here in Mid Wales the hills are a tad too steep, however I am sure if I lost some weight it would get me up the hills.
When choosing a motor it is probably important to know that different motors and different controllers behave differently, especially on steep gradients. Obviously there are hub motors and mid drive motors. Mid drive motors take advantage of the gearing of the bike, thus they are usually good on hills. The downside ist that they are relatively clunky and because they use the transmission of the bike chain wear may be an issue. As the motor sits on the pedals shifting gears can to a degree become an aspect, too, especially with hub gears as hub gears prefer not to be shifted under load. Also mid drives are wider than normal bottom brackets and thus your feet are wider apart than on a normal bike - a fact that you may not recognize or dislike massively (individual taste). Pedaling w/o support often feels a little odd with them. On many folders retrofitting a normal mid drive is not a good option as it may conflict with folding (i.e. on a Brompton with folding the rear wheel under the frame). Retrofit options come from a couple of companies with Bafang and Tongsheng being the most popular ones. Both work fine. Motors from brands like Bosch, Brose and others are not retrofittable as they need a special frame to take the motor, so these are for factory builds. You should have in mind that electric motors are still evolving technology. One downside of a frame proprietary for i.e. a Bosch, Brose, Shimano, Yamahe or Panasonic motor is, that the motors may change over time and if in ten years from now your motor dies and you need a new one with some bad luck the fitting may have changed and in the worst case you can only scrap the frame as no fitting motor is available. This is not only phantasy as i.e. Continental did produce mid drive motors with proprietary frame fitting and went out of the electric bike motor business like two years ago. I you happen to have a bike with a motor from Continental you may look very sad in a couple of years when/if you need a spare part or replacement motor.... Generic motors for ordinary bottom brackets like the Tongsheng or some Bafang models do have an advantage here as the type of bottom bracket they are made for has been a standard already for decades also for manual bikes.
When it comes to hub motors, front or rear wheel, there are two main kinds: Direct drive and geared drive. Direct drives are more silent, typically stronger, reliable and a pleasure to ride in the flat as they do react instantly. However: They are heavier and, due to the lack of gearing, they depend from the right amount of speed (or more precise the optimal rotation speed of the hub) to work efficiently. Thus they are not optimal on steep hills as, with the bike becoming too slow, they run outside their optimal rotation speed with consequences for their efficiency. Heat may build up, power consumption goes up while supporting power goes down. These kinds of motors were dominant a decade ago with BionX being the most popular brand. Since then middrives took over, mainly Bosch, and direct drives in the rear wheel lost popularity. There are a lot of reasons why and only some of them relate to advantages of the mid-drive. Today BionX went bust already a while ago and others like GoSwiss went out of the motor business. Direct drives are still available from companies like Albers with it's Neodrive, Heinzmann and ebikes.ca/Grintech with the Crystalite Motors. However, those are not on the very cheap side. There are others as well, but not very commonly known. Personally I still consider a rear direct drive the most convenient and natural way of electric support. It just feels totally natural when riding. Total silence and long term reliability/robustness add to that. The only downsides are weight and the hill issue (with the "hill issue" often being totally overrated, it is in practice way less bad that what rumors claim). However, if I lived in the Alps and were to buy a mountain bike I'd probably prefer a mid drive. On less massive and constant gradients it is usually less of a problem. Furthermore i.e. Crystalite offers a special torque version with more power on hills.
Pedaling a direct drive w/o motor support is fine with one issue: As the motor ist relatively huge and heavy it adds to the rotating mass. Therefore coming up to speed w/o motor support needs a little more effort than with an normal unmotorised bike. Again your mileage may vary and different motors behave a bit differently regarding the extend.
The other option are geared motorised hubs. They have been and are by far the most popular option for retrofitting mainly because they are cheap. There are plenty of manufacturers and models, almost exclusively coming from Asia and those are the motors you find in most retrofit kits. A front wheel drive is also easy to retrofit, that adds to the popularity. However, one should be aware that forks of older bikes have not been designed with a motor in mind. As with retrofitting of disk brakes force may show up in places the constructor did not have in mind and the fork may be able to handle that or not. Due to the gearing they are relatively noisy - they produce the typical tram sound. Because with most motors the gearing is made of plastic and they are cheap mass production products many may not last forever if you do a lot of miles. The internal gearing has three effects apart from the noise: The motor can be smaller, so the hub is lighter. Because the motor is smaller it typically has way less torque than a direct drive or a mid drive. They are often better on hills than a direct drive (but worse than a mid drive) because they can rev higher, but then they have less torque which limits their climbing abilities. Basically the problem of optimal rotation speed of the motor persists but is mildered by the gearing. The lower torque compared to direct drives may become a problem. The upside of folding bikes is that most of them have smaller wheels which is a plus when it comes to hub motors in hilly areas - they will perform better than with big wheeled bikes. This also applies for direct drive motors.
