Picture source: Wikipedia.
A recent piece in The Guardian on solar energy  made two claims. One was fair comment but the other was very misleading.
First, it said that the present government is unsympathetic to green issues. I think that many people may have deduced that voting Blue does not lead to very Green policies. In the ‘small print’ of this year’s budget, Mr. Osborne covertly abolished a significant subsidy to renewable energy.
Second, it suggested that solar photovoltaics (PV) in the UK can already generate cheap energy. In support of this, it cites a generation cost of 8 pence per kWh (p/kWh). This is, of course less than the price that most domestic electricity consumers pay, which is nearer to 15 p than 10 p/kWh.
But this is incomplete and misleading. The majority of our energy is needed as heat or transport fuel, not as electricity.
In terms of electricity, solar PV produces masses of it on a bright, cold April day, much less on a grey day in November. To produce a stable electricity supply, such a source can only contribute a modest proportion of total electricity consumption; i.e., a few tens of percent. If it is to contribute more, electricity must be stored for use later.
Long-term bulk electricity storage is expensive compared to storing energy as chemical or thermal energy; i.e., as fuel or heat. One can safely say that the more outlandish concepts put forward may never work commercially.
For stability, today’s national grid needs short-term electricity storage in pumped storage plants like Dinorwig. These can be turned on instantly to replace the loss of a large generator elsewhere on the system or to meet a sudden, unexpected load.
The grid also needs spinning reserve. These are fuel-burning generating plants that are kept warm, with the turbine blades rotating in case a capacity shortage develops.
The grid also benefits from having interruptible loads. In return for payments, some large industrial consumers agree to be cut off for up to 20 minutes in emergencies; e.g., if a large generator fails. 20 minutes gives time to start up diesel generators and keep other consumers supplied. Many smaller loads could be made interruptible; e.g., industrial cold stores and even a portion of the lighting load, utilising dimmable ballasts.
The above measures are needed even when 75% of electricity is generated by burning fossil fuel in despatchable plants. For further security after fossil fuels, we would probably need the above plus power-to-gas (PTG) plants which turn surplus wind or solar electricity into fuels such as methane . Such fuels can be piped around, stored cheaply and indefinitely from year to year and reconverted to electricity when needed  .
The round-trip efficiency of PTG would be lower than that of pumped storage, which is around 70%. But chemical energy is a lot cheaper to store than potential energy; i.e. water in two ponds at a height difference of several hundred metres.
So, 8 p/kWh is good news for PV but it is a desperately incomplete figure. PV electricity produced on a bright day in May could probably be free and still the cost of supplying solar power year-round to consumers to run their lights, domestic appliances, office equipment, electric railways, etc would be higher than the cost of gas-generated electricity today.
If money matters to people, measures costing 3 p/kWh look a considerably higher priority than electricity supply costing 8 p/kWh at the point of generation and more with the national grid’s costs added. My blog of 1 February 2014 set out how the more efficient use of electricity could substantially reduce consumers’ bills.
 www.theguardian.com/business/economics-blog/2014/mar/20/george-osborn e-budget-kill-renewable-energy-revolution-tax-break?CMP=twt_gu.
 Just as natural gas can be stored. Long-term storage is needed to provide a secure energy supply from a mix of intermittent and variable sources like wind and solar. The output of wind energy is particularly variable between years. Although the round-trip efficiency is significantly lower than pumped storage, once the fuel is produced it can be stored indefinitely without losses.
 Or used in road transport. This operates more easily and cheaply on portable fuels than on electricity.