The size of your battery should be based on how much energy you use at night, not your solar system size.
You’ve had a solar system installed for a little while, and you’re wondering how big a battery you would need.
The number of batteries you need is dependent on how much energy you use at night. Usually, we would think that it is tied to the size of the solar system. But this is not the best way to size your system.
Typically, your solar array is sized to how much roof space you have, and how much money you can afford. The maths shows that even if you oversize the solar array and generate far more than what you need, the payback period is still short enough that it is a good investment.
So the solar system size is usually independent of your energy consumption.
And the one big problem is that the panels cannot generate energy while the sun is down. So we want to charge the battery during the day, and use the energy at night time.
But first, we need to find out when night time is.
Daytime and night times are different between seasons. Summer has the longest daytime, and winters have the shortest.
According to https://www.timeanddate.com/sun/australia/brisbane, Daylight is
So any energy use outside of those times will be energy taken from the Grid, and not your panels.
As a result, the best sized battery would be one that will sufficiently support your night time energy needs.
But how do we know how much energy you use at night?
There are 2 main ways to find out how much energy you use at night. You can get your data from your smart meter through the energy wholesaler, or through your solar monitoring app.
The first and most accurate way is to download the data from your electricity distributor if you have a smart-meter. In Queensland, there are only 2 distributors.
If you don’t know who your distributor is, you can find it on your energy bill.
Once you find out who your distributor is, you can create an account with them, and download your latest energy use. This in usually in a .csv file and has your energy use per 30 mins, and looks something like this:
So from understanding your smart meter energy table, you can see exactly how much energy you use during the night time when no energy is being generated by solar.
This is probably the most accurate you can get.
The second option is to get the monitoring data from your solar monitoring device.
This is an example of what you might see from a solar monitoring app such as SolarEdge Monitoring
Just from the top right hand corner, you can see how much you have imported from the Grid. In this case, this person imported 100.66kWh in the span of 1 week.
If most of your days are roughly the same, then you can divide that power by 7 and that’s how much energy you would import on a daily basis. In this scenario, it would be 14.38kwH per day.
Taking the previous example further, we notice that they are consuming 14.38kwH per day that they are not generating with panels. This will be mostly at night time.
For this person, we would size an array of batteries that would cover for around 17.5kwH. This is 20% on top of their expected usage.
We really do not want to undersize the battery for their use.
The reason is that undersizing batteries increases the risk that you may drain the battery to flat.
Typically, this does damage to lithium ion batteries, which most home batteries are made of.
Nowadays, most batteries have safety features that ensure they are not completely drained, and have a minimal amount of charge remaining to reduce damage.
But we don’t want this to happen consistently.
In addition to a flat 20% buffer, there are other factors that you should consider when sizing a battery for your solar array.
Batteries are still very expensive. To most people, their return on investment is too long for them to consider it a good place to put their money.
As a result, we should always put budget as a big reason for thinking about how big the battery should be sized to.
There are ideal figures, which are at least 20% of usage at night. But there are also realistic figures, which some people may simple not be able to afford.
You should also consider the number of days that your batteries can’t be charged. You need extra batteries to handle a heavier load when it becomes vital to power some specific devices for a particular timeframe without any form of disruption.
For example, you could be living in areas that have high rainfall, consistently overcast and cannot generate the energy you need during the day.
Where you live has a big impact on the size of battery is suitable. If you live in a hot or humid area, this impacts on the heat dissipation of the battery. It also affects the efficiency of the battery changing from AC to DC, or backwards.
Here’s some further reading if you want to know where to install the battery for ideal operating conditions.
The discharge rate is how quickly your battery can discharge the energy inside. The slower your solar batteries may drain, the more time you'll get from them.
The equipment or appliances you have has a big impact on this.
The discharge rate of batteries may be determined by checking for the number indicated (C-?). If the inscription reads (C-8), it indicates the battery will completely discharge in eight hours. If the inscription reads (C-10) it implies the battery will entirely drain in ten hours.
Off-grid systems are a completely different kettle of fish. If you plan on being completely off grid, this section is for you. There are many other factors for consideration, but this will give you the basics.
Previously, we talked about sizing the battery based on an extra 20% of your load as a buffer.
This won’t be enough. Because we still assume that we are always connected to the grid, and if we do drain the battery flat, we can always rely on the grid to help us out.
For off-grid systems, we cannot rely on this anymore.
In addition, we have to make a large assumption that our solar system is sized extremely conservatively and produces much more than what we would typically consume during the day. Otherwise, an off-grid system will not work.
Taking into account all of this, we would recommend oversizing your battery by about 200-300%.
So taking into example of the user before that imports 14.38kwH per day, we would advise a minimum battery of at least 28kwH, and preferably 42kwH.
You oversize off-grid solar systems by an extra battery capacity of 50%
Sizing a battery for your home is not depending on the solar size array. In fact, there are some homes that have batteries but do not have a solar system.
Rather, a battery size is dependent on a number of other factors. The most important being how much energy you use at night time.
Calculate how much energy you use, with either of our two accurate methods, and add 20% on top.