Energy storage and solar system basics
Grid-tie PV inverters
This type of system supplies households with solar power during the daytime for self consumption and selling excess energy back to the grid. Households still rely on the grid at night and at times of poor solar yield. The system is wired in parallel to the existing household loads and any shortfall from solar is topped up from the grid. In the event of a grid failure, the system will disconnect from the household/grid supply and as a result typically cannot be used as a "backup" power supply.
A typical system comprises of a grid-tie PV inverter and solar panels. The PV DC is inverted into AC which can then be used by loads in the household. AC-coupled battery storage can be retrofitted and other forms of storage such a hotwater cylinders can also be incorporated to allow greater levels of self consumption.
An alternative to string inverters is micro-inverters where a small inverter is attached to each individual solar panel. This allows inivididual panel performance monitoring, management and high levels of redundancy as every solar panel in effect becomes a small solar system in its own right.
As LifePO4 batteries became more mainstream, affordable, safer and the incentives to sell solar energy back to the grid were substantially reduced, households realised the benefits of having energy storage to harvest their own energy for backup use or night time consumption.
The market for retrofitting energy storage to existing grid-tie PV installations has boomed and seen many offerings from various manufacturers, although many are actually OEM rebadged and customised products.
AC coupled storage functions very much like grid-tie inverters other than that typically a 48V DC battery is connected to an inverter/charger, which then supplies the AC to the loads on demand at night or when solar output is lower than the household demand. Batteries are also available in high voltage e.g. 100, 300 or 400V, although inverter choice still limits the available system options in this market segment.
The charger allows the excess energy from solar to be stored in the battery, which can then be used at night or at times of poor solar yield or to charge the battery from the grid as required. Some batteries can optionally, only if equipped to do so, be used as backup power systems, although separate essential load circuits will have to be provisioned. These will be isolated from the household during power outages for safety and legal reasons and will prevent power being fed back into the grid. Note that very few solutions are suited to prolonged use in this way and battery warranties may be invalidated if manufacturer guidelines are not adhered to.
A modern ESS can function without solar and even be used to harvest cheap off-peak grid energy for later consumption, although due to the initial huge capital outlay required, it is seldom economically viable from an ROI perspective and much depends on the types of flexible tariffs that may be available to you and our daily household consumption.
AC-coupled storage offers an easy way to retrofit storage to an existing PV system without having to replace existing hardware or impact exiting FIT contracts, although it will take up a fair bit of space for the inverter/charger and batteries and being AC coupled, a G99 application and DNO approval will be required as it will typically exceed the 3.68 kW limit for a G98 certified installation when added to an existing grid-tie inverter's capacity. (UK specific)
Batteries can come with varying warranties of up to 10 years when used in line with the manufacturer's guidelines. This means they may guarantee that after e.g. 6000 cycles a battery will still retain 60-80% of it original designed capacity if e.g. a DOD of 80% is not exceeded.
Cycles are counted when a battery is discharge/charged beyond certain levels, but that will depend on the manufacturer. The internal battery management system (BMS) will monitor and log these cycles and some manufacturers may also expect the battery to be connected to a portal in order to meet the warranty terms and conditions. In the absence of realworld case studies however, there is still much to learn and batteries are also evolving constantly.
ESS retrofitted to grid tie PV
Where an ESS (energy storage system) is retrofitted as in the example below, both the inverters are AC coupled in parallel to the household loads and the battery is charged from the AC supply with excess solar or even grid power as required. Both systems will disconnect from the household/grid supply the event of a power failure unless specific provision has been made for essential loads capability.
Hybrid inverters can probably be best described as all-in-one systems, which incorporate the inverter, charger, solar charger (MPPT) and DC battery connection. Apart from a relatively small footprint when compared to a grid-tie PV and AC coupled storage system, only one inverter is required as opposed to two, battery charging can be as much a 98-99% efficient being PV DC to battery DC, which results in very small losses as opposed to grid-tie PV DC to household AC, then AC charger to DC battery. Naturally various factors will determine, which system fits your requirements best and why starting with a good design based on a well defined set of requirements is important if you are designing a new system.
Our flexible Victron powered ESS systems offer the best of all worlds
The diagram shows a combination of all the types of systems combined together, also allowing essential loads to be powered in the event of grid failures. Where grid-tie PV inverters are combined with essential loads, it is important to note that it's output cannot exceed the maximum rated output of the Multiplus-II in this example, referred to as the 1:1 rule.
Victron offers a Multiplus-II, Multiplus-II GX and Easysolar-II GX which allows more design flexibility than any other product known to us today and systems can be tailored to very specific requirements. Being a popular choice in domestic settings, the Multiplus-II 3 - 5kVA are G98/G99 type tested and certified, but always check the ENA register or Victron website prior to committing to a purchase as Victron offers up to 15kVA single phase inverters, which while G59/G83 certified, may not have G98/G99(G100) certificates issued.