AC/DC – which way should I go? Does it matter nowadays?
First and foremost, I want to be clear that as cool as AC/DC are we are sadly not talking about the rock band… Just the electrical concept of Alternating Current or Direct Current and how it relates to battery storage.
Key points –
- Most people are fine with AC or DC, rarely does it matter that much nowadays.
- Every time you convert from DC to AC and vice versa you can lose efficiency in the conversion cycle and end up with a lesser value or energy after the conversion.
- The choice of DC storage may affect your Feed-in-Tariff by more than the benefit it gives – be aware.
- AC connected systems normally offer simplicity of install, flexibility, safety and reliability and as such are nowadays an accepted choice compared to previous years when DC was preferred.
In Summary, DC connected battery systems are typically connected directly to the solar panels and will use the same inverter as them for convert to AC power. AC battery systems will be separate from the Solar PV system and a self-contained installation
If the battery is purely for solar storage and maximising self-consumption then certainly, a DC system probably offers all you would ever want and quite possibly has a better total round cycle efficiency than a competitor’s AC one. If, however, you wish to have the ability to potentially charge from the grid and cycle energy through the battery taking advantage of possible financial savings from tariffs then an AC system is your choice.
A DC connected battery that loses 4% efficiency on its cycle means that when the generation meter registers your solar production you will be 4% down on whatever cycled through the battery compared to what you originally generated. This is due to the cycle loss of the battery occurring BEFORE the generation meter, as the DC battery is connected between the panels and the inverter. Whereas an AC battery is pulling charge after you have inverted and your generation meter has logged how much you produced and as such does not affect your Feed-In-Tariff.
Fitting a DC connected battery to a Solar PV system also has to be carefully considered because if that battery is capable of being charged using electricity from the Grid rather than only from the Solar Panels, you could lose the Feed in Tariff eligibility for the Solar PV system. This is because FiT is only available for purely renewable electricity passing through your generation meter, if the battery has changed from Grid then some of that energy isn’t renewable when it is later discharged through your generation meter. You will be responsible for informing your FiT provider of the installation of a battery storage system and wil have to declare that it’s FiT compliant or not, so be sure to be sure what you are buying.
It’s a technical point but very costly to get wrong, particularly for DC battery additions to older installations where a high FiT rate could be lost as a result. Check with your installer that they are specifying a battery that only charges from the Solar DC.
All of this means that when you decide whether to go AC or DC you need to look at the cycle loss of the DC one, what your Feed-In-Tariff is as a monetary value. Multiply the annual loss of metered production by this value and compare it to the total cycle loss that the AC option is giving at current electricity costs and decide if there is any financial merit one way or another. And be very sure that your FiT accreditation won’t be affected. Check in advance with your FiT provider if necessary.
Traditionally if you are installing batteries as a retro-fit to an existing solar installation then you will go AC for ease of installation and because it is totally separate from your Solar PV system and won’t affect your FiT payments. The only time this is irrelevant is of course if you are installing batteries without a renewable generation source in which case you are clearly AC only as you have no DC power source. The reason you may do this is for a UPS – Uninterruptable Power Supply e.g. for medical equipment in the home orsome requirement commercially, or purely to charge using a cheap rate at night and use that energy when electricity is more expensive during the day.
In summary the choice of AC or DC, can potentially have big consequences if specified wrong, but properly chosen and installed wouldn’t make that much difference nowadays – however, I hope to have given you the tools to analyse this for yourself and ensure that you are happy with the choice you make.
Written by John Rowlatt