Steve Fetter (physicist, and Professor at School of Public Affairs,
U. Maryland) has posted a critique of space solar power here:

http://www.fpsboard.org/forums/showflat.php?Cat=&Board=pands&Number=84&Main=81#P\
ost84

(click on the Attachment link) where he claims:

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SSP could compete with earth-based solar power only if all of the
following conditions are met:

* solar supplies ~100% of total electricity demand (SSP cannot compete
if solar supplies less than 20%, even if launch costs are free);

* the cost of space-based solar arrays is reduced to $1000/kWp
and that earth-based arrays do not cost less than space-based arrays
(SSP cannot compete if earth-based systems cost 30% less);

* SSP transmission costs no more than $0.01/kWh and is no less efficient
and no more expensive than storage or intercontinental transmission of
electricity generated by earth-based systems;

* SSP operation and maintenance costs no more than $0.01/kWh and
is no more expensive than operations and maintenance of earth-based
systems; and

* launch cost to low-earth orbit (currently about $10,000/kg) is reduced
by a factor of 40, to less than $250/kg.

The probability that all these conditions could be met simultaneously
within the next 50 years is vanishingly small.
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Arthur