Solar - battery storage explained

Posted on 18 Oct 2017 by
  • Have you got solar panels on your roof and are wondering about installing batteries to store power?

  • Want to say “goodbye” to expensive power suppliers and take responsibility for your own power?

  • Got an off-grid property where grid power is just too expensive to install?

  • Not keen on burning fossil fuels to create the electricity for your house?

Or maybe you just need to read a bit more about solar, to try and cut through all the hype and misinformation.

If one or more of those seem to fit the bill then this blog is for you. It’s written in a simple manner, by someone who actually does this stuff for a living, rather than by an enthusiastic blogger!

So, a quick health warning. The opinions in here are mine, and laws change, technology changes, and other people might have different ideas. I reckon that what you’ll read below is correct when it is written, but the internet has a long memory…

Burning fossil fuels?

Nope, I don’t like burning fossil fuels to make electricity either. There is a myriad of reasons why we do it – efficiency, immediacy, convenience, questionable political agendas, but:

About 4.6 billion years ago, an extremely powerful nuclear reactor was created about 149,600,000km away, which is a much better place than Chernobyl, Three-Mile Island or Fukushima.

Light takes 8 minutes to get to us from this reactor, but it’d take you more than a lifetime to get there flat out in a Holden Commodore even if you didn’t have to stop for petrol!

Now this reactor is going to run smoothly for about another 5,000,000,000 years, and nobody charges you anything for the energy that you can freely collect from this reactor (although it wouldn’t surprise me if a politician tried to find a way).

Oh, and you putting up some solar panels will only have a positive effect on global warming and solar panels pay back the energy/pollution created in making them within weeks, despite rumours spread to the contrary (by people who make money from coal sales largely), and the economic payback on an installation is pretty quick too.

So, let’s assume that you agree that solar power is a good idea, and move on with the “how”.

How much solar power do I need?

You’ll find thousands of different approaches to this on blogs and websites the world over, so let’s add another one.

A good starting point is your average daily power consumption. You can find this on your electricity bill, if you have one, or you can make some calculations based on what you run and for how long each day if you are setting up an off-grid system. If you are making your own calculations, don’t be optimistic and make sure you allow for everything in the most extreme conditions, and then allow a bit of slack on top. As a guide, a 4-person house without a pool or spa might need about 20kwh (20 kilowatts for an hour, 1 kilowatt for 20 hours etc) per day. My two-person house with a swimming pool chews through about 26kwh per day.

Then you need to size your solar system. A 5kw system is pretty much standard, nowadays, as a starting point. That means, theoretically, it will put in 5kw per hour in peak sun conditions and, when the sun is hitting the panels at angle below 90 degrees, or when there is a bit of cloud or pollution, or when the neighbour’s tree has dropped sap on your panels, the production will be much less. In fact, the real-world output of a solar install is typically around 80% of the rated output at best, with occasional glimpses of better. Also, in the real world, you might get 5 times your system’s rated output in summer, and 3 times in winter so, for example, your 5kw system might deliver 25kwh in summer (note that I say “might”) and more like 15kwh in summer.

So, bottom line, a 5kw system might just about produce enough to run a 2-person house with pool in summer, but this ignores inefficiencies, clouds, bird poo, losses through heat and resistance, losses through charging and conversion of ac to dc, dc to ac, low volts to high volts and back again. In winter, you’ll need 60% more in the way of solar panels to get the same output, so the 5kw isn’t going to be enough unless you are especially frugal. So, fit as much as you can in the way of solar panels. You are unlikely to overdo it.

If I am fitting batteries, how much capacity do I need?

IF you have enough solar power to run everything during the day, and also enough spare solar power to fully recharge batteries and replace the power you took from them last night, then your batteries only need to get you from dusk to dawn (so you want to be using the dishwasher, washing machine, pool pumps and other power-hungry appliances during the day, and not all at the same time).

So, let’s say you’ll need about half your power overnight (because that’s when you have the TV, set-top box, computer, lights and other stuff running), then you might need 12kwh overnight. Depending upon your battery choice, you probably want that to equate to about 30% of the battery capacity, so you have enough spare for tomorrow if it is a dull day, and the night after, and you aren’t working the batteries too hard.

As a general rule, and with only one exception that I can think of, batteries work best if they are cycled less deeply ie they get recharged before you have taken much out of them, and the less deep the regular cycling, the longer they will last. If you need to work them hard from time to time, no problem, just get them recharged quickly (by solar, mains or generator power).

So, if we say 12kwh is 30%, then your batteries need to store about 40kwh, preferably more. That’s 3,333ah at 12v, or 1,666ah at 24v. Brace yourself, and brace the floor too, because that’ll be about a ton of lead acid batteries!

