Expected rectenna array sizes are expected to be in the square km range.
But compare that to the size of solar arrays necessary to provide the same level of power, and generate excess capacity to be stored for nighttime, foul weather, and changing seasonal influx levels.
About 3 to 4 times the physical size plus massive storage facilities. And the whole lot is as toxic as Hell itself. Either during fabrication or inherently as with most battery technology.
The simplest rectenna element is a piece of bent wire and a fast diode rectifier. More complex ones with multiple elements and a backplane can achieve efficiencies as great as 90% with a receiver cost of just a handful of dollars per square meter.
Solar cells are expensive as hell. It makes sense to place them where the maximum wattage can be squeezed out of them. If it's affordable to do so.
And the effect microwaves have on the materials, items, critters and people in their way are very very dependent on the chosen wavelength. For fairly obvious reasons the first limiting factor is that for the purpose of power transmission the beam be as minimally affected as possible by the Earth's atmosphere. Thus it's affect on the atmosphere is equally minimal. One major element in the atmosphere, is water thus one frequency that certainly won't be chosen is the one used in microwave ovens. So strangely enough it's not going to speed cook any people or birds that manage to get in the way of a beam. At worst it would amount to a slight warming effect as some lesser element or compound in the body does adsorb a small proportion of any beam.
As for total waste heat input into the Earth's biosphere, it would be no different to today. That energy is generated and turned into useless heat regardless of it's source. Fossil energy sequestered in fossil fuels is the least of our problems. It's the greenhouse gases which modify the Earth's atmosphere to trap solar energy all over the many millions of square kilometers that face the sun every single second. The few terawatts we dirrectly add to the environment with our activities are nothing against the hundreds of terawatts (petawatts?) we're now adding indirectly through globlal warming.
And perhaps, one day as we get the hang of this strange idea of cooperation we can think about retuning a few transmitters to react with water, not to harm, but to tinker with the weather to redirect or sap the strength of dangerous storms and to create and/or steer beneficial rains over crops and catchment zones.
With abundant and sufficiently cheap energy virtually anything becomes possible. Even if we limit ourselves to the physical resources of Earth, magnetic separation of ionised sea salt could provide almost any rare element we could ever need. It's woefully inefficient but if the energy is cheap enough the rest is very basic engineering. Bucket particle physics.
My fear is not that launch system limitations will prove too expensive, but that they will become cheap too quickly for us to learn the lesson of cooperation properly if at all.
I once saw an article quite a few years ago that spoke of a microwave beam climbing launch vehicle. The basic idea was to focus part of the beam with a parabolic reflector to create a shockwave in the air ahead of the vehicle and creating a partial vacuum in front of the craft. The remainder of the beam was rectified and used to power a ring current around the perimeter of the craft which accelerated heat ionised air past the craft to create forward thrust. Rather ironically the shape of craft was two shallow bowls face to face, with a parabolic bowl inset into the upper surface. Claims were made in the article that the concept worked in wind tunnel tests. If it scales up and ever works in the real world launch costs would plummet to the point where virtually anyone could establish an orbital presence.
A recent development of solar cells which can be printed onto a substrate strike me as making the idea of SBSP even more attractive as solar cells can be rolled onto spools in thousand metre lengths and unwound onto gossamer frames in orbit. My guess is between and acre and a hectare of 12-14% efficient solar cells per ton mass.
It really is starting to look like it is doable, and may well be done soon. Leave it to the military alone and problems will ensue. If it happens, it must be with full international cooperation.