Solar Batteries 101, Part 1: Understanding Batteries

By Finn Peacock – Chartered Electrical Engineer, Ex-CSIRO, Founder of SolarQuotes

Last Updated: 13th Sep 2021

Are you thinking of going solar and wondering whether you need a home battery too? Or maybe you have solar panels installed and want to know what’s involved with adding residential energy storage.

I’ve developed this three-part ‘Batteries 101’ beginners guide to get you up to speed as quickly as possible:

  • Part 1 – Understanding Solar Batteries – this page – goes through the fundamentals of home energy storage.
  • Part 2 – Buying Batteries – is full of useful information to help you buy the right solution for the right price.
  • Part 3 – Owning Batteries – sets expectations for your energy storage-powered lifestyle.

If understanding home energy storage is not a priority for you, skip to part 2 of my guide for practical advice on buying a solar battery.

  1. Why get a home battery?
  2. How do batteries work? Anatomy of a battery storage system.
  3. Power vs Energy – Do you need a sprinter or a marathon runner?
  4. Lead-acid? Lithium-Ion? Flow? Which technology to choose?
  5. Adding a battery to your solar system – AC vs DC coupling.
  6. How do solar batteries save you money?

1) Why get a home battery?

Use your solar after sundown: Batteries enable you to store solar energy to use in the evening and through the night. With a home battery, you’ll import less energy from the grid and pay less money to your electricity retailer every quarter on your power bills.

Benefit from Time Of Use Tariffs: Time of Use (ToU) tariffs are becoming more popular (and may even become mandatory). These tariffs slug you with high per-kWh charges during the late afternoon and evening (e.g. 5pm – 9pm) when demand on the electricity grid peaks.

If you don’t have a battery, Time-of-Use tariffs are likely to increase your bills. But an appropriately-sized battery with enough solar panels can power you through these peak periods so you can dodge most of the peak pricing. If the evening peak empties your battery, you benefit from off-peak pricing through the night.

Some battery systems (e.g. Tesla) are smart enough to top up with off-peak grid electricity where it makes sense. For example, if it looks like there won’t be enough sunshine to charge it the following day.

Backup: Some (but not all) batteries will also back up important circuits in your home. If a blackout hits – you can ride it out in style:

Finn's house during a blackout

My house was the only one on the street with the lights on in a blackout – thanks to my Tesla Powerwall.

2) How do batteries work? Anatomy of a battery storage system.

A battery is an electrochemical “sandwich” used to store energy. One bread slice is the anode, and the other is the cathode. Between them is a filling called the electrolyte and a separator.

It’s easy to remember the cathode is positive because many people love cats. They’re positive about them. The anode is negative, like my Aunt Ann, who complains about everything.

Negatively charged electrons concentrate at the anode. Because opposites attract, they want to go to the positively charged cathode. The filling blocks electrons from taking the short path through the battery.

Connecting the anode and cathode with wire allows electrons to flow through it. This flow of electrons is what we harness as electricity.

How a battery works

In rechargeable batteries, you use an external energy source to reverse the flow of current. This stores that energy for later use.

There are many ways to arrange the sheets of cathode, anode, and separator in a modern lithium-ion solar battery. They are usually constructed like a jam roll inside metal cylinders called cells. A home energy storage system can have thousands of these cylindrical battery cells.

18650 battery cell diagram

3) Ensure your battery has enough power AND enough energy.

In part 1 of my “Solar 101” guide, I explain the fundamental difference between power (kW) and energy (kWh) – and it’s important to understand this.

When it comes to batteries, a useful analogy is water flowing through a pipe into a container.

  • Power (kW) is how fast the water flows through the pipe, into or out of the container.
  • Energy (kWh) is the amount of water the container can hold.

solar batteries - power and energy

Again, understanding the difference between power and energy is super-important. It can mean the difference between choosing the right home battery for your needs and a dud.

There are all kinds of solar batteries out there; each with its own combination of power output vs energy stored. As an example – compare the time it takes for these batteries to discharge fully:

Home battery discharge rates

You’d have a tough time running any major appliance (aircon, etc.) off a single Enphase battery.

Most solar batteries have a maximum continuous power output of 5 kW. My Tesla Powerwall, for example, has a 5 kW output. If I ever want a 10 kW power output from my battery system, I will need to add a second battery.

Be aware of the power and energy needs of your home when choosing a home battery.

If your solar battery can only provide 3 kW and your home needs 5 kW, you’ll need to get the power shortfall from the grid. For example, I have a Finnish sauna in my home that pulls 7 kW. I can’t run it entirely off my Powerwall, because that only outputs 5 kW. That means I can’t have a sauna in a blackout. What a hardship.

4) Lead-acid? Lithium-ion? Flow? Which technology to choose?

Until 2015, if you wanted to install energy storage, you were most likely living in a rural area and looking to go off-grid.

The dominant technology up to then was lead-acid. It required a big, heavy bank of batteries installed in a dedicated room (usually a shed) and regular maintenance – not exactly ‘set and forget’.