What type of batteries?

There’s a bit of choice here but, for simplicity and unless you live in a submarine or warship, it comes down to lead acid (cheaper) or lithium (longer lasting), with a few zany technologies emerging around the edge. The choice in lead acid can be broken down again, and typically comes down to absorbed glass mat (AGM) batteries, which are safe, non-spill, don’t need topping up and, if they are the right model, will last very well, or gel batteries, which are much like AGM but tend to be a little more expensive and a little more tolerant of deep discharges, or wet batteries, which, for domestic installations, should be of the type that can (and must) be topped up with distilled water every few weeks, and should live in a well-ventilated place that isn’t in the house – because acidic fumes and people don’t mix.

Now, for me, the wet lead acid batteries often make most sense. They are cheaper than agm or gel, much cheaper than lithium, should have a long warranty and will last many years if well cared-for, as long as you buy a decent product. See our other blogs on cheap batteries. Good quality is an investment that will pay back in spades.

My personal preference is to use traction batteries (also used in forklifts) and in cells that are, each, 2v, wired together in a series to produce the required voltage. You can use 6v, 8v or even 12v cells for smaller systems, but a big system will go well on 2v cells, and I’d be aiming to use the larger cells and, personally, I’d build two banks of 12 cells in series each, so two banks of 24v, with a switch on each (so one back can be turned off and worked on without upsetting the system). In this case I’d be using cells of about 1000ah per cell, so each one stores 2kwh (2v times 1000ah) and the 24, in total, store 48kwh, a little bit more than we thought we’d need. Better more than less!

Why 24v? Well, power supplied is measured in watts which is “volts multiplied by amps). Double the voltage and you halve the amps that need to be carried through the cables, and the thinner the cables can thus be without resistance affecting the voltage (out of scope for this article). That’s why 240 volt cables are very thin, but carrying the same power (wattage) at 12 volts would require hugely fat, expensive cables.

24v is a compromise. 48v is better, but there aren’t many things that run on 48v so you end up having to convert it back to 12 or 24v elsewhere. If, though, you are only going to run 240v appliances then go with a 48v battery bank and 48v inverter/charger. In the example, above, we’d just wire all 24 cells in series for one bank of 48v instead of 2 banks of 24v each, and go without the luxury of being able to turn off one bank if we needed to.

Completely off grid?

So, your property is in the bush, and you’ve decided on batteries, and solar panels. What else do you need?

Actually, it’s nowhere near as complicated as you might think. You need some fuses, a big switch (for the batteries), and then something to turn the solar panel output into a battery charging voltage (a solar regulator), and then whatever wiring you need from the batteries to your low voltage appliances (so if you have a 24v battery bank, you try and use as much 24v power as possible – fridge, tv, lights, fans etc are readily available in 12v or 24v configuration).

Then you need an inverter (that inverts 24v dc power from the batteries into 240v power for mains appliances) and you need an electrician to wire your 240v circuits so you can run coffee machines, hairdryers and the like. Cook and heat with gas if you can, and don’t go overboard on the size of your water pump, and you’ll have a simple, basic system.

See diagram 1 – no generator.

Want to add a generator to give you some backup power? Then also swap the inverter, above, for an inverter charger, and you can then start the generator and supply power to the house AND charge your batteries. Great when you’ve had a few overcast days or need a bit of extra power from the system, and small generators really aren’t expensive.

See diagram 2 with a generator.

Got a house with 240v from the grid, and solar panels on the roof?

So, does a battery system make sense?

First of all, one of the fancy lithium battery systems is the easiest to install – almost plug and play – and Tarquin and Bianca swear by theirs, and the brand name is all the rage, so they must be good.


Have a look at the cost, power available and the payback equation. Some of the sexiest lithium battery products have quite limited battery capacity, and very limited power output, so you’d need more than one to see you though a typical night, especially if you might have the kettle and the oven on at the same time, or the washing machine, tumble dryer, dishwasher, hairdryers, fan heaters…forget aircon and underfloor heating with one of these.

The all-in-one approach comes at a cost, with typical payback periods of 15 years plus and typical life expectancy of 10 years. If you know the parable of The Emperor’s New Clothes then you’ll know where these things fit in. If you really must have one or two to keep up with the neighbours, make sure that ‘keeping up’ is worth what you’ll lose overall, financially.

In fairness, there are some lithium systems that are a bit more practical, usually modular ones that can be expanded as needed, and can have core components replaced as they fail and, as battery prices reduce, little by little, some of these might make sense. Might, in certain circumstances.

