That's part of the question. What degree of protectiveness would be realistically feasible on a Power Armor? I imagine that protection against things like sabot and HEAT would be superfluous, as the Power Armored soldier can make use of mobility and cover to avoid direct hits. Protection against secondary damage from heat and explosion would be highly desirable, on the other hand.
The armor strength would also depend on what you think the primary type of missions power armored soldiers would be involved in. In urban warfare clearing insurgents from buildings? Or assaulting enemy positions while weathering hostile artillery or air cover?
Maybe superflous against HEAT rounds of artillery size. But once heavy armor become widespread on infantry soldiers, would it not then become profitable to develop rifle caliber HEAT rounds just for antipersonel use? Also, once heavy body armor become really widespread, it would probably become worthwhile to equip rifles with flechette rounds similar to anti-tank sabot rounds to pierce the armor.
It is said modern HEAT and SABOT technology can punch through essentially any material up to a thickness equal to 6-7 times the diameter of the charge. If one were to incorporate HEAT technology into a 5.56mm round, the soldier would need to wear a suite 2 - 3 cms thick. If one were to go back to 7.62mm rounds, the soldier would need protection about 4 cm thick.
3 cm of steel will protect against splinter penetration by a 155mm round exploding 2 meters away.
But I am not sure if armor will help enough in protecting against the concussion and blunt impact of the soldier being thrown by the force of the 155mm round exploding 2 meters away.
To fully protect a human would require a minimum of about 2 square meters of covering. A suit made of steel 4cm thick covering a human completely would weigh 750kgs (3 time the load carrying capacity of a WWII jeep). If the protection is restricted only to the vitals the suite would still weigh around 300 kgs. Using ceremic a vitals only suite would still weigh about 150kgs. This is if the amor only protects the man.
If you put power source, the actuators, the electronics, etc all under armor, the armor would evidently weigh more.
If the powersource, actuators, electronics are outside of armor, and weigh about the same as the armor, then the kit will weigh about 300kg, in addition to the weight of the men and his weapon, which would likely amount to another 150kgs. The total is about 450kg for a single soldier in a partially protected powered armor suit, or about 3 times that of a normal soldier.
A head to toe suite that also cover actuators and vital electronics would probably weigh 3 times more. Put weapons also under armor and the weight go up still further.
Once the soldier and his kit weigh 1000 kgs the temptation must be great to load him with more than just a 4kg rifle. One the caliber of weapon increases the need for more armor also increases, so the weight spirals up again.
At some point in weight and complexity the greater automotive efficiency and simplicity of conventional wheeled or tracked modes of locomotion will start to outweigh the flexibility of legs.
Pretty soon you have a one man tank.