Following my previous post there was some discussion about the need for grid backup of solar PV to deal with extended periods of overcast weather. It’s obvious that storage will help with this to some extent, since batteries can store electricity from the grid as well as from distributed solar. I thought I would try to put some numbers on this (slightly changed from last time to simplify the numbers).
I’ll focus on 1 kW of solar PV generating an average of 4.8 kWh per day, with (as before) 2 kWh of storage. If there is zero solar generation and no demand management, the entire 4.8 kWh per day must be drawn from the grid. In the absence of storage, we might suppose that 1kW of backup capacity is needed to match the peak output of the solar PV system. But with storage, all that is needed is enough to supply 4.8 kWh over the course of a 24-hour day, that is, 0.2 kW.
The optimal backup choice is a fully dispatchable technology such as hydro or gas. Hydro resources are pretty much fixed, so I’ll focus on gas. According to the US Energy Information Agency, the capital cost of gas-fired power plants is around $1000/kw so our grid backup will have a capital of cost only $200 for each kW of distributed solar. There’s also the need to take account of fuel and distribution costs. Fuel costs will be low since the system is only used as backup, while distribution costs will be around 20 per cent of what would be need if peak loads were to be met by centralised generation.
To sum up, if battery storage becomes available at a sufficiently low price, there’s no obvious problem with a system in which over 80 per cent of capacity, and an even larger proportion of generation is distributed solar PV.