Since then, advances in lithium-ion tech has resulted in Li-ion dominating the home energy storage market for a few reasons:

  • Superior performance (better power output and depth of discharge)
  • “Set and forget”, maintenance-free operation.
  • Longer warranties
  • Competitive pricing
  • Smaller, lighter

Because of this, most solar battery installations for homes are using some variant of lithium-ion tech these days.

The two major lithium-ion technologies are Nickle Manganese Cobalt (NMC) and Lithium-Ion Phosphate (LiFePO). The Tesla Powerwall, for example, uses NMC cells.

Pro-tip: People always ask me about alternative technologies, like nickel-iron or flow batteries. I’m yet to see an alternative technology beat lithium-ion in both performance and price.

So – what battery technology should you choose? Unless your home has specialised needs, lithium-ion will be the way to go.

5) Adding a battery to your solar – AC vs DC coupling

As explained in my Solar 101 guide, solar panels output DC electricity. But the appliances in your home use AC electricity. The job of a solar inverter is to convert the DC from the panels into AC used in your home.

Batteries charge and discharge DC electricity. So – how do you integrate home energy storage into a solar system?

There are two ways:

DC coupling: You use a hybrid inverter, which contains a DC-DC converter. This converts the DC current from the solar power system into DC that can charge the battery.

Hybrid inverter - DC coupling

Hybrid inverters, used for DC coupling, are more expensive than regular solar inverters.

Hybrid inverter example

The Fronius Gen24 hybrid inverter

AC coupling: A special “battery inverter” converts the AC output of the solar inverter back into DC to charge the battery.

AC coupling - solar and battery inverter

Some home energy storage options, such as the Tesla Powerwall, come with a battery inverter built-in. The Tesla Powerwall can only be AC coupled. Others, like the LG Chem, need an external battery inverter to be installed on the wall next to them.

Battery inverter example - SMA Sunny Boy Storage

An example of an AC coupled battery inverter – the SMA “Sunny Boy Storage”

Advantages of DC coupling: there are ‘fewer stops’ along the way with a DC-coupled system. Fewer stops = fewer losses = higher efficiency.

Disadvantages of DC coupling: batteries pair with specific hybrid inverters. So a spiffy new future energy storage product may not be compatible with the hybrid inverter you buy now. This isn’t an issue if you’re planning on buying a solar + battery system in one hit.

Advantages of AC coupling: it is solar-inverter agnostic. You can retrofit an AC-coupled battery to any existing solar power system.

Disadvantages of AC Coupling: There are ‘more stops’ with the DC->AC->DC conversion, so it’s a little less efficient. Another drawback of AC coupling is rules on system sizing from your local electricity network.

For example, SA Power Networks (SAPN) allows a maximum of 10 kW inverter capacity per phase. Crucially, unless you can get an exemption, SAPN counts battery inverters towards the 10kW limit.

Let’s say you have an existing 7 kW solar system with a 6 kW inverter. You want to add a Tesla Powerwall, which has a 5 kW inbuilt inverter. SAPN may say you can’t because 6 kW (solar inverter) + 5 kW (Powerwall battery inverter) = 11 kW total inverter capacity.

DC coupling bypasses this limit because there’s only one inverter that handles both the battery and solar power.

Pro-tip: My battery storage comparison table lists the prices of various options. Importantly, it does not include installation costs. The cost of extra electronics, such as a battery inverter, can mean a $6k home energy storage system winds up costing $10k installed.

6) How do solar batteries save you money?

Home energy storage systems store surplus solar energy for use at night; meaning you charge your battery with ‘free’ solar electricity generated during the day. At night, instead of drawing from the mains grid, you use the energy stored in your battery. If you are on a time-of-use electricity plan, you may be able to dodge the evening peak tariff period altogether.

With a big enough solar and battery system, you may never pay an electricity bill again.

If you’re part of a Virtual Power Plant (VPP), your batteries also support the grid and earn credits (read: bonus feed-in tariff) for doing so.

But what’s the catch in all this?

Well – the cost of enough solar battery storage to completely cover your electricity bill is high. The bigger your bill, the bigger the energy storage system you’d need.

I go into more detail in part 2 of my battery guide. But to use a simple example – a $15,000 Tesla Powerwall will save, best case, $900 per year. It is up to you whether it is worth spending $15k to save $900 annually and have backup.

The next step

I hope you found this guide to the basics of home energy storage useful. Next up is part 2 – covering everything you need to know when buying solar batteries.

If you have any questions after reading this – feel free to reach out to me directly at [email protected]

If you’re considering installing solar panels or energy storage for your home or business, SolarQuotes can help you get quotes from high-quality, trusted installers quickly and easily:

Finn Peacock

About Finn Peacock

I’m a Chartered Electrical Engineer, solar and energy efficiency nut, dad, and founder of SolarQuotes.com.au. My last “real job” was working for the CSIRO in their renewable energy division. Since 2009 more than 496,000 Australians have used my site to get quotes for high quality PV systems from pre-vetted solar installers.

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