David,
Today the grid is the prime source. Tomorrow with more self-sufficient homes, businesses and devices - offline generation would be the first choice supply - with the grid standing by to support peak demands or shortfalls.
What I've learned though is that the VP of sales and marketing has more control over all this than the technology gurus (unfortunately!).
Notice cell phone plans as an instructive model of what happens. People have plans that guide them to use offpeak minutes for their calls - and to buy group plans and committed usage plans - with penalties for exceeding contract amounts.
All these of course happen in the cellphone world with the consumer at the maximum disadvantage - not being provided with devices that show exactly what status and charges are.
So we may want to have our devices be able to deliver power in informed ways - but those VP sales and marketing are out there making sure that never happens!
Reality is probably somewhere inbetween - where market forces power some degree of consumer enablement.
Again lessons learned - PIPE XML was designed to support a supplier / distributor (re-seller) / consumer model.
How much we can actual create a supplier / (smartgrid) / (consumer|supplier) model remains to be seen!
Notice the missing piece in your cellphone is accounting / finances. In Africa now - people can exchange money via their cellphones. Banks on reluctantly support this - with controls back to their brick and mortar branches of course.
In case of the Smartgrid - this beckons too - where it may accumulate use credits for your local systems that contribute back to the grid. But who functions as the "banks" and the auditors, or do you need those at all - if secure deposit and drawdown is built in? When can you convert credits to actual money, or not (only certain entities can qualify as paid providers and have contracts accordingly)?
Making this all secure so the opportunities for "gaming" the systems is always a challenge too (as other posters here have already noted!).
Thanks, DW