Honestly the electrical energy output from tankless is pretty close to the gas needs from a tank, or was at least when I replaced mine a few years ago. Tankless is great for sure, but you'll likely need additional plumbing to add it, which is another cost to consider. It's not a slam dunk environmentally, but it's probably a step in the right direction if you're ok with installation costs. Maintenance costs on tankless are also much higher.