In either type it is the joule/energy rating that gives the jolt and keeps them in after the first time. Electric fencing is a psychological barrier rather than a physical barrier.  You can have high voltage but little energy and its like rubbing a balloon on your hair - lots of volts but no current.  Joules are what make the first touch memorable.

Best is a charger with voltage in the 6-9 kv range and also 6 joules.  Voltage is cheap but joules are expensive in a charger.

I wouldn't use a charger with less than about 6 joules.  I'm not sure a solar can get that high but as close as you can get or go 120v.

Jim at Dawn

PS having grabbed a 7kv + 6 joule wire that I was sure was disconnected but was wrong (back fed thru a slinky gate far away), I would say that contact was VERY memorable. So much so I am afraid that anything much bigger than that might cause heart stoppage on a person with a weak heart or a child that accidentally touches it.

I find 6-9 kv + 6 joules is a balance between keeping the cattle in and safety for people. A hot wire from this charger on standoffs on both sides of an old 4-barb wire fence worked for keeping a neighbor's "high strung" Jersey bull and my Hereford bull apart.