Home Battery Basics: Fundamentals You Need To Know

Don’t go in blind when buying a solar battery — first understand these important fundamentals to help ensure you get the system you want (and need).

Battery Power Vs. Energy

When selecting a battery, both its power and its energy need to be specified. To help easily understand the difference, let’s use an analogy of a tank that’s ‘filled’ with electricity through a pipe.

Power

The size of the pipe into the tank determines how fast the tank can fill (charge), or be emptied (discharge). A tank with a skinny pipe will fill or empty more slowly (low-power ) than one with a large pipe — a high-power battery.

A battery’s power is measured in kilowatts (kW).

Energy

But how big a battery is (the capacity of its tank) is determined by its kilowatt-hours (kWh) rating. The “hours” distinguishes energy from power.

Power And Energy Together

As an example, a Tesla Powerwall 3 is a 13.5 kWh, 10 kW battery.

• It stores 13.5 kWh of energy.
• It can discharge that energy at a maximum rate of 10 kW (power).

Some household loads may consume significant amounts of *energy* over an extended period, but also require a lot of *power* throughout, or at points during that period. For example, a large ducted air-conditioner as it cycles or a pool pump (very briefly) as it starts up.

So, if your aim is to minimise electricity from the mains grid, you also need to think about power and not just energy.

Battery Chemistry

10 years ago, if you wanted home battery storage, it would be based on lead-acid chemistry. You’d have a lot of these batteries, and they would require routine maintenance. These were mostly used by off-gridders.

But 2015 saw the dawn of the lithium-ion residential energy storage age, which offered:

• Better performance.
• Zero maintenance.
• Set-and-forget operation apart from an electrical checkup every few years.
• Longer lasting batteries.
• Cheaper per kilowatt-hour discharge.
• They don’t need their own shed!

Given the advantages, it’s little wonder lithium-ion tech blasted past lead-acid for residential energy storage, with the latter mostly being the domain of die-hard off-gridders these days.

The Main Chemistries: NMC And LFP

There are different types of lithium-ion chemistries, with the most common currently being:

• NMC – Nickel Manganese Cobalt
• LFP – Lithium-Iron Phosphate (aka LiFePO4)

LFP batteries are generally regarded as safer in terms of thermal runaway (catching fire) risk than NMC. However, SolarQuotes founder Finn Peacock says:

“I will stress any well-installed battery from a major manufacturer is safe. I’ve got NMC batteries on my house and I sleep fine at night.”

Connecting A Home Battery And Solar

Batteries store Direct Current (DC) electricity, and solar panels produce DC electricity. The cheapest and simplest way to hook up a battery to your home and solar system is via DC coupling. This only requires one inverter — a hybrid inverter — to convert the DC current into AC electricity suitable for use by appliances in your home, or export to the grid.

But you’re generally locked into whatever models the hybrid inverter is compatible with.

For greater choice, that’s where AC coupling comes into play; which requires separate battery and solar inverters. However, while providing a greater range of storage solutions, the AC coupling approach also costs significantly more.

Batteries And Electricity Plans

If you’re on a flat-rate electricity plan where the cost per kilowatt-hour for grid electricity doesn’t change no matter the time of day, the battery simply charges up from your solar panels during the day and it’s used instead of grid power overnight until discharged.

If you’re on a time-of-use (ToU) plan where grid electricity is usually significantly more expensive from 4pm to midnight and cheaper at other times, things should work a little differently.

While the battery will still charge from your solar panels during the day, it can also charge from cheap daytime electricity when there’s not enough solar to fill it — such as during winter, overcast days or when a solar system’s output is being used for charging an electric car. It can even charge up again after midnight on cheaper off-peak rates.

“But this behaviour is dependent on good battery control software,” says Finn. “Watch out for the cheap, ‘dumb’ batteries that don’t have the software to do this, and watch out for inexperienced installers who won’t configure your battery to save you the most money.”

More Information

If you’re keen on further detail, see Finn’s full guide to understanding home batteries, and its companion guides on buying and owning solar batteries. To see how various home energy storage solutions available in Australia stack up side-by-side on power, energy, features and price, see our home battery comparison page.

And when you’re ready to get quotes for good batteries that are properly installed and configured by installers who care, SolarQuotes can organise up to 3 quotes from trusted, pre-vetted installers.