Gas or Electric Tankless?

I would rather go for tankless water heaters. Although gas is more energy-efficient than tankless, tankless is easier to maintain and has a 5-6 GPM, which saves me a lot of money compared to my old water heater.

In Ukraine where I live gas taxes are so high atm so in such a case I really recommend the electric one

For your situation, an electric tankless water heater would likely be the best option since you're adding solar panels to your house. This aligns well with your goal of minimizing operating costs. In terms of determining the right size, or gallons per minute (GPM), you'll want to calculate the peak demand in your household, especially considering the possibility of three simultaneous showers. While the chances of this occurrence might be low, it's wise to plan for peak usage to ensure sufficient hot water supply. Consider factors such as incoming water temperature, desired outlet temperature, and flow rates of your fixtures to determine the appropriate GPM rating for your tankless heater. Consulting with a professional plumber or an expert in water heating systems can provide valuable insights tailored to your specific needs and circumstances.

nice work going on