• Climate hub
  • 11 Nov 2021
  • 6 min read

Key things to know about home solar battery storage

By Joanna Jefferies

Home solar energy storage
Home solar energy storage

Are home solar batteries worth the investment?

Without an energy storage system, you’ll lose all the energy you generate from solar panels that isn’t used by your home appliances. But are home solar batteries worth the investment? Here’s some key answers to common questions about home batteries.

In many New Zealand homes, solar panels generate energy when it is least needed–during high sunshine hours in the middle of the day. However, integrating home battery storage with a solar panel system is a great solution to store unused energy, which can then be used at night, on days with low sunlight and when utility lines are down.

Here are some answers to commonly asked questions about home battery storage to help you decide if it’s a good option for your home.

Related article: 6 Questions to Ask Before You Get Home Solar

How does home battery storage work?

The purpose of home solar battery storage is to store energy for later use. The electricity generated by solar panels from the sun is passed via a direct current (DC) into an inverter, allowing it to generate alternating current (AC) electricity, which is the electric current needed to power your home appliances. The electricity not used by your home appliances charges your battery so that when the sun goes down, your appliances are powered by the stored energy in your battery.

Related article: How does solar power work?

What types of solar batteries are there?

The two most common types of home storage batteries are lithium-ion and lead-acid. But what are the differences between the two?

Lithium-ion batteries

Lithium-ion is typically more expensive than lead-acid, but it’s the most popular option to tie to a solar power system. It has a higher maximum depth of discharge (DoD), which means that more of the percentage of energy stored in the battery can be discharged relative to the overall battery capacity, and it can charge at a much faster rate.

Lead-acid batteries

Lead-acid batteries are more popular for off-grid set-ups. They have a lower maximum DoD, which reduces their longevity, however, they tend to be cheaper and are more tolerant of over-charging.

When selecting a battery, you’ll also need to know the system type and configuration you need, because not all batteries are compatible with all systems.

You’ll need to know what batteries your inverter is compatible with, or if the battery has a built-in inverter (such as the Tesla Powerwall 2).

To properly evaluate a battery, you should consider:

  • Its storage capacity (kWh)
  • Efficiency
  • Cost
  • Inverter and system compatibility
  • Output power ratings (continuous and peak)
  • Expected lifespan and warranties

Solar battery storage system

How much power do batteries store?

When you refer to battery storage capacity, you’re talking about how many hours a battery can deliver power, rather than the continuous output capacity, which describes the maximum level of instantaneous power the battery can output (when the panels are in full sunlight).

If your battery’s capacity is 6kWh it means that it can produce a maximum of 6kW of instantaneous power. However, its storage capacity (measured in kWh) will be a lot higher.

Common battery sizes

Common capacity sizes for home batteries are 6kWh, 10kWh and 13kWh. Consider that an electric jug, if switched on for an hour, would use around 2kWh and the average AC unit would use around 1.5kWh.

One option for increasing storage capacity is to “stack” multiple batteries. If you’re likely to need increased storage capacity in the future, make sure you purchase a battery that is compatible with stacking.

How much do home storage batteries cost?

Solar battery prices are generally between $10,000 to $20,000 depending on the battery’s capabilities, the type of battery, its output and expected lifespan. Batteries with high output and lifespan are naturally at the upper end in terms of cost, but you should base your decision on the needs of your household rather than what is cheapest. It makes sense to weigh up the purchase price of the battery against the cost of the solar panel system over both of their lifespans.

Are home energy management systems (HEMS) compatible with home battery storage?

HEMS are a developing technology in managing power usage at home over the course of the day. These smart systems are designed to intuit the household’s power usage and distribute the power based on the time of day and the amount of energy required across the household’s appliances.

While this is still an emerging technology in New Zealand, it’s highly compatible with households that are grid-tied and use home battery storage.

What size of battery is needed for a typical family home?

Your choice of battery will be based on your daily household electricity consumption (you can check this on your current power bill) and how many solar panels you have installed. Households that use more electricity during the daytime will need a smaller battery, while houses that need to store more capacity for peak times will need a battery with higher storage capacity.

Average daily household electricity consumption

The average four-person household in New Zealand uses between 15kWh and 28kWh per day, with around only a third of that used during daylight hours. Therefore, if you’re at the lower end of that scale, a 5-7kWh battery will suffice. The remainder of electricity will need to be purchased from the grid.

If you’re planning to go off-grid you’ll need to stack multiple batteries to cover your electricity needs, with sufficient solar panels to generate the electricity required.

You can increase the life of the battery by …

The DoD (depth of discharge) is an important factor in the longevity of a battery. A lower DoD with a larger capacity battery bank has a longer battery life.

A battery requires replacement every five to 12 years depending on the type of battery and how it is used. Overcharging is the main culprit in reducing the lifespan of a battery, so batteries that automatically monitor the charge level will last longer, as opposed to manually monitored batteries.

Can you feed power back to the grid?

In New Zealand, grid-tied solar power systems require the installation of an export meter that measures how much power you’re selling back into the grid. Solar power systems can be grid-tied whether there is battery storage or not, and the advantage is there’s back-up supply for when you don’t produce the electricity you need.

However, it’s not currently a great money-maker as feed-in tariffs are generally a fair bit lower than the cost of the electricity you receive from your retailer. That may change in the future, though, so watch this space.

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