Just like for Solar panel sizing, battery sizing begins with first working out our typical energy usage per day. For this you can refer to the article How much Power does 12Volt stuff use?
This figure of Ah in a typical day is essential for determining both battery size and solar size, so it’s worthwhile spending a bit of time to make sure it’s reasonably accurate.
In the post on How much Solar? we used a typical example consisting of a fridge, a TV and some LED lights. We’ll use the same example here, so our typical day’s consumption is going to be 45 Amp-hours per day. You can refer back to that post for details of how we got to 45 Amp-hours per day.
This figure gives us a practical example to work with, so we can now determine what size battery is going to suit the 12 Volt system.
The battery’s job is to store the solar energy we have gathered, for night-time use and for when the panels are not producing enough to support the 12Volt system. A rule-of-thumb that has stood the test of time is to provide enough energy in the battery to last 3 days.
Without going into the technicalities, we originally arrived at this figure after statistically analysing the patterns in solar irradiation data. This 3-day figure provides the battery capacity to ride through a typical succession of cloudy days. Over the years this figure has proven to be both practical and reliable, and designs based on this have resulted in realistic and dependable solar-battery systems.
Of course there will be some debate around this figure, and those wishing to go bigger or smaller will find no argument from me. I’m just passing on what I have found to work well in practice.
So 3 days’ typical usage in our example is simply 3 x 45Ah = 135Ah, and that’s the size of the battery we need to support our 12 Volt system properly. It really is then as simple as looking for the nearest size battery to that, and deciding whether you have space to go bigger.
This 3-day figure is also handy in that the typical day’s usage will only take one-third of the energy from our battery, so most of the time our battery will be 60-70% full. This is useful in terms of extending the battery life of your 12 Volt system. This is also covered in Batteries: their quirks and characteristics, but it’s worth bringing in battery life here too, when we’re deciding on the size of our battery.
Some have asked why we can’t simply put in a nice small 45Ah battery which would cover the typical day’s usage in this example, and that should be just fine? Well you can, but here’s the rub. A lead-acid battery has what is known as a cycle-life, and typically it will last 250-350 cycles of being fully-discharged and then fully-charged again.
So if this battery is in daily use in an off-grid shack, then a full cycle will happen every day, and our battery will be gone in less than a year – ouch! Of course in a caravan or motorhome which is only used occasionally, the effect won’t be so bad, but you get the point – undersizing just buggers your battery life.
Now, if we only discharge our battery only half-way down each time, the cycle-life doubles. And if we only discharge it to one-third down, then we can double that figure again – so now we’re well above 1500 cycles. So even doing that every day, we’d now be looking at 4 to 5 years of battery life – much better!
If we now look back at our battery sizing for 3 days of typical usage, we can see that on a normal sunny day we will use up just one-third of the battery’s capacity. This puts us up at the high end of 1500 cycles of battery life. So by sizing our battery using the 3-day figure we have automatically done ourselves a favour in terms of longer battery life too. All good.
In short, when it comes to battery life, bigger is better! Of course practicalities like size and weight also factor in, and of course cost. And this is where personal choice takes over, so I’ll confidently leave the rest to you to decide.
Batteries and battery types, plus things like connecting them in series and parallel, are discussed in more detail in the section on Batteries: their quirks and characteristics.
So there we have it – we now have a way to size the batteries so that they keep our 12Volt system working reliably, even when our solar is not producing power.