I have a Solis 3.6 KW inverter which works well until it exceeds about 2000W when it shows "over grid voltage" and shuts down. This does not happen if something is switched on in the house which draws a large amount of current such as the immersion heater. I have a voltage meter in the house which can be showing around 240V, but as soon as the inverter starts up, the voltage climbs until it reaches around 258V. As soon as the inverter shuts down, the voltage returns to normal. The company who installed the inverter insist this is a grid problem and there is nothing that can be done about it. Perhaps they are right, but the 6mm cable which connects the inverter to the consumer unit is about 30 metres long and is about 40 years old. Could this be adding to the problem, or indeed causing it? The cable can't be shortened, but would a thicker cable be able to absorb more volatage and stop the overload?