‘Next Best’ Component Failure

A year or two ago, I designed a solar power system "for Africa": one which could be built without tools or test equipment, with components (hopefully) readily available: see African Solar. Last week, I built my own system, yet within a week it failed (several similar systems are still going strong). It was due to something I hadn't reckoned with when thinking "Africa". When I examined the failed circuit, I found that the electronics store had given me next best components, presumably because they didn't have stock. I re-examined another system which had failed a little too soon. I discovered that the store had done much the same there. OBSERVATION: It was critical components (Zener diodes) which cost a mere R5.00 / US50¢ each. In one case this had expensive consequences.