What about lead acid systems then? The batteries are certainly cheaper initially, not expected to last quite as long as lithium batteries (although they can if you care for them properly), and there’s a good deal more flexibility. You’ll want the batteries to be outside if they are the wet acid type, whereas gel and agm batteries can live inside, in a safe place away from inquisitive hands.

What about the environment though? Well, lead acid batteries are recycled when they fail, with 97% recovery of the lead and casing so, actually, pretty good.

So, let’s suppose that you already have a functioning grid-tied solar system, with a PV inverter on the panels that feeds lovely 240v power into your house, and the surplus back into the grid for about 1/6th of what you are paying the grid to get it back. Can you just add batteries and a solar regulator to charge them?

No…and there are two key considerations before we get to the technical stuff.

Firstly, your system is designed so that the solar inverter stops working if the grid stops providing power. This is an essential legal and safety requirement. If it didn’t then, when the grid failed (as it does once in a while) your solar panels and everyone else’s solar panels would continue feeding power back into the grid, and the poor old grid technician would get a terminal electric shock when they touched the wires to fix them. Not good.

The second consideration is the law around this sort of thing. Now most laws are written by people who feel inspired to bring their enormous intellect to bear on regulating the daft behaviour of the great unwashed – you and I for the most part. In the case of solar, it’s pretty clear that the people who drafted the laws on this sort of thing were outside of their area of expertise – outside, out of sight, in another universe. The wording of recent regulations is loose, open to interpretation, but basically limits how much power can be connected to the grid by your domestic installation and, depending on interpretation, it’s pretty likely that a 5kw solar install and a bank of batteries with a 240v inverter will break the rules. Or maybe not. But let’s not put it to the test eh?

So, what you do instead is buy something along the lines of a Victron Multigrid to link your lovely new battery bank into the system. This piece of kit is fully compliant with the mysterious regulations and what it does is as follows:

The mains power connects to the input side of the Multigrid
A fancy colour control panel connects to the Multigrid
The batteries connect to the Multigrid
The solar PV inverter connects to the output side of the Multigrid
Your house connects to the output side of the Multigrid too

When the sun shines, your controller tells the Multigrid to power the house, and use some surplus to charge the batteries, and anything left over is sent back to the grid for beer money.

If you need more power than the sun is providing, the controller tells the Multigrid to take it from the batteries and, if even more is needed, to go to the grid and take a bit from there.

If the grid fails, the solar PV unit keeps working because it is getting power from the output side of the Multigrid, so your solar keeps working and the batteries kick in to help, BUT it doesn’t allow 240v back to the grid, so no smell of burning electrician.

When the sun goes in, the Multigrid draws from the batteries, but if you need more power, or the batteries are running low, it can go to the grid for some power too.

If the batteries are very low, and the solar isn’t producing the goods for one reason or another, the Multigrid can grab some power from the grid to charge the batteries.

And, once it has been programmed, all this happens automatically.

See diagram 3.

What about cost and payback, I hear you ask?

So let’s say the batteries and Multigrid will add up to about $10,000 or so, plus some bits and pieces and some labour to get it all installed. How much is your electricity bill per quarter? If it’s $500, you might get it down by between $300 and $400 (you’ll still need some grid and you’ll still need to keep connected for emergencies, unless you are really feeling like cutting the cord!). So let’s say you save $1600 a year, and the system set you back $12,000 all in. The cost of interest on that $12,000 is about $600 a year, so you are really saving about $1000 each year net, so 12 years to payback, unless electricity prices go up (surely not, when does that EVER happen?), but in 12 years or less you’ll be buying new batteries and other bits so it doesn’t make sense on economic grounds alone.

On the other hand, if you have a larger bill each quarter and can save $600 a quarter, then your payback might be somewhere around 6 years if electricity prices climb gently, and much quicker if prices rise more steeply.

In summary

The jury is still out, as you might expect. If a battery installation made obvious economic sense then everyone would be doing it, right now. I’d argue that it would make most sense if it could be bundled into a home loan when people move into a house, and would certainly add a lot of value if you were selling a house with lovely low electricity bills.

Dinosaurs were, allegedly, wiped out about 65 million years ago. So it’s interesting to see how many ‘dinosaurs’ there are in positions of power today – Presidents, Senior Ministers, Corporate Magnates and the like, people who see their own agendas as a higher priority than the clear and irrefutable evidence of global warming. If we’re not careful, these 21st century dinosaurs will wipe us all out, which is a little ironic.

If you, on the other hand, feel that it’s time to help heal Mother Earth and you see this as more than a purely economic decision, you might just want to use all that lovely wireless power from the perfectly safe nuclear reactor at the centre of our solar system over the next 5,000,000,000 years. That’ll give the latest dinosaurs long enough to get a grip on reality.