For people who already have solar PV on their roof and want to know if batteries are a good investment, these simple four steps will tell you how many years batteries will take to pay for themselves. The default figures on page load are the 2018 data from my own installation.
Look at your last 4 quarterly energy bills and add up how much energy you still imported annually. It's worth cross-referencing the total figure with your inverters import meter.Peak kWh
Enter your Time of Use rates per kWh
How much energy have you exported during the last year and how much have you been paid for it? You can get this data from your export meter and cross reference it to the same four energy bills where it'll appear as something like "Retailer Solar Buy Back" as a credit to your account.
Exported energy (kWh)
Your Feed-in tariff (FIT) $
Obviously the larger the battery, the less imported energy you are likely to use.
Estimated battery cost $
How much of your imported energy (step 1) do you think your battery will replace? %
Subsidies, incentives and rebates $
Since I don't yet have battery data, the above is only an estimate, but it more-or-less aligns with the outcomes when I use the more sophisticated Battery Calculator from Solar Choice so doesn't look particularly compelling.
However, it's not completely out of the ballpark either and here are a few factors that could tip it to break-even pretty quickly:
It’s probably a combination of all of the above that will make batteries compelling. Try some of these in the calculations above to see the effect.
From a purely commercial perspective, logic would suggest get solar PV now but wait for batteries.
Last year I estimated it'd take 50 years to pay for a battery. It turns out I didn't understand Time of Use in sufficient detail, probably because it isn't noted on my bills, isn't controlled by your energy retailer and how they allocate the different times of the day changed in 2018.
So instead of estimating that I use 15% Peak and 85% Off-Peak, when I go back to the energy data, it turns out that even after installing solar, the actual figures are still Peak 19%, Shoulder 41% and Off-Peak 38%. This affects the maths and the outcomes significantly.