Good news! An array of solar batteries are now available in New Zealand, allowing customers freedom of choice; something that wasn't plausible only a few short years ago. By now I'm assuming you've heard of the Tesla Powerwall, a proven workhorse from Elon Musk's forward thinking company. But what other brands of solar batteries are worth your time and money? Which ones provide the best value, and which one will best suit your needs?
Each battery comes with specifications in the form of a datasheet. Most branded batteries have similar datasheets that provide detailed figures such as nominal power, round trip efficiency and peak power etc. It can get confusing. We aim to demystify the specs in the hope that things will be a little easier to understand so you can make your own assumptions on all the facts and figures.
Below you'll find the more important specifications I've selected to discuss, and I point out why they are important to you, the consumer.
Warranty
Quality lithium battery storage systems generally come with a 10-year warranty. Batteries are likely to outlast 10 years, but after the 10-year mark, there is no guarantee. The batteries performance is expected to reduce overtime, as you may have experienced with a lithium powered laptop or similar device that's battery has decreased over the years. Make sure to compare performance warranties along with battery life warranties. For example, the Sonnen Hybrid battery warrants that the battery retains at least 80% of the nominal energy for 10 years after the date of the initial install. Therefore if the battery is performing at 50% after 9 years, for example, then the warranty can be claimed.
A battery manufacturer may also state how many cycles the system is warrantied for. One cycle is the battery fully charging up and then discharged. For example, the Sonnen battery is warrantied for 10,000 cycles at 100% depth of discharge.
What System Size to Choose
The average New Zealand household uses 20 kWh of power per day, therefore if a household is able to self-consume 50% of the solar generated from solar panels on the roof, then a battery with 10kWh of usable capacity would be appropriate (If the aim is to consume the majority of the solar power generated). System sizes as small as 1 kWh are available on the New Zealand market, on the other end of the scale you could have 4 Tesla Powerwalls on a garage wall, which would be an impressive 54 kWh. Some battery storage systems are scalable, you can add more at a later date should you require more storage. Some systems can be 'daisy chained', adding more storage units onto the original unit to increase storage capacity.
Power Output
This is a factor that varies between solar battery brands and model. For example, one Enphase AC battery has a power output of 260Wh, whereas the Tesla Powerwall 2 has an output of 7kWh. If the power output is too small you might not end up using all of your power each night, which would reduce the value of the battery.
Round-Trip Efficiency
Round-trip efficiency is shown as a percentage, it is the ratio of power going into the battery compared to the amount of power that leaves the battery. The percentage is never 100% due to heat loss and other inefficiencies. The higher percentage, the more value you will get out of the battery.
Hybrid Models
Not all battery storage systems are hybrid models. A hybrid system means getting the best out of a grid connect system and off-grid system; you can still export excess solar power to the grid and use energy from the grid, but you can also operate in off-grid mode. If there was a power-cut or a blackout, you could use solar power directly from the solar panels, charge your batteries and/or use power from the batteries themselves. Batteries without hybrid modes mean you will not be able to use any power in blackouts or power-cuts.
What we've just discussed are just a few things to look out for when comparing battery storage systems. When you get 3 quotes for a solar power system with battery storage, solar installers should provide you with datasheets that include most of this information. Want an easy way to compare batteries? My Solar Quotes has also made it easy for you to compare solar battery models here.