We recently published an article on the topic of how much money a household could save with a Tesla Powerwall 2. Soon thereafter, we received two separate requests to do a similar analysis for Enphase’s AC Battery – also one of the most popular battery storage offerings available in Australia, but without as much media hubub associated with it. How much money can you save with the AC Battery, and is it worth it?
About Enphase’s AC Battery
The Enphase AC Battery is unique in the Australian market in that it is very small and completely modular: A single unit (weighing 25kg) stores 1.2kWh of usable energy and contains its own microinverter, which manages the conversion of AC to DC electricity and vice versa.
Their small size means that it’s easy to choose the number that is most appropriate for your energy needs and budget – whether that be 1x unit, 5x units or more. It also makes them a great, ‘entry level’ battery system for those who desperately want batteries (or a battery) but don’t have the budget for a large unit, or those who have only a small solar system (less than 3kW or so) and low daily energy consumption.
A robust battery
Using robust lithium-iron phosphate (LiFePO) chemistry, the AC Battery can last up to 7,300 cycles and has a warranty period of 10 years (roughly 2 cycles per day) while degrading to only 80% of the original storage capacity within that time. (All batteries other than flow batteries degrade over time – it’s just a question of degree.) With a high cycle life & low degradation rate, there’s a pretty good chance that AC Batteries installed in your home will endure beyond the 10 year warranty period – especially in light of the fact that you’re unlikely to use up all 7,300 cycles within the first 10 years.
An example of lithium battery degradation (loss of capacity to hold energy) for example Enphase AC Battery. With 7,300 cycles, if charged/discharged (‘cycled’) only once daily the batteries could feasibly last well beyond their 10 year warranty (denoted by spot where line turns from orange to blue).
An example of lithium battery degradation for an example AC Battery when cycled 1.5 times daily. Here, the possible life beyond the 10 year warranty is shorter but still significant.An example of degradation in a lithium battery with an end of life retained capacity of 70% and cycle life of 3,6500, cycled once daily. Here, the line stops just as the 10 year warranty finishes up – ‘extra’ cycles beyond the warranty period are more questionable than with a battery with higher cycle life (such as Enphase’s AC Battery.)
Low power output & slow charging speed
One of the ways that Enphase manages to get this considerable longevity out of their batteries is by ensuring that they are handled gently – each unit has power output capacity of only 0.275kWh (or 0.825kWh for 3x units). To put this into context, Tesla’s Powerwall 2 (13.2kWh storage) has an output capacity of 5kW – more than enough to run the average home in the evening. By contrast, you’d need 18-20 Enphase AC Batteries (21.6kWh-24kWh) to get 5kW of instantaneous power.
Low power output is often coupled with slow charging speeds. You can think of the charge speed (as well as the power output) as being like a straw – the larger the straw, the faster the energy can flow. All in all, if your battery charge/discharge speeds are too slow, you’ll fail to capture and release energy at the time when it’s needed.
How important are the charge/discharge speeds?
This issue, however, might be besides the point if all you’re looking to do is reduce the overall amount of energy that you draw (and purchase) from the electricity grid. Although it would be helpful to have a battery bank that can quickly ramp up to meet your home’s electricity demand when you switch on multiple devices at once (think: vacuum, washer & electric oven all at the same time), it’s not strictly necessary, especially if you’re not going off-grid, you’re on a flat rate tariff and have only a small battery bank in the first place.
In fact, as long as the battery’s charge/discharge rates are fast enough, the savings achievable would probably be similar for most ordinary households. Why? Because once evening rolls around and you’ve got multiple devices drawing power over the course of several hours, you’ll probably empty your batteries before you go to bed (or before ‘peak time’ ends) regardless of their output capacity. (This theory goes out the window, of course, if your home does use short, sharp spikes in electricity use from lots of devices simultaneously.)
Example of battery charge & discharge for a 5kWh battery with output/charge rate of 0.825kW. Note that the battery doesn’t reach ‘full’ state of charge on at least two of the days examined (a random week selected from November).
Example of battery charge & discharge for a 5kWh battery with output/charge rate of 3kW. Note that the battery reaches & maintains full state of charge more (white line in lower graph) than with only 0.825kW.
The point being that there is definitely an advantage to having a battery with higher instantaneous power output, but in homes with only moderate levels of energy consumption (say 15-25kWh per day and at least 5kW of solar), it probably won’t make a huge difference. Initial estimates by Solar Choice show that in the scenario above the difference could be as low as 1-2% less energy passed through the 0.825kW output battery in the first year.
Check out our Solar & Battery Sizing & Payback Estimator tool to see some of the numbers for yourself.
How much does it cost? Also – how much can I save and is it worth it?
So many questions!
