How much Battery Storage do I need for Off-Grid Solar?

07 Sep 2022

 

An often-wondered question. Though it may seem simple enough, whatever your energy requirements are for the period that you’re not generating power, it can be deceptively difficult to determine. What’s your plan if you don’t have consistent sunlight for a couple of days? How much of your energy usage occurs during daylight hours versus the evening? Is there a battery technology that’s better for storage? Plenty to consider, so let’s have a look!

 

First, let’s look at what your actual total usage would be over a period. Depending on whether you’re connected to the grid currently, you can just use your usage that your electricity company provides you, which will be over a quarter. Otherwise, you’re probably powering your premise with a generator, and as such you would most likely be able to determine your approximate usage from your fuel consumption and specific generator’s fuel efficiency.

Once we have these figures, we’re looking to reduce them down to a daily average. Using my own usage as an example, over the quarter my household used 1159kWh, and the quarter covered 94 days. This gives us a daily consumption of 12.32kWh.

To recap, our algorithm currently looks like “Usage over x number of days ÷ x number of days = Daily usage.” Pretty simple!

 

Now that we have this figure, we can look at how much we are using during periods where your panels are unable to generate sufficient energy to power your household e.g., night time. Simply, this is done by determining an approximate percentage. As an idea, a fair estimate would be considered to be 30% used during the day, and 70% during the evening for a standard residential address. A business premise will of course be different to this, however would be determined by your opening hours and total usage.

So, bearing this in mind, we’re going to work with 70% for our example, giving us a total of 8.624kWh, used outside of daylight hours. Per evening, I’m using approx. 8.624kWh!

 

As a final part of our equation, we need to look at how many days’ worth of storage we would need. When looking at this, think about your area. During the rainy season, what’s the likelihood that you wouldn’t get sufficient sunlight to charge your batteries for a couple of days? What about 3 or 4 days? In periods of heavy rainfall, you may go a week or more without enough sunlight to charge them depending on your area. This isn’t to say that most people need that much battery storage. Generally, we would recommend a minimum of two days of storage capacity.

Looking at my example, that would be about 17.25kWh. Considering how close we are, I would probably push that up to 20kWh of storage, however, you’re probably going to be bound by what increments your preferred battery comes in. We’ll go into more detail on that in a moment, but first, let’s recap our equation!

In order to determine your battery storage requirements for off-grid solar, use the equation: “(Daily usage kWh x night-time usage %) x storage days # (min 2)”

 

Okay, finally, let’s talk about battery types and how that can affect your choice and storage needs. When looking at most modern solar batteries, you will see a lot of lithium batteries. Occasionally, you can see AGM, Gel or Flooded batteries used for solar applications, however the market seems to be primarily moving towards Lithium as the gold standard. There are a number of reasons that lithium is considered superior for this application, but the major reason is usable capacity.

When looking at batteries, often you will see the capacity described in amp hours (AH), however this is somewhat misleading. Different battery technologies will have different percentages of capacity that they are capable of discharging to. With lithium batteries, they can discharge significantly lower than what other battery technologies are capable of. On top of that, when compared to other technologies, lithium is capable of a significantly high number of cycles before the capacity is depleted. As such, theoretically, these batteries are capable of longer lifespans as compared to competitors.

 If you’re looking for a recommendation, I would recommend the Pylontech US5000Cs! Ongoing testing indicates that these batteries lose very little capacity over thousands of discharges, and have an impressive 5000AH capacity!

 

There you have it, that’s how you work out an estimate for your battery storage requirements for off-grid solar. Unfortunately, if you’re not currently connected to the grid, you may have a harder time determining your storage requirements as you’re relying upon petrol consumption as your guide, but this should still be able to give you a pretty good idea of what you will need to power your home.