There are also higher quality geared hub motors like from Mahle or the motor Brompton is using for the Brompton Electric but most of them are not available for retrofit. But even apart of them there is a huge range of quality and price and the differences in quality are not easy to distinquish. The more as the steering by the different types of controllers and their programming vastly adds to how a motor feels. The very same motor can behave and feel totally differently, depending from the controller that is used and how it is programmed. I.e. the kits from
https://www.cytronex.com/ use a geared front hub and are considered very good - but also not cheap.
Modern geared motors can be ridden w/o motor support more or less w/o oder w/o much additional effort. You could probably measure it, but in practice you won't feel much difference to a normal bike with most motors. Again: There is a spread between different models.
Obvious but possibly still worth mentioning: If you use a rear wheel motor you cannot use hub gears as the hub is already used by the motor. Also, while better rear hub motors take cassettes (but often only 8-10 speed ones) the cheaper rear hub motors of use screw on cogs with 7-8 speeds which are still available but on normal bikes outdated technology for more than 20 years already. Still rear drives are good option as you have no influence of the motor on the steering of the bike and apart from that it feels more natural than front wheel drive.
The above section about motors is clearly simplified plus there is more complexity to the topic like i.e. brushed motors vs. brushless and there are other types of motor as well like friction motors but as I do not want to gain a doctor's degree the above may be sufficient to get an idea of the topics complexity. Next point:
As said a couple of posts before, there are different ways of modulating power through the controller. The most natural are torque sensors, rarely seen in retrofit kits (though offered by some). With a torque sensor the electric motor "simply" adds a constant percentage to the power the rider provides. The power levels are different percentage levels. I.e. if you pedal along with 100Watts level one would provide you with another 30% while level five would provide you with additional 200% to your own power. As the torque sensor measures your power in real time riding with a torque sensor feels in principle as if you would have a really good day (on lower levels) or as if you gained super power (on higher levels). Being dependent from what you yourself do and provide riding with a torque sensor feels totally natural as it mirrors your behavior in real time - if you push harder the motor pushes harder, too, if you soften your effort support will decrease, too.
Obviously torque sensors make mounting more difficult and they add to the price while you have to understand their advantages first to be willing to pay the premium for that. That's not the case for mass selling of cheap kits via internet and technical knowledge is not what many buyers are after.... However: Most of the middrives are torque controlled, factory ones as well as the Tongsheng. The popular Bafang BBS01 does lack a torque controller and that is a major difference to the Tongsheng. Direct rear hub drives usually do have a torque controller as well, with the sensor often being part of the hub internals. With geared hub drives a torque sensor is an extra piece of hardware which is rather rarely seen though they are generally compatible. However, the controller (the tiny black box with the electronics that steer the motor) has to support it and - as you can imagine - those who do are more expensive than those who don't.
There is one downside to torque controllers: As they do relate to the power you provide if you are totally unfit the motor will reflect that as well. You can compensate that by choosing a higher support level, but if being very weak is a notorious and constant pattern (i.e. if you are disabled) possibly a torque controller is not the optimal choice. In all other cases I would prefer it.
Anyhow: When people are baffled about the differences in behaviour between a Bosch-equipped bike and a cheap folder the main reason for the difference in behaviour is the lack of the torque sensor on the cheaper bike. But obviously the typically strong Bosch motor with well developed steering also adds to the package to a degree and beats cheap solutions from china in terms of impression. One should not underrate the integration aspect - the whole thing is more than the sum of it's parts and systems like Bosch, Shimano and the other big names have more resources, develop fast, are better at integration with other bike components than the typical chinese brand or a kit manufacturer and the distance is constantly getting bigger. The downside with them is the price tag and that you are locked into a closed, proprietary system plut no option for retrofit.
If you lack a torque controller how do you choose how much power the motor should deliver? You have to have kind of "powersteps", represented by the support levels. If you remember your school days: Power = Current * Voltage. So there are basically two ways: You can modulate the power via cutting/modulating either the voltage or the current from it's possible maximum to a certain level for each level of support. Cheaper controllers simply cut the voltage with the effects that I described for the Swytch a couple of posts above: Only the highest power level supports up to the max speed. The reason for that is that each motor is rated and designed for a certain voltage, typically 24V (rarely seen today), 36V (most common today) or 48V (gaining more market traction). If you do not reach this Voltage (you "undervolt") the motor stays below it's possibilities.
Small excurse: This is btw. what happens as well when your battery goes flat: Starting with a fully loaded battery voltage will go down along with the usage. If you start with let's say 38V on a fully charged battery the motor will cut out when you are down to let's say 30V to avoid damaging the battery. Obviously there is a time before that during that you are below the nominal 36V of the motor. The further you are below the lower the speed up to which the motor supports. Interestingly this issue is - at least in my experience - more obvious with retrofit kits than with high level factory motors like the Bosch and others in that category. They seem to have a better control of their electronics than the cheap retrofit kits. Which means: In practice you probably will not be able to use the full capacity of your battery but only 90%. And if you do that you should be aware that with many bikes with retrofitted motors after like 60% the supported top speed is slowly decreasing.
Back to the topic: If you modulate the current instead of simply cutting the voltage all power levels may support up to max speed (just with different amounts of additional power) which is clearly more desirable (and to be honest what I would expect as an innocent person).
