Mild Hybrid doesn't run in electric mode.. it's a very small electric motor and battery. In most cars, it's really just an electric stop-start. In a few, it adds a couple.of horsepower to help with acceleration. In any case, it has no real benefit to the driver; it's just there to improve MPG and rescue carbon emissions under test conditions.
This is the one where there seems to be most confusion over, certainly going by comments I read on the forum. Even with some regular members (who I'd assume to know better) I've read comments like, "mild hybrid wont drive very far on electric power". As you say, a mild hybrid (MHEV) won't drive any distance on electric power alone, though it may coast for a bit with the engine off and the engine may switch off as you slow to a stop.
In ascending order of electric power you've got:
MHEV, mild hybrid electric vehicle(!). As mentioned above, this will not drive anywhere under electric power alone, though you may get some engine off coasting and/or slowing down.
HEV, hybrid electric vehicle. This is a self charge hybrid like the Yaris, capable of driving up to a couple of miles at a time on electric power. The battery recharges very quickly, so rather than thinking about how far it will go under electric power, think about how often. Thats why, for someone who does most driving in urban environments, the benefits can be very large.
PHEV, plug-in hybrid electric vehicle. This will have quite a big battery and some examples can drive more than 30 miles under electric power. Obviously (as the name implies) they need to be plugged in to recharge (though they will recuperate power through braking the same as an HEV, with such a big battery the percentages gained are very small) and they are also very expensive.
EV, full electric!
More confusion also comes directly via the manufacturers who bandy about terms like "hybrid technology" (in reference to MHEV's) and often have badges on the car saying so. The cynic in me says that this is simply because the term "hybrid" sells cars and people latch on to it. They buy MHEV's and assume it is the same as an HEV when it isn't. While it is a bit of a con, it isn't that much, because it isn't actually hard to find out whether the car in question is MHEV or HEV if the person can be bothered to look it up.
But it is also completely pointless to pontificate between a solely ICE car and MHEV. If buying new you don't have the choice, and if buying used just go for the youngest version you can afford and ignore whether or not it is MHEV.
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