The economics of battery storage for solar power don’t yet stack up for the grid-connected, but the price is dropping all the time.

Does it make sense to soak up the sun and use the energy later?

The Achilles' heel of self-contained solar systems is batteries. The extra cost doesn't usually pay for itself if your home is already connected to the electricity grid; wired electricity is cheaper than stored.

Yet there is a growing number of off-grid homes that make economic as well as environmental sense if the alternative is paying for a new grid connection.

Built four years ago, Michael Carpenter's large family home in the Manukau hills was off-grid from the start. Over two kilometres from the nearest supply, grid connection cost was prohibitive. "It was going to be very expensive, with ballpark figures around $100,000."


His 4kW of solar panels and 1000Ah battery bank would have cost less than the $60,000 he paid if built now, due to falling panel costs over the past few years.

Batteries the big factor

The battery bank can be the costliest component and needs replacement when it wears out. Marijn Weehuizen's Waipu home has just been treated to new batteries after seven years - a $9000 bill.

Carpenter believes it's worth it in the long run. "What we would pay in power bills is more than enough to set aside for new batteries," he says.

There are a number of battery options, but the most popular is lead acid. While your car battery is 12V and perhaps 60Ah, solar battery banks tend to use 48V DC and store 10-20 times that energy.

These are deep cycle batteries, designed to slowly release much of their energy. However if you take too much charge out of them it does shorten their life so they need to be used carefully.

Flooded cells need regular electrolyte checks, while costlier sealed units are maintenance-free. Other technologies are coming; "Lithium batteries are starting to come into play and will be quite popular," says Kevin Hunter of Cellpower NZ. "For now they are at quite a high price."

Stanton offers NiFe batteries. "They are about twice the price, but virtually indestructible." They also have a long cycle life, he says, and you can replace the electrolyte around five times so they last a lifetime.

Going off-grid

Both Stanton and Hunter estimate the break-even cost of going completely off-grid is when you face a $20,000 bill for connection. Sue Pugmire, whose Manawatu home has been off-grid since 2000, reckons for their set-up $15,000 is nearer the mark.

With a 5kW array and $27,000 spent, she estimates her lead acid and ex-telecom gel cells can hold four days' supply. Her Nissan Leaf electric vehicle holds four days (of average motoring) in its on-board battery if required.

Installers often recommend a generator, helping to keep battery banks from working too hard. Carpenter recommends switching on the generator manually rather than using an automatic kick-in, to save the unit starting up early on a sunny morning. Next time he would buy a smaller generator, one suited to slowly recharge the batteries.

Lower panel prices may eliminate this element. "You can spend $10,000 on a good generator, or just get a bigger array," says Stanton.

Living off the grid

An off-grid home generally uses less power than average; perhaps 5kWh per day instead of the New Zealand average of 21kWh. The main difference is heating and water heating, usually with a wetback-equipped wood burner and, in Michael Carpenter's case, solar water heating. "But we're not skimping on everything - we still use an electric toaster and iron our clothes," he adds.

Energy saving includes LED lighting throughout Campbell Simmond's seven-year-old off-grid home in the Tukituki Valley, Hawkes Bay. "We do have to watch our power use as we depend on the batteries," he says.

His 48V lead acid bank is showing some signs of age already and he is expecting to replace it in two years. And, next time, good quality sealed units would be his choice. "At the moment we have to top up every month or so and keep an eye on the specific gravity."

One thing everyone asked can agree upon; get good quality panels, electronics and batteries. Your lifestyle depends upon it.

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