I've had a 2015 Nissan Leaf as my daily driver for a little over two years. In that timeframe, I've charged multiple times from every flavor of electric car charger that the Leaf supports. It's not surprising that I often get asked which kind of electric car charger (actually EVSE; the charger is in the car while the charging station is a smart extension cord) one should buy, for home, office, retail business, etc.

The answer, as for so many things, is "it depends". What kind of electric car charger(s) you should install at any given location depends on multiple factors. At the minimum, you ought to be able to answer the following questions:

• Who are your charging constituents? (Customers, employees, yourself, etc)

• How long does a visit at the charger location last? (a few minutes, a couple hours, a workday, overnight)

• During the time span of a visit, how many miles of range is a "reasonable" amount for your constituents to pick up during a charging session?

• Do you plan to charge for electricity or offer it for free?

• How many simultaneous users? This dovetails with the length of a visit question.

• How much power is available at your location (more in the details below).

• What's your budget?

• What's your motivation? (Making a green statement for a business provides a different context and budget than what you might set up for yourself at home)

So that we're all on the same page, here's a little bit of electrical engineering background. If you already know the relationship between Wh and J, you can skip the next four paragraphs.

The joule (in my East Coast USA accent, prounounced indistinguishably from "jewel") is the SI standard unit of energy in general electronics applications. A joule can be described as one watt of power for a period of one second. Note that when we talk about energy, we are talking not only about power, but about time.

What's a watt then? It's one volt (of electromotive force) flowing at one ampere (of electric current).

One watt is LED flashlight territory, and one second is an awfully short period of time for getting any kind of actual work done, so just as we buy gasoline by the gallon (or litre for my non-US friends) instead of the shotglass-full, so too we like to talk about more manageable units for energy.

We could use "megajoules" (MJ) and be entirely correct, but the more popular way to think about meaningful amounts of electricity is "kilowatt-hours" (kWh), just as it appears on your electric bill (so it provides us a convenient cost reference). Which, since there are 3600 seconds in an hour, is exactly equal to 3.6 MJ.

Electric cars have "fuel economy" just as internal combustion engine vehicles do. Just like a traditional car, the observed fuel economy is variable by several factors including driving style (speed), outdoor temperature (batteries hate the cold, and there's no waste heat for the heater), etc.

In my Leaf, I've observed fuel economy ranging from 3.2 miles per kWh to 4.9 miles per kWh, the extremes being a day when it was 7F out and driving 70 mph to work on a limited access highway, and puttering about on country roads on a day when it was a pleasant 78 degrees out.

So the Leaf gets about 4.2 miles from a kWh of electricity. A Tesla or a BMW i3 will each get somewhat less miles per kWh because they're bigger vehicles. But a nice round number for our discussions is 4 miles per kWh for popular electric cars.

There are three broad classes of chargers available; each can put electricity into the car at a different rate (which varies from bottom end to top end by about two orders of magnitude). Remember the questions about how long your clientele are hanging around and how much range you want them to be able to pick up in that timeframe? These numbers will guide your decision here.

• A portable Level 1 EVSE usually is supplied with the car. This is a charger for plugging into a normal electrical outlet and typically draws about 1200 watts. That is about as much electricity as a toaster draws. You can't plug two of them into the same circuit at the same time or you'll have the same result as when you try to run a toaster, a waffle iron, and a mixer all at once on the same receptacle. Since the power rating here is 1.2 kW, that means that a car will be able to pick up about 5 miles of range per hour that it charges.

• A Level 2 EVSE is a small box that mounts on a wall, post, or other permanent object and is often hardwired to the electrical supply. It is always on a dedicated circuit. These are available in lighter-duty 16 amp versions and heavier-duty 32 amp versions. The EVSE tells the charger in the car how much power it can draw and the car behaves accordingly. They are also wired to 240 volts (if at a home) or a single phase of a 208 volt service if in a heavy commercial building or shopping center.

Assuming 240 volt power, a client plugged into a 16 amp EVSE will be able to pick up 15 miles of range per hour.

Likewise, on a 32 amp EVSE that comes out to 30 miles of range per hour if the charger on board the car is capable of drawing the full 7200 watts. The charger in my Leaf is only rated at 6.6 kW, and thus I can only pick up about 26 miles of range in an hour.

On 208 volt power, those numbers are more like 13 miles, 26 miles, and 22 miles (in the Leaf) per hour of gained range, respectively.

