In most places, there is a peak in electricity demand in the late afternoon and early evening.  Solar PV panels clearly cannot generate beyond dusk, which leaves a big gap in the early evening.  However, 4 hours of storage allows daytime solar PV output to be spread to cover the evening peak. [br][br] There are options for the connection between the solar and the storage as follows:- [br][br]

• Independent – the battery storage system is completely independent of the solar PV farm and does not share a grid connection.  There are no constraints on the charging or discharging of the storage.
• AC coupled – the battery storage system and the solar PV farm share an AC grid connection with limited maximum capacity (both outwards and inwards).  But otherwise, the two can operate independently.
• Loosely DC coupled – the two share a grid connection and a bidirectional DC to AC inverter.  The storage can be charged  from the solar PV output via a DC to DC connection, and from the the grid via the inverter subsystem.
• Tightly DC coupled – the two share a grid connection and a DC to AC inverter.  The storage can only be charged from the solar PV output, via a DC to DC converter.

[br] Although storage is expensive right now, compared with solar PV technology, a few things work in its favour, though detailed costs matter a great deal. [br][br] Firstly, for utility solar PV, DC capacity is normally configured considerably higher than the AC grid connection capacity – a ratio called the ILR (inverter loading ratio).  Because the sun isn’t always shining directly in daytime, the optimum financial return from a project for US fixed tilt C-Si systems in 2021 used an average ILR of 1.38. [br][br] The implication is that, around noon on a sunny day at mid summer, nearly 40% of the possible solar power will be clipped by the inverters, and go to waste if there is no storage.  Adding storage to a financially optimised solar PV project captures this energy, otherwise wasted, to charge the battery, then sold at peak evening electricity prices.  The more sunlight in a location, the lower the cost of the stored electricity. [br][br] For ILRs of 1.3 or less, only a few percent of DC power from the solar panels will be wasted due to clipping. [br][br] Secondly, solar panels are less than half of the total utility solar PV farm capital cost.  In particular, DC to AC inverters and a grid connection are quite expensive, and some configurations solar plus storage enables these and the AC grid connection to be shared between the solar and the storage. [br][br] Note this is only an advantage for storage connected by a DC link to the solar panel output.  If a solar plus storage configurations is connected by an AC link, then the installation carries the cost of independent, high quality, DC to AC inverters for the battery storage.  However, the grid connection can still be shared. [br][br] Thirdly, in regions where a lot of solar PV capacity is installed, such as California, wholesale power prices are generally higher in the evening than during the daytime.  Even if power from storage is more expensive than power direct from solar PV, it may well be worth selling it if prices after dusk are a multiple of noon prices. [br][br] And lastly, some auctions are open only to solar PV systems of a maximum AC MW capacity.  Provision of storage enables more power to be sold for the same connection MW capacity. [br][br] The financial optimisation of a DC coupled solar PV + storage system, including the ILR ratio, will be different from that of a standalone solar PV system.