Finding the right battery size for your home

Size matters when it comes to batteries. Too small, and a battery won’t offset enough evening electricity consumption. Too large, and you’re paying extra for capacity that’s never used.

Both sides of the battery size spectrum have pros and cons, so let's break it down so you are better equipped to understand the size that would work best for you.

To properly size a battery you need to know your consumption patterns

Before jumping into quotes and rebates, the first step is understanding how you use energy throughout the day. It's the first question on the battery journey: Do I actually need a battery?

The graph below shows the real consumption and solar export patterns of a home I analysed. These are their energy patterns averaged into a single day. What we see is a typical pattern, an overabundance of solar during the day with peaks of grid consumption in the morning and evening.

At first glance, it seems that a battery could help this home. But we're missing one more puzzle piece, the price of their energy and the time of day their pricing changes. For this home, most grid usage occurs during their off-peak period, which runs from 0:00 to 15:00 and again from 21:00 to 0:00. Therefore, offsetting the consumption that occurs between 15:00 and 21:00 would yield the greatest savings.

Fortunately for this home, its grid consumption is moderately low during peak hours, but this also means the potential savings of a battery will take a hit. The ideal scenario is having larger peaks during the peak period, which is where a battery would really shine, offsetting higher demand at peak times.

But maybe there is still enough grid consumption happening in off-peak hours for a battery to be worth it, it's just not quite a slam-dunk analysis. So we need to keep digging deeper.

Simulating how the battery could behave for every hour of the year

This is where it gets more interesting.

Now we use the historical consumption and export data of the home for every hour of the year to see how a battery would perform through hundreds of charge and discharge cycles. We want to know how much of the home's energy consumption a battery could offset. Then, finally, how much is that offset actually worth in cents?

We ran a simulation to see how a 10 kWh SolarEdge Home Battery would perform in the home to calculate how much money the home could earn per month based on its consumption.

We also ran multiple other simulations for a Tesla Powerwall 2, a Powerwall 3, and a Sigenstor 16 kWh system. For each battery, we modelled charge and discharge patterns throughout every hour of the year.

At first we tried to optimise the battery to only discharge during the time when the home was paying peak prices. But it quickly became obvious the battery was being underutilised. In summer, the battery was barely discharging before the peak period ended. Below you can see the green lines sit mostly at the top of the chart to the right, this is because in the summer months the battery is not discharging fully before the next charge cycle begins.

What we really want is a graph that is full of green, which is a battery that is being utilised to its fullest extent. Charging up to its full capacity in the afternoon and fully discharging each night so the home can offset as much of its grid energy as possible. So we removed the restriction for the battery to only discharge during peak pricing hours.

A fairly dramatic change, the battery now has far deeper discharge cycles in summer and overall the battery is being utilised far more often. A change that translates into more money saved for the home. One takeaway here, a battery can be over-optimised for peak pricing especially in Summer, when excess solar energy is in abundance don't be afraid to let the battery discharge well into the night. This battery was also forecasted to save the home more money as it was due to offset more of their grid consumption.

Battery utilisation and the state of charge over the year

This is one of the more useful but less talked about parts of battery analysis, utilisation.

The utilisation rate gave us insight into whether the battery would just sit idle or do some real work across the seasons. We discovered that with the higher utilisation rate, for this home, a 10 kWh battery could save between $615 and $800 per year, depending on how the home was consuming energy. Pretty good, but not through the goal posts either.

It’s also worth noting that as feed-in tariffs continue to drop rapidly. It’s more important than ever for the battery to increase the home’s self-consumption, reducing reliance on peak energy rather than chasing solar feed-in credits.

It's more important than ever to find a battery that is the right size for a home. Because a battery that is too small will result in missing out on offsetting peak consumption. Too large, and the battery spends long stretches just sitting idle. Understanding the utilisation helps expose that sweet spot in between where you don't go too far in either direction.

How to understand how a battery can benefit your home

If you're interested in understanding how a battery system might benefit your specific situation, we can help. We can analyse your energy consumption and solar exports for the past two years and calculate the potential savings from a battery. If you already have quotes from installers feel free to send those over. If not no worries, we can help you understand which batteries out there are going to be around the right size for you and your home. To get started we don't have to visit your home or connect a device, we can access your energy remotely by connecting to your current energy provider and requesting the consumption data from them.