Originally Posted by IanB

Your electrical system should have no issues powering factory equipment, even with an aftermarket deck (which I power straight off the battery on a fused line personally), if the amp is stressing the system, then you need a cap. If the load is a concern, then the solution is to get a cap, not a gauge that tells you that you need a cap. You can check the status of your system with a multimeter if that's all you want to do.