Re: new guy
The problem with electric is
1) the weight and cost of the batteries, and
2) the short range and low speed between charges
While it may only cost pennies to charge an electric, a vehicle the weight of yours would typically only travel at 15-20 mph for about 10 miles give or take 50% depending on many things.
Lead acid batteries are the cheapest at about $100 to $400 depending on capacity and voltage and you are looking at 50 to 100 lbs in batteries. Their typical life is about 2 years. Lithium batteries are less than half the weight but are 4 times the cost and at least double the life if you don't burn them out, which is very easy to do. So batteries are the big cost and problem and weight.
To keep all this weight rolling at 15mph you are going to need 500w to 1000w min, ideally geared to your pedals so you can use all the power on hills. A hub motor or hub drive isn't going to cut it.
Gas engines are not all equal. The China Girl motors are cheap and simple and complete, easiest and lightest to hook up. They are not the best word in efficiency and you have to be a bit of a hobbyist to get the most out of them. Most utility 4 strokes are only moderately efficient, but easy to hook up and you are only looking for 3-4hp for 15mph which is all you want with a bike and trailer. Again, ideally it would be best to have it geared or hills are going to be a problem, but then, you only have 10-25 lbs more.
I guess distance is the deal breaker. Longer distance you need gas, urban short distances make electric king.