Most of the issue with power is moving it. Generating power is not trivial, but it's easier than moving power, which is still very inefficient. Distributed solar panels on the roofs of Sears facilities might offset their own carbon footprint, but it's unlikely that the power they generate would have a significant impact on the surrounding area.

This is the reason we don't have giant floating solar panels in the ocean, for example.

For solar, wind, and tidal power, you get what you're given, and deal with additional demand by building overcapacity, through dispatchable power (conventional plants or biofuels in addition to hydro and geo), or by building storage.

Building a nation-sized battery is a pretty heady task. There simply aren't sufficient mineral resources for many electrolytes (including lead) to build sufficient capacity, though some abundant salts and metals may be suitable. Efficiency matters far less than scale and cost.

Floating solar hasn't been built because solar panels have until recently been more expensive than conventional sources, and more significantly, because marine environments are unbelievably harsh and electrical equipment will degrade quickly. It's far more cost-effective to simply build them on land. There's plenty of rooftops and unused acreage.

To increase the output of a thermal plant you need to increase the heat production, wait for the steam pressure to increase, then open the steam tap to increase the turbine power.

With the exception of New Zealand, which is ~80% hydro, most countries use coal for base-load, and hydro for peak generation.

Other peaking options are natural gas turbines and diesel generators, both of which are quick to start, and quick to throttle.

Additionally, Hydro power is easy to store - just leave the water behind the dam. Hydro will probably provide the national grid sized battery we will need to balance out transient generation like solar and wind.

It's the fact that they can be throttled. You cannot control the throttle on wind and solar. You can control the brake (how much of the generated electricity is released onto the grid). But if the motor ain't running you can't make it go. And if much of your power is dynamic and non-dispatchable, you've now got variance on both the demand and supply sides of your grid.

Hence: the need for storage. If you can take the surplus available at times when you don't need it and store it somehow, then you can match grid demand.

The problem with hydro is that there's simply not enough capacity. A small number of nations (New Zealand, Switzerland, Sweden, Brazil) have large hydro capacities (and relatively small populations for the most part). For the US, and much of the rest of the world, most available hydro sites have been built out.

Other options include geothermal. There's a vast potential under Yellowstone National Park, though it's politically untenable to even contemplate tapping it (a USGS survey of US geothermal resources didn't even mention it): http://pubs.usgs.gov/fs/2008/3082/

Iceland, Hawaii, Japan, and Kenya also have very significant geothermal potential.

http://en.wikipedia.org/wiki/SolarCity#Commercial_solar

Does SolarCity participate in the resale of power to the grid? I wasn't aware that they were an energy broker.

The building itself could use it, and if there was excess the neighboring buildings could use it.