I haven't seen any numbers, but I have owned one pair of front panniers since 1999. I'm sure that except for the hook hardware, they would have no trouble dong a tour loaded with water. So, say 10" wide, 8" deep and 16" high, that's 2/3 of a cubic foot or 46 pounds of water. I have loaded them to 20 pounds a bunch of times. Go 10 pounds more with the rears and that would be 100 pounds for your shopping trip with 4 panniers. (I do the 4 fronts every Saturday at our local farmer's market. No cans or liquids so I rarely get past 30 pounds but those panniers have also carried books, cans, etc. Only weight concern was loading them equally for balanced steering. (Lowriders.)

Ortleibs are made by a company that does river rafts. The panniers use the same materials and construction. They can take the same abuse. (Falling off at 25+ mph and sliding doesn't phase them at all. (From not noticing I hadn't snapped the hooks shut at 4:30 am in my not so well lit garage and being in a pre-caffiene stupor. Happened many times.)

For serious non-car living - get the Ortleibs and just figure that you might want new hardware every two or three years. Probably $30 a shot. Any bike shop can order it. I'll bet you can also do it directly. I get the impression from the shops I have dealt with that Ortleib is a good company to do business with. And replacement hardware 8 years later on early Ortleibs was no big deal and went on easily.

Bigger issue: get racks that are strong enough. The original Blackburn Lowriders are marginal. They flex a lot and break at the fender eyes. The breaks are not catastrophic and usually show up when messing with the bolts, but for car free living those Blackburns are not up to the job. The Jahds (sp) are far better.

I lived car free until I was 32. Long before Ortleib but I did spend the last 5 years using Lowriders. (Life with truly waterproof panniers is SO much better! Thank you, Ortleib!)

Ben