'Save on electricity bill costs' is dramatically being oversold.
People should really only consider the FIT's payments and 50% export, and, until studies have been done, treat the rest as 'bonus'.
To get the best 'electricity bill savings' requires people to match their usage with their generation. Ok for businesses when the majorioty of the usage is during the day, for domestic, people will be lucky if they use 50% of what they generate, so just because you are generating mores, you can't say that they will save more. Daytime generation curve is near enough a sine wave (see the SMA video on OptiTrac) Daytime domestic consumption has a peak early am, drops dramatically during the day, and peaks in the evening, long into the dark.
Check out the 'Average' Domestic profile not on economy 7 (it's what the 01 means on the MPAN )
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Usage of generated power will depend upon the size of their system and their consumption. Iin an 'Average' domestic situation, spread over a year, where say, they will generate 2500kWh and their bills say they consume 3500kWh, most domestic users will be lucky to use 1000kWh of what they generate, put a 3500kWh system on the roof, and they will still only use 1100kWh of what they generate.
The problem is a bigger system just generates more when it's not being used, hence the discussions about monitoring export and the work we're doing with CurrentCost.