There are Level 2 EVSEs that are designed specifically for Teslas which offer somewhat more power than the more universal J1772-standard EVSE which I mention above. While a fine addition to one's existing electric car charger fleet if one's clients mainly drive Teslas, this is a less than satisfactory solution for an "only" charger. Teslas come with an adaptor that allows them to use "normal" J1772 chargers, but the reverse is not true; I can not use a Tesla charger with my Leaf.

Unless one is exclusively serving plug-in hybrids (which have smaller battery packs and lower spec chargers onboard) and have no plans to serve pure electrics now or in the future, the marginal cost savings of going with a 16 amp Level 2 EVSE vs. a 32 amp EVSE are not worth it.

• A DC Fast Charger (DCFC) is sometimes referred to as a Level 3 EVSE, though it's not "level 3" and it's not "EVSE". They gain their speed by bypassing the built-in charger in the car and charging the battery directly. The tradeoff is size and cost. These are large devices, the size of a soda machine, with an equally hefty price tag approaching the cost of the car. Unlike the Level 1 and 2 EVSE, for which there is a single standard J1772 that works either with an adaptor that comes with the car (Tesla) or directly plugs in (everyone else), there are three popular standards for DC fast chargers: Tesla Supercharger (Tesla), CHAdeMO (Kia, Nissan, Mitsubishi, Subaru, Toyota), and CCS (Audi, BMW, Daimler, Ford, General Motors, Porsche and Volkswagen). These run on 480v 3-phase circuits and can charge at a rate of between 25 kW and 120 kW (with even faster on the horizon), which can get a properly equipped client vehicle up to around 170 miles of range in a half hour... if the battery pack is capable of storing that much energy (most aren't).

It is possible to get a DCFC that will do both CCS and CHAdeMO.

UPDATE: If you're throwing tens of thousands of dollars at a DCFC, the cost of an adaptor from CHAdeMO to Tesla is inexpensive in the grand scheme of things; maybe get one and lend it out under the "hand me your driver's license for collateral" scheme?

Let's talk about some possible use cases and appropriate technology for them:

Case 1: At my day job, we were in the somewhat ironic position of telling our green committee that we did not want EVSE as such in our garage, that instead we would much prefer if a 100 amp 3-phase branch circuit were installed and 18 15-amp circuits hung off it, with one dedicated normal receptacle per parking space in the electric car parking area. This is much more cost effective than installing even a couple of EVSE devices and the cost savings allows us to have a sufficient number of seats at the table. Since we spend 8 hours at the office, even a 5 miles of range per hour charge rate gains 40 miles over the course of a workday. Another advantage if your management/building owner/insurance company is wringing its hands about unauthorized persons on your property, slow-charging infrastructure is a disincentive to "drop-in" charging by random people since you have to be there for so long to take advantage of it.

Case 2: Organic grocery stores in Herndon and Reston VA each have a couple of 32 amp Level 2 EVSEs on 208 volt power out front. A Leaf driver who comes in and shops for 45 minutes can get an easy 18 miles of range added. Two chargers is probably not enough for an organic grocery store; they periodically have contention difficulties.

Case 3: Reston Town Center has four 32 amp Level 2 EVSEs on 208 volt power in one of their garages. You can't count on getting one if you arrive after 11:40 in the morning - the lunch crowd takes them all and clearly four wasn't enough. But if you come in your Leaf and stay for an hour and a half for lunch and coffee, you can count on 39 miles of gained charge (or a message on your phone saying that your car has finished charging if it's only 30 miles from your house to your office to RTC). RTC also has a very interesting cost model for free/pay charging, which I detail below.

Case 4: A drug store in Arlington has one of the aforementioned ABB dual standard DCFCs, and an organic market in Frederick, MD has a CHAdeMO DCFC. Even a 10-15 minute quick drop-in can get you enough range to get home again after driving into the city for coffee or up to Frederick for a minor league ball game.

There appear to be three models for pricing access to charging infrastructure. The first is to give away the electricity for free, which is a nice a goodwill gesture and attracts clientele. The second is to charge usurious rates in an attempt to recuperate the cost of the equipment, pay for the electricity, and even perhaps turn a profit eventually (in the case of certain charging networks) by making electricity for cars cost the same per mile as gasoline at $3/gallon. Needless to say, this approach is a substantial disincentive to actually using the charging infrastructure. The third approach is to consider the EVSE to be a sunk expenditure and charge cost-recovery for the actual electricity, at a rate approximating normal tariffs. It is rare to see the third approach in practice; I've only seen it in municipal garages where there is a regulatory/legislative requirement to not give away government property for free.

