The most important consideration for the correct sizing of your battery bank is your estimated Load Evaluation.

This is an estimate of how much electricity your home is likely to use over a typical 24-hour period. For an off-grid system, we’re particularly interested in what that looks like during winter.

Getting this reasonably accurate is important. And, after literally doing hundreds of Load Evaluations from Stewart Island to Pukehui, we’ve gained some serious insights along the way that we can pass on to you.

If we underestimate your electricity use, we risk designing a system that is too small. Overestimate it substantially and you can spend thousands of dollars on equipment you may not need.

There is quite a bit involved in developing a good Load Evaluation, so we won’t repeat it here. You can read our detailed guide to Off Grid Load Evaluation.

For this article, we’ll assume we’ve already done that work.

Let’s use a fairly typical example of an energy-efficient off-grid home using gas for hot water and the cooking hob, and maybe a wood burner for heating.

We know that might not apply to you, but let’s just use that as an example for now.

Let’s go with estimated winter electricity consumption of 8 kWh per day.

The next question is: when, over a full day in winter, is that 8 kWh being used?

Once we have an estimate of how much electricity you will use each day in winter, the next thing we look at is when over the course of the day you are likely to use it.

During daylight hours, your solar panels will normally supply some or all of the electricity being used in your home directly. As your panels produce less and less as the afternoon progresses, your home increasingly draws electricity from the battery, which then supplies power to your home’s fuseboard to run your nighttime and overnight loads.

Let’s go back to our example of 8 kWh per day.

If we estimate that around 40% of your electricity is used during daylight hours, that leaves approximately 60%, or 4.8 kWh, being used outside the main solar production period.

Battery Sizing for After Dark
That 4.8 kWh is important because it gives us an estimate of how much energy the battery needs to supply before the sun comes up and meaningful solar production starts again.

Of course, not every household uses electricity in the same way. Someone who works from home may have a very different consumption pattern from a household that is empty during the day and busy from 5pm onwards.

So even if both homes use 8 kWh per day, their battery requirements may not necessarily be the same.

As a general rule, the more of your normal electricity use that happens while your solar panels are producing power, the less energy needs to be stored in your batteries for later. This is one reason why when you use electricity can influence the amount of battery storage you need.

Next, we need to look at where in New Zealand the system is going to be installed.

If your curious about the different sizes of off grid solar battery banks, take a look at our Off Grid Systems & Pricing Page.

Where you live in New Zealand also plays a part in deciding how much battery storage you need and sizing.

Solar production varies considerably around the country, particularly during winter. A system in Nelson or New Plymouth will have different winter sunshine hours from the same system installed in Southland.

For off grid solar battery sizing, what we’re interested in is how reliably the solar panels can put power back into your batteries after you’ve used it.

This becomes particularly important during periods of poor weather. Your batteries may have comfortably supplied your home overnight, but if the next day is dark, they may not get fully recharged before you need them again that evening.

That’s why we don’t look at battery capacity in isolation.

Your location, solar panels and batteries all need to work together as a system.

We’ll look at the relationship between the solar panels and batteries next.

s mentioned earlier, it can be tempting to think about battery size on its own, but your batteries are only one part of the system.

Batteries store power. They don’t create it.

Once you’ve used some of the power stored in your batteries, your solar panels need to put that power back again.

This means the amount of solar you have available to recharge your batteries is also part of the battery sizing discussion.

Adding more batteries to give you a larger total battery bank can give you more backup, but those batteries still need to be recharged.

This is really important in winter, when there are fewer sunshine hours available each day.

So when we’re deciding how much battery storage you need, we’re also thinking about how many solar panels you’ll realistically need to keep that battery capacity charged.

Next, let’s put the pieces together using our 8 kWh per day example.

Let’s go back to our example home using 8 kWh of power per day in winter.

We estimated that around 40% of that power would be used during daylight hours, leaving approximately 4.8 kWh to be supplied by the batteries overnight.

If the next day is dark and the solar panels produce very little power (or even no meaningful power, which is not unusual), our example looks like this:

• 4.8 kWh to cover the normal overnight use
• 8 kWh to cover the following day if there is very little solar production
Giving us about 12.8 kWh of usable battery storage

That gives this example home roughly one day of battery backup, based on the way we’ve estimated power will be used.

A different household using the same 8 kWh per day could end up needing an extra battery module because of their location or when they normally use power over a 24-hour period in winter.

And of course, you can add more battery storage to give yourself more backup.

The question then becomes: how much backup do you actually want — and how much is it sensible to pay for?

That’s where battery sizing starts to get really interesting.

epending on the size of your inverter/s (which is a subject for another day), you can keep adding batteries to increase the amount of backup your system has.

In our example, we have allowed for roughly one day of battery backup. We could add more batteries and design for two days, three days or even longer.

Your whole battery bank works together as one battery storage system. In normal day-to-day use, you’re only using part of its total capacity overnight before the solar panels start charging it again the next day.

As you increase the size of the battery bank to provide more days of backup, that additional storage capacity may only be needed during longer periods of poor weather.

So you’re progressively spending more money on battery capacity that may be used less frequently.

Solar Panels & Battery Sizing
There is another consideration too. The more battery storage you have, the more energy you need to put back into it. In some situations, increasing your battery backup may also mean increasing the number of solar panels so you have enough solar available to recharge those batteries.

That’s not necessarily a bad thing. A larger battery bank means you’re normally using a smaller percentage of its total capacity each day. This can reduce the depth of each battery cycle and potentially extend the working life of the batteries.

You can learn more about battery cycles and battery lifespan in our in-depth article about off-grid solar batteries.

But there is still a price versus benefit trade-off. As you add more storage, you’re spending more money on battery capacity that may only be needed occasionally, and the additional benefit becomes harder to justify.

None of this means having more battery backup is a bad idea. You may simply want more autonomy and are prepared to pay for it.

At some point, it can make more financial sense to have enough battery storage for your normal needs and use a backup generator occasionally during longer periods of poor weather.

We’ve written an indepth article about off grid solar generators.

The best system isn’t necessarily the one with the most battery storage. It’s the one that gives you the level of backup that makes sense for you and your budget.

here isn’t one right answer to off-grid battery sizing.

Two homes using the same amount of power each day can quite reasonably end up with different battery banks. Where you live, when you use power, the number of solar panels you have and how much backup you want all play a part.

That’s where our Free Design Service can help. It can give you peace of mind that these things have been considered before you decide what system is right for you.

Get in touch to learn more.

We also don’t believe off grid solar battery sizing should start with a calculator or a standard number of batteries.

That might be a fair starting point, but as you’ve seen, there are other factors to consider.

We start with understanding how you intend to live off-grid.

From there, we can estimate your power use, work out when you’re likely to use it, take your location into account and then design the battery storage and solar panels around your actual needs.

The goal isn’t to give you the biggest battery bank possible.

It’s to give you enough battery storage to live comfortably off-grid, without spending thousands of dollars on capacity you may not need.