that means you overpaid on solar, if you have 10kWh day excess (even sometimes), you paid good 15k for energy you don't need and can't store. so the cost is not zero, you overbuilt the solar farm. if you have the system set correctly, you don't have much excess you can't store, certainly not 10kWh per day, even if there are some exceptional days. also, in many countries, there's a limit of 10kWp, if you go more than that, you enter into commercial producer area - regulations, taxes, ...