John,
That is sometimes offered as a reason for better emissions but it isn't always the case, and whether it is or is not comes down to a ton of factors. On one end of the scale if the marina (or your garage) was powered from a solar or wind station then even with all kinds of inefficiencies in the system there would be fewer emissions involved. At the other end if the power is coming from a coal plant hundreds of miles away then you're probably emitting less just burning the fuel where you are. This comes up when trying to figure out whether a gas range or an electric produced more emissions, if the power plant is burning the same natural gas as the stove is then its going to be more efficient to just burn it at the stove than to burn it at the power plan, convert it to electricity, send the electricity down the lines, and then convert it to heat, better to just make the heat at the point of use. It feels intuitive than an electric application produces fewer emissions than a fuel burning one but that's largely because the emissions aren't right in front of us with an electric.
The case of plug-in hybrids is similar, if your power comes from coal (like most of ours in the US does) then you're probably making more emissions with a plug-in instead of just burning it where you need the the energy, you could start calculating your mileage in tons of coal instead of gallons of gas. Just as an experiment a few months ago I worked through some of the equations to try and figure out whether my gas law mower was producing more emissions than my neighbor's electric law mower. Our municipal power here comes from a coal plant. I burn about 2 gallons of gas a year in my mower. I didn't have the emissions levels for the local plant or any idea of the efficiency of the grid bringing the power over to our houses so those were just estimates in the equation. The solution was he was probably producing more emissions using an electric mower than I was with my gas mower. His mower uses less power to do the job because of the limits of 120V electric outlets, but the exhaust from my gas mower makes a lot less of the bad emissions that a coal plant does. The equation could easily have tipped the other way with a more efficient power plant or a better distribution grid, but it would still be pretty close. Of course, the guy down the block who mows once a year does better than either of us!
Its certainly true that a power plant can be more efficient with its fuel use per useful energy out, but most power plants aren't that modern and even most of the new ones aren't using the best technology either, even with the greater efficiency you have to take into account the transmission of the energy, which can sap over half of it in some cases.
I guess the short answer is if you're burning fuel where you need the power then you know exactly what you're using, if you're plugging in then you have no idea, without a lot of work, how much energy is being used or what the emissions might be.
David
On the issue of emissions, the whole point of plug-in hybrid cars is that the energy you get from the electric grid is generated more cleanly per kWh than from an ICE due to more efficiency in a large-scale power plant. Shouldn't the same hold true for a plug-in hybrid boat?John
Sent from my iPhoneAs for reducing fuel consumption for a short outing. If you're charging the batteries with the generators then you're not saving anything, if you're charging at the dock then you're just displacing the emissions and again, it might be more or it might be less depending on distance to the power generation state, efficiency of the power grid, type of power plant, efficiency of battery chargers, etc. In general, point-source fuel use creates fewer emissions, but like I said, there are so many factors involved it can go either way.
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