If you decide that your model doesn't require charging for electricity, you can save a substantial amount of money by not getting a retail style EVSE with an RFID reader and all that stuff. A light commercial / residential EVSE costs a lot less than a retail one, to the point where you might be able to install three or even four for the same price as retail. Always having a charger available is great for the goodwill scale, so to a point more chargers = better.

Remember my comment about the office and dedicated normal 15 amp receptacles to plug your Level 1 charger into? If you have any Teslas coming by, you can provide some extra spaces for them for cheap (assuming you're not planning to charge for electricity) since the EVSE that comes free with a Tesla is also capable of charging from a 208/240v 50 amp circuit on a NEMA 14-50R receptacle - the same sort of receptacle that is standard at campgrounds for hooking up shore power to a camper. These receptacles are incredibly cheap compared to EVSE, and worth installing as "overflow Tesla capacity" if you are so inclined.

It should be clear at this point that the sweet spot in charging infrastructure cost vs. benefit is the 32 amp EVSE, with a nod to standard receptacles (both NEMA 5-15R and 14-50R) for special cases.

You'll want to talk to an electrician about calculated load if you're installing more than one or two of any type of car charger. The reason for a formal load calculation is that you can expect that they will all be used at once along with everything else that exists on the electrical service. It would be potentially embarrassing to have installed multiple electric car chargers only to discover that the electrical service at one's location can't handle the total amount of power draw of them all running at once and attempting to do so pops the main breaker.

If you're looking for specific model recommendations for (no fee) Level 2 EVSE, I've got those too.

The EVSE that I have at home is a Bosch EL-51254 which has a nameplate rating of 30 amps (not 32 oddly enough) and a 25 foot cord. The only reason I have the 25 foot cord is that the parking arrangements at our place are weird and require it. In all probability you can get by with a Bosch EL-51253 which is the exact same thing but with an 18 foot cord instead of 25, and save the $200.

The EVSE that Nissan sells with the Leaf is private-branded (with a Nissan logo) Aerovironment EVSE-RS with an 18 foot cord although a 25 foot cord is available for a $150 surcharge.

ClipperCreek makes several residential, commercial, fleet EVSEs - one that bears attention is the HCS-40. They also have buyers' guides, an evse selector tool, and such on their web site - good reading.

If you're looking at putting out a couple of NEMA 14-50R receptacles for Teslas, an RV campground pedestal may be just the thing. Note that these are loop feed with a breaker on each box for the receptacle on it. Milbank is also worth checking out - they make a lot of electric meter and service entrance things for the trade and their stuff is high quality.

Tesla referrs to their dedicated bolted-to-the-wall EVSE as the "Tesla Wall Connector" and they're even kind enough to provide their installation manual for your perusal.

It's really not necessary to charge for electricity, by the way. A typical electric car user dropping in for a charge is going to eat less than a dollar worth of electricity. You probably pay way more than this for coffee for your employees, advertising for your business, etc.

But maybe you've decided that you want to be on one of the pay-to-charge networks. There are actually some interesting use cases for these that don't involve usurious pricing - Reston Town Center allows the first four hours of charging for free, 5th hour is $5/hour, the 6th hour is $10/hour, the 7th hour is $15/hour, and so forth, all the way up until you're paying $100/hour for bogarting the EV charging spaces. The message is clear - charge your car and then move it somewhere else and let the next guy have a chance.

The popular networks around here (Virginia) are, in no particular order:

• SemaConnect - these are the people who supply the funky charging model for RTC, if that floats your boat.

• -chargepoin+ - these folks are the ones who are known to support the municipal garage cost recovery model over in Bethesda

• EVgo - the only folks I know of who are all in on public-for-pay CHAdeMO and CCS chargers.

I have RFID cards for all three networks in my car. It's the new gasoline company charge card.

Oh yeah, a word about signage. You probably want to put up signs that say "electric vehicle charging only", not "electric vehicle parking only". Believe it or not there are some entitled @#$^@ who think that they're getting a reserved spot as a perq for driving an electric car, not as an exigency for charging because not every spot has a charger in it. People who park electric cars in an electric car reserved spot and then don't use the charging facilities are no better than people who park internal combustion engine cars in those spots (or park in handicapped spots without having an actual handicapped person in the car, or abuse the parent-with-small-children spots, etc etc etc).

One last thing... The government doesn't hesitate to extract maximum value from you in terms of taxes; you shouldn't hesitate to extract maximum value from the government in terms of energy-efficiency tax credits, grants, and the like. The folks at the Department of Energy have curated a nice list of various federal and state incentives for installing EVSE.