New electrical install problem. Low Current?
You need to perform measurements while the loads are applied. There are plenty of power test devices that are pass-through: one end plugs into the outlet, and the device itself has an outlet you plug the load into. It'll display the real power, the mains voltage, etc. If you worry about any voltage drops/sags, you'll want one of those. One brand (with some copies) is Kill-A-Watt. It appeared first on the US market, but you can find very similarly looking devices for the UK market. One of them is called Hopi, as used by bigclivedotcom.