God Save Me From the Type IIA

[Schroeder]

Going on electric engines doesn't conserve fuel. After you surface, your diesels will have to recharge the batteries. And by doing that they burn more diesel than if you had sailed on diesels alone (and while you are recharging your Type II it will only run on one propeller and thereby lose speed.).
Think about it. When going on diesels the engines turn fuel into movement (turning the propellers). When you go on battery power your diesels will have to transfer fuel into movement (running the diesels) and movement into electrical power when you recharge the battery (driving the generator). Later the electrical power has to be turned into movement again (running on electrical engines). Whenever you change one energy form into another you will always "loose" energy (actually energy can't be lost but you will turn it into unusable waist products like for example heat...).

So stay on your diesels whenever possible for maximum range.