Here's my experience/opinion.
The only way to really be successful is to go all-in on generator sales & service. Its not a secondary service you can offer and make money. We sell Generac and the margin is about 4%, and then you have to pay freight to get it to you. If you're a high-volume dealer its better around 10-15%, and you're ordering a whole truckload of them to offset the per-unit freight cost. You also can't compete with Home Depot & Lowe's or any of these online dealers. You can't sell for less than MSRP and make money. I can go to my local supply house and buy a Generac for less than what it costs me as a low-volume dealer because I don't have to pay the freight.
The money is in service. If your goal is to sell anything less than 100 a year, you're wasting your time. I'm certain others will disagree. You're selling a product that REQUIRES maintenance and attention. You need a tech that is hands-on with them every day that can fix them quickly, not working on 2-3 a year.
I started selling them because existing customers kept asking me repeatedly about it. I've sold maybe 2-3 dozen in the last couple of years and all I did was create more work I didn't have time for. I don't have time to run around and check these things every time a storm is coming, or do the oil changes, but I have to because I sold them. Now I just flat out refuse new sales and send them to a friend of mine that is a really high volume dealer.