Cheap and badly programmed controllers feel unnatural and sometimes like riding a moped as the amount of power provided is only loosely coupled to your own behaviour (if at all), sometimes they do have a lag in reaction with adjusting power and they clearly do not modulate power closely along to your behaviour but rather kick in with a more or less constant amount of power as soon as you move., independend from the power you provide.
The controller has to be programmed and how it is programmed determines how the motor behaves, how the power evolves and in the end how it feels to the rider. With factory solutions there is often a bit of configuration possible for the user but no "programming". Same goes for most retrofit kits: You may be able to adjust some settings, but will not be able to groundbreakingly change the behaviour. With some Bafang and Tongsheng mid drives this is possible by using a computer and a special USB cable - you can totally change the behaviour of the motor to your personal taste and while this is complex this often makes it a way better experience than from factory.
A bunch of manufacturers like i.e. the menitioned Vekkit and Cytronex do have a fancier way of programming by kind of emulating a torque controller w/o having a real one and this seems to work surprisingly well. A good controller that is well programmed is at least as important as the motor, it will influence how an ebike feels to the same degree or even more than the motor itself does - a good motor with a bad controller will deliver a bad user experience. A cheap motor with a well programmed controller may deliver a good one (but may i.e. be limited in power oder long term endurance).
Other than motor, controller and eventually a torque sensor (or a cheap cadence sensor) the parts you additionally need for an electric bike apart from a bunch of cables are a display and a battery. The display is the thingy where you have your user interface: You set the power levels and some times you get displayed various information via an LCD. This is also what you use to program the controller within it's paramters if you do not use a computer and usb cable for a "limitedless" programming. As you may guess there is a huge variety in features and price with displays and also in compatibility.
Regarding the battery you have the obvious choice of different shapes and capacities and the less obvious one of higher and lower quality. The latter is again not always transparent, obvious or recognizable but often (but nor always) reflected in the price. Batteries for Bosch and many other "branded" motors are proprietary and often very expensive. But typically they are of high quality and can be refurbished once they degrade. Still im most cases you have to pay a hefty premium for proprietary batteries just because they are proprietary - you have no choice and the manufacturers not only know that but conciously designed it that way to create additional profits. With most retrofit kits there is little intelligence in the battery and you can choose and combine whatever battery you prefer (but face the challenge to find a decent quality one for an adequate price).
If you look at the kits on offer and look at the cheaper ones you will recognize a pattern: Cheap geared motor, cheap controller with limited abilities, very basic programming, no torque sensor, cheap display, cheap batteries. So clearly targeted at reaching the cheapest price as the single goal. Still often advertised with formally impressing data like battery size. The sellers often have little or no own competence but simply import kits from Asia and resell them. Some import components and combine them like lego - the result is most of the time still far from perfect. Other ones import basic components like the motor and combine them with own developments - either a controller and other parts they develop themselves or at least sophisticated programming. I'd say the Nano Kit for the Brompton lives in that area. But as we see from the Swytch even with some own developments (like Swytch did with their mounting kit) the result is not necissarily convenient. And then there are the ones like Cytronex or Vekkit where the own development is clearly the dominating part of the kit and typically you'll end up with concious development and good to decent quality with those.
As pedelecs are a hot market even a lot of the newer bike brands that only were founded during the boom of electric bikes do offer basically cheap, lower quality bikes combined with cheap and basic electric components, today often nicely integrated and designed and marketed as modern hipster bikes. Looks shiny but has a Lada inside. Still they do charge a huge premium for that, in my eyes often not justified and overpriced.
On the other ends of the spectrum you have on the one end cheap pedelecs where you have a really lowest end bike, cheap components, all coming from china and rebranded under various names. Cheap, but with low quality and limited functionality, at least if you want a bike. And then on the other end you have the premium spectrum with wellknown brands, today mainly using Bosch, Shimano or Yamaha motors and sometimes Panasonic, Mahle or Neodrive (there are other decent smaller brands of motors as well but with way less market share). Often very expensive but of decent quality.
When it comes to folders you'll recognize that there is not much choice from factory if you look for wellknown brands: There is the Brompton Electric which surprisingly, despite hitting the market seven years later than planned, was one of the first in the folding sector. You have the Tern Vectron with a Bosch mid-drive, heavy as a lump. And just a couple of weeks ago finally Dahon joined the party, I think using a Bafang. There are a bunch of smaller companies like Ahoga and Vello, but none of the bigger brands. And then there are all the el cheapos with frightening quality. So for a folder retrofitting a motor often is a good option if you do own a good quality folder that you like. Choose sorrowly for quality and your needs, don't go for the cheapest offer and you may end up being a happy camper.
Same goes btw. if you want a light "normal" electric bike that offers support when you need it but mainly is a classic bike. Hard to find in our days, electric bikes become more and more SUVs: Fat, heavy, unagile. So again a use case for retrofit and this time one where either small hub motors or friction drives like the
Gboost come into play in combination with a small battery.
