"How far can you REALLY go?"
My wife and I both drive electric cars, so we get asked that question a lot.
It is almost identical to the "what MPG will I get?" question in a gas car, which also cannot be answered with a single number. The same things affect it, although some in different amounts. It depends on speed, acceleration, wind, temperature, terrain, cargo, tire condition, accessories and other factors. Top Gear got 18mpg running a 48-mpg-rated-Prius around a track, and hypermilers have seen 90mpg--so the variation can be very wide.
So I cannot give a simple answer; but I will try to give some simple-but-rough boundaries to set expectations. This chart is for the all-electric Tesla Roadster. True, the Roadster is not ideal for the mass market; but it has been available for a couple of years so there is a lot of data for it. Its rated range is 244 miles; actual range will usually fall somewhere in the colored portion of the chart below.
How fast do you plan to drive? Find that on the chart, then look up at the top and bottom of the colored area above it. Your range is likely to fall in there somewhere, depending on how many accessories you have turned on. This is mostly about A/C and heat--headlights, wipers and radio don't make much of a dent.
For a given speed, the best range (the top edge of the colored area) is Tesla data; it can be found at http://www.teslamotors.com/blog/roadster-efficiency-and-range. Tesla owners driving on a nice day, flat road, at a constant speed with all of the accessories turned off get this range. Note that if you go less than ~55mph in these conditions, you will get more than the rated range; faster, you will get less. At 70mph your range is only 3/4 of the range at 55mph.
The lower edge of the colored area is the Tesla range data minus my calculated draw for running all of the accessories at the same time. After speed, climate control is by far the largest power draw--especially in the Roadster which has a resistive heater (many EVs use heat pumps that are much more efficient). This is what you can expect if it is a really cold, rainy evening, and you have the heat on full blast and A/C running to clear the windows, headlights, radio, etc. Going slower means you spend more time running accessories, so there is a larger effect at slower speeds.
There are other factors that could stretch out this chart. Some, like wind, can help as much as hurt. Others, like quick acceleration, the driver has control over and can avoid when range is an issue.
Worst-case range "rules"
The chart is broken in to three areas, based on the types of roads you are driving on.
If you are like most people, your eyes were immediately drawn to the lower left of the graph. The 244-mile Roadster might only get 85 miles of range! (Few notice that it is also possible to go over 400 miles). That sounds like a deal-breaker, but remember that at that end of the graph you are moving very slowly. Even with all accessories on full blast the whole time, it would take 8.5 hours to drain the battery. At these speeds, the amount of time in the car is more likely to limit a trip than the range--especially given that time and range both shoot way up if you do not use the accessories heavily.
On the right end of the chart, at freeway speeds, accessories do not make as much difference--the big hit there comes from pushing all the air out of the way. Between 60 and 80mph, accessories take an additional 26 to 34% toll. Let's say they could drop your range by up to 1/3.
In the middle of the chart, at arterial speeds, the top line slopes quite a bit, but it is all above the rated range. We will focus on the bottom line, which is fairly flat. From 35 to 55mph, the worst-case range varies from 170 to 182 miles. 170 is still more than 1/3 less of the single-number rated range.
In an attempt to make this simple, we will ignore city driving where range varies the most, but is least likely to be a concern. Considering only arterial and highway driving, then: Expect the rated range at 55mph in good weather (significantly more if you slow down). Deduct 1/4 if you want to go 70mph. And, worst case, deduct up to 1/3 in extreme weather for heat and/or A/C.
Reality check: how far do our cars really go?
Our experiences match these numbers very well. At 55mph, I have seen the full 244 miles of range on a 70-degree day and an almost 1/3 reduction in a cold heavy storm where I had both heat and A/C on. At 70mph, I see an almost 1/4 reduction. (I have not yet done both at the same time).
At lower speeds, I am usually just driving around town and in no danger of stretching the range, so I go ahead and pop the top off (worse aerodynamics) even in very cold weather and crank the heater. I also accelerate quickly whenever traffic is clear. Despite not being careful, I have still never seen more than a 1/3 total reduction in range in over a year of driving.
Things are even better in my wife's 100-mile RAV4-EV, where the worst range we have ever gotten was a 1/4 reduction. That was at 60mph, with 5 adults and cargo, in heavy rain so all accessories were on (although the heat wasn't all the way on).