Well, I did a little research and came up with the following figures:
A 53 foot long by 8 foot wide reefer container has 424 square feet of roof area.
Typical solar panel efficiencies are in the range of about 10 to 14 watts per square foot.
So we'd get about 424 x 12 = 5,088 watts per truck roof. Assume an average full sun equivalent of 4 hours per day.
5,088 watts x 4 hours = 20,352 watt-hours per day.
Reefer compressors consume 15 to 20 thousand watts when they are on.
So, a truck roof completely covered with panels would provide maybe an hour of compressor use or approximately 3 hours of refrigeration, per day. If you could save a full day's worth of solar collection, and had 100% efficiency - but most inverters and other components add up to maybe 80% total storage/recovery efficiencies. OK, maybe we can get 14 watts per square foot... Total system installed price, with really decent wholesale costs for the equipment, maybe as low as $40,000 to $80,000 per truck. About 80 panels, about 200 watts each.
Average reefer fuel costs run in the range of $6,500/year per truck at $5.00/gallon, (online calculator), with solar providing 1 hour/day of a typical 8 hours per day of diesel (Note: It seems they are basing this on the estimate that the compressor runs about one third of the time, or 8 hours per day. See calculator.) So solar would provide about (1/8*100) 12.5% of total energy consumed per day: $6,500 x 12.5% = $812.50 savings on diesel fuel per year with solar. The break even would be: $40,000 divided by $812.50/year = 49 years... (without any tax breaks or subsidies or accounting for possible interest earnings if the $40K was kept in the bank, or the interest on a loan for that amount) but the cost of fuel will continue to increase, so the value of the system will also. OK maybe a 30 year payback without tax breaks or depreciation write offs... about the same result as putting panels on a house or business. What we need is a carbon tax and/or tax breaks and/or subsidies to make this a go... before the diesel is gone. But... by then survivors might just be growing most of their food locally, so the whole point may be moot.
Meanwhile this has been a great example of how much we take the enormous power of cheap fossil fuel energy for granted. 15 to 20 thousand watts to run every refrigerated truck on the road. Doesn't even include hauling up the hills and mountains, that's just for the refrigeration!!! Imagine if it all had to come from bicycle powered generators! That's a lot of hungry slaves pedaling really, really hard for their 1,500 mile iceberg lettuce salads!
Gentlemen: Start your gardens.