Okay – so now I’ve laid the groundwork for understanding how the AC battery, its value proposition and how it can save you money. Let’s dig into some of the financials.
How much does it cost?
Roughly speaking, one AC Battery unit costs about $2,000 installed. If you do not already have an Enphase system on your home, there’s an additional one-off cost of about $1,000 to have Enphase’s communications gateway installed – which is necessary for the different batteries to function as a whole.
(Please note that these are figures we have observed across several sources – including pricing from our own installer network database. That being said, make sure that you shop around and consider other options as well.)
That means that you’re looking at about $3,000 for the first battery, and about $2,000 for each additional battery: $5,000 for 2x units, $7,000 for 3x units or $9,000 for 4x units.
How much can I save?
Let’s assume you install 3x Enphase AC Batteries at a cost of $7,000, and that they are installed alongside a 5kW solar system.
Your actual savings will depend on your own circumstances, including how much you pay for electricity, how you use electricity throughout the day and whether you’re on a flat rate tariff or time of use tariff (<– If you don’t know what this means, or which tariff type you’re on, definitely read this article.)
We’ve worked out that having 5kW of solar and 3x Enphase batteries will save the average household about $1,600-$1,700 per year in select capital cities (as per the tables below). The savings are marginally higher for households on a time of use tariff (Adelaide & Perth were the best two cities for it). Keep in mind, however, that the vast majority of the savings comes from the solar system; the savings attributable to the batteries is in the region of $400 annually (again, higher on a TOU tariff).
(If you’re curious about the assumptions made here, we have posited a north-facing solar array, with electricity prices as listed here and solar system prices as listed in our October PV Price Index. We also make the conservative assumption that the batteries charge only with the sun – no ‘pre-charging’ with off-peak electricity for TOU customers; this would likely improve the savings a bit for TOU customers.)
Estimated first year energy bill savings, plus payback period (in years) & internal rate of return (IRR) for a 5kW solar system with 3x Enphase AC Batteries (3.6kWh). Select cities, flat rate electricity tariff. (Click to enlarge.)
Estimated first year energy bill savings, plus payback period (in years) & internal rate of return (IRR) for a 5kW solar system with 3x Enphase AC Batteries (3.6kWh). Select cities, time of use rate electricity tariff. (Click to enlarge.)
As we’ve pointed out in our Powerwall 2 analysis, the question of ‘savings’ – especially first year savings – doesn’t necessarily tell you everything you need to know about a battery as an investment. Figures like payback period (or even ‘break even’ period – which should happen before the warranty expires) and internal rate of return (IRR, which is like ‘return on investment’ but better) are much more important, as they give a better idea of how your batteries will perform as an asset that works to save you money.
Furthermore, we’ve also noted previously that there are two ways to look at an investment in a solar + battery system:
- You can lump the solar and batteries together and think of them as a single, ‘whole’ system; or
- You can look at the solar and the batteries separately and evaluate them individually as an investment.
If you’re very keen on batteries for the sake of energy independence and are looking for a justification to get them installed with your solar system, option A – the ‘whole system’ approach – will probably be good enough for you. If that’s the case, look for a whole system payback period that is shorter than the 10 year warranty period and an IRR of over 5%.
If, on the other hand, you want to evaluate a battery purely on its own merits (as most people will – especially if they’re considering retrofit for an existing solar system), use method B – it is a much stricter way of evaluating a battery’s investment-worthiness. Just as with method A, you’ll want to look for a battery that passes the 10 year payback test – but it’s much harder for the battery to do this without being ‘aided’ by being lumped in with the solar system.
Which, on a related note, warrants a reminder that solar is a fantastic investment on its own – without batteries.
If you’re using the ‘whole system’ evaluation method, you may be convinced that it’s worth kicking in an extra few thousand dollars for some AC Batteries – whole system payback periods are under 10 years on both tariff types in all of the cities we looked at minus Brisbane (where it was about 11 years).
That being said, if you’re looking only at the value directly associated with the batteries (‘battery only’ evaluation method), it’s highly unlikely that you’d bother with them. The payback periods for the batteries on their own are over 15 years in every scenario we examined – well beyond the 10 year warranty period and very close to the optimistic, non-warrantied ‘expected lifespan’ of roughly 15 years.
We do encourage you, of course, to do your own research on the topic – including crunching the numbers for yourself and looking for the best possible deal (you might be able to find prices that beat the ones used for our modelling!) Check out our Solar & Battery Sizing & Payback Estimator tool, and/or get a free & impartial Quote Comparison for solar & batteries from installers in your area.
© 2019 Solar Choice Pty Ltd
He is now the communications manager for energy technology startup SwitchDin, but remains an occasional contributor to the Solar Choice blog.
James lives in Newcastle in a house with a weird solar system.
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