Then there is the round-trip efficiency, e.g. how much power you lose in charging batteries and then inverting it to 240V. While good batteries will have an efficiency factor of 95% or more cheaper ones have around 80%.

Then there is the cycle warranty – the number of times a battery can charge from “0%” (it never truly goes to 0%) to 100% expressed as in kWh based on one cycle-per-day. Some have as low as 10,000kWh cycles while others have up to 50,000kWh. This can mean a cost difference per warranted kWh from 20 cents to over $1.

Then you have construction – Lithium (and all its variants), Aqueous Hybrid Ion, Graphene and Zinc Bromide to name the main ones. Locally made Zinc Bromide (Z-Cell is really looking the most promising).

This also impacts on the kW load it can steadily deliver. A cheap 10kWh battery may only be able to continually deliver 30% of its rated output and a good one close to 90%.

So, we have two million households generating their own electricity during the day. But what about the night?

Back to our example – solar will cover the day, but you need a battery if you want to cover the night. That 5kW on-grid costs around $2.50 per day or in round figures $1,000 per year.

Solar batteries vary in cost – the cheaper ones (with shorter life and recharge cycles) may cost $800 per kWh to the best at $2,000 per kWh. That 10kWh battery that ranges in price from $8,000 to $20,000 – can take up to 20 years to deliver a positive ROI. Catch 22 – the battery usually lasts up to 10-15 years – this is not cost-effective.

You can explore batteries here.

But I can sell electricity back to the grid (feed-in tariff or FiT) and reduce costs

Yes and No. Currently, the only way to sell power back is via your energy retailer (the one you pay the daily supply charge of between 70-100 cents). And you need to install a smart meter and other switching gear.

Remember, Australia is already over-supplied with solar power during the day. In my opinion, that is hurting the delicate grid balance because it needs less power during the day and a huge ramp-up at night when solar does not work, but batteries do.

The retailer will pay you around a paltry 7-11 cents per kWh whenever you export to the grid. BTW they usually resell that power for ‘many’ (at least four-to-five) times what they are paying you.

A householder currently can’t sell directly to the grid, but there are investigations into neighbourhood and regional co-op type schemes that make the aggregated supply into a mini-generation station. If this gets off the ground (and I am sure GadgetGuy will be first to report on it), then you may see power purchased at 40-50cents or more per kWh. That brings the payback time to a few years.

I know I am using strong words, but the last company you should sell your power to is the energy retailer or the battery maker that takes control of your battery. Those schemes are fraught with often impossible to meet conditions and hidden charges. For example, 

“You agree to allow us and our service providers access and control of your battery, at our complete discretion and as we deem necessary.”

Summary – When is it time to go solar?

If you buy the right-sized, quality solar system for your immediate and near-future needs, it is a no brainer to do it now providing you realise that pay-back time depends on knowing your real use, not some theoretical salesperson’s spiel.

If you buy a larger system than you need then you extend payback periods considerably – there is no value in that unless you intend to put in a battery to cover night use and FiT.

There is no such thing as a free lunch, and if you get a bargain, it is at the expense of efficiency, reliability and longevity. This is a 25 year+ purchase, not a five-year purchase when dodgy installers won’t be around to honour warranties.