x 20%]
  • 10 kWh + 2 kWh
  • 12 = Adjusted energy requirement

Step 2: Multiply your adjusted energy requirement with how many days you need backup power

If you only need backup solar power for one day, you can skip this step. Your adjusted energy requirement will already be the size of the solar battery you need.

If not, you simply have to multiply the days you need backup power by your adjusted energy requirement. Yup, it’s that easy.

To make it even simpler, though, here’s the equation and an example:

• Equation: Adjusted energy requirement x days of backup power = Solar battery requirements
• Example: 12 kWh X 2 days backup power = 24 kWh

Sample solar battery requirements

Before you read further, the values below assume a few things:

1. Your prospective solar battery has a DoD of 80%. I’m assuming this because modern lithium solar batteries have DoDs of 80–95%.
2. Your current solar system is large enough to fully charge your solar batteries.
3. Your energy usage is the same as that of the average Australian household. The averages I’m using for my computations are based on CSIRO’s data.

That being said, the table below should give you a rough idea of what size solar battery your home needs.

Remember that this computation is a simplified version.

You should only use the values and computations listed above as references.

This is because there are still other factors that are specific to your solar system that I can’t possibly account for, including the efficiency of your solar panels, the size of your solar system, shading issues, your home’s usage patterns, and other things.

For a more accurate estimation, I highly recommend speaking with a qualified installer. That, right there, is where we can help.

We have a network of CEC-accredited solar installers who, needless to say, are experts in solar batteries and other components. Whenever you’re ready, we’ll connect you with our installers and get you 3 FREE quotes for free.

FAQs:

Conclusion

The equations weren’t too hard to follow, were they?

In any case, I hope this helped clear things up for you. But, more than that, I hope you now have a rough idea about the size of the battery for your solar panels.

To recap, you first need your average daily electricity usage figure from your electricity bills or your monitoring system. Next, you need to adjust that value based on your prospective battery’s DoD. Finally, multiply that value by how many days of backup solar power you need.

If you still find it confusing, don’t worry. We can always connect you to our network of pre-vetted installers. They’ll have you sorted in no time.