"Low-power" is human-power or the equivalent. Electric motors for bicycles should also be considered low-power. Less than "very fit" humans may, at times, need e-motor assist. "Cycle tech" is short for bicycle technology, whether one, two, three, or more wheeled. About the only common feature that defines "bicycle technology" is the use of the lightweight wheels that come in various sizes characteristic of modern bicycles.
While large direct drive wheels were once used (the penny-farthing), today's wheels are 16 inches to 28 inches (317 - 635 mm) in diameter. Vehicles that use such wheels approach the highest possible efficiency (least power required) for land transportation on roads.
There are as many designs as designers, so "a guide" is one designer's guide. Who the designer is, perhaps, becomes relevant and some personal history must creep in. I joined the International Human Power Vehicle Association a few decades ago due more to an interest in cycle tech than in human-power per se. The promise of human-power tech, on land or sea, seemed compromised by the fact that average humans come nowhere close to putting out the power of their athletic brethren whose abilities make bike tech so marvelously functional (see The Limits of Human Power). So long story short, forty years thinking about cycle tech and the reader may consider or not the following:
Realize that for many who are clinically "fit", human only power is not enough to make bicycle technology practical. Those who desperately need to increase their activity level (their health and longevity are at stake) need assisted power such as small electric motors can provide. For those who have enough human-power, no e-assist is needed, but for the not so teaming multitudes, pedal assist is needed except on flat land when there is little or no headwind. Pedal assist (pedelec) means the rider pedals and the motor adds extra power in proportion to the amount of human-power produced. Stop peddling and the motor stops. Over time, a 20W "unfit" human will become fitter and health will more likely be upon them.
The need to be minimally active is the starting point realization. If walking is enough, walk. If not walking enough, consider cycling which is up to four times more efficient at getting people out and about. Activity is a need. Transportation is a need. Consider combining the two.
The weasel way to avoid offending any special interests would be to note the myriad options in bicycle-based offerings, from unicycles to velomobiles, and conclude "to each their own". But this is a guide, so let's talk about the needs of the majority and how best to meet them. To be excruciatingly clear, "needs" refers to our need for transportation and to achieve a minimum activity level (walking, cycling, otherwise moving about for about two hours cumulative a day). Self assessment is easy. Wear a pedometer and if it says 10,000 steps at end of day, you're reasonably active. Otherwise, consider cycling as an alternative to running or swimming or climbing trees. Cycling could get you 20 - 40 miles (32 - 64 km) a day and brachiating will not. A car may get you further and faster, but stomping on the accelerator and then on the brakes does not qualify as exercise.
First: sit on a padded post? The force athletes put on pedals largely cancels their body weight on the seat. Pedal hard enough and you won't need a seat, but few pedal hard and non-stop. For most, for easy pedal pushers whose entire weight is nearly supported by the seat, a padded post can be brutal, can be a deal breaker. How about sitting on something comfortable, as in chair-like?
Second: human-power involves sweating, so sit on a non-breathable seat surface or a mesh one? Answer, for most: chair-like mesh seat.
Next: one wheel, two wheels, or more wheels? Steve Gordon (USA) once rode a unicycle non-stop 68 miles (109.4 km)--BACKWARDS!, while Lars Clausen (another Yank) went 9,125.97 miles (14,686.82 km) by unicycle. But let's talk the greatest practicality for the greatest number. While four or more wheels may be practical and may or may not be legal, let's consider the more commonly available two and three wheeled options. The ever common bicycle lets you follow deer trails or heard cattle, but the phrase "to drop" applies to bi and not tri-cycles. For transporting a fully exposed human, bicycles do keep it simple.
Add weather protection, however, and a recumbent trike serves well. For practical purposes, considering that weather happens, let's not belabor the agony of indecision and just go with trike over bike. Upright cyclists often opine that trikes are too low, cannot be seen, and riding one is near certain death. Consider a safety flag as standard equipment and if that's not enough, add flashing lights. Other advantages, which will become obvious, happen to follow.
For weather protection, add a partial or full fairing. A fairing also helps your low-power to go further and faster via improved aerodynamics while shedding rain and, with a cover, provides shade when too hot the eye of heaven shines. For better weather protection, attach a body sock to a partial fairing. Add a face shield and something practical emerges.
There are many trike designs.....and the winner? Greenspeed Magnum. But other designs will do.
Two options: hub motor or through-the-gears mid-drive motor. Direct-drive hub motors have one moving part, the wheel, which has to move anyway, so non-geared hub motors add no moving parts. KISS defined. And, like muscle power, they provide silent power. Geared hub motors and through-the-gear (mid-drive) motors whine. If used to go were no bicycle can go, geared motors can more effectively drag you up a hill, but otherwise create political issues. Spandex wearing human-only powered cyclists do not want grandmothers whining past them while not pedaling. To fit in, to use bicycle paths that are of, by, and for bicyclists, those needing e-motor assist need to pedal, to be quiet, and not go faster than fit humans. The rule to consider is never e-motor past a human-power cyclist. Turn off the motor and if you can't pedal past them, then don't pass them. If you have a non-geared hub motor, and are pedaling, only close scrutiny would reveal whether you are using the motor or not. Only simple hub motors used sparingly allow for incognito, non-offensive blending in with normal cyclists. Don't expect to be allowed to join a bicycle race (or get a prize if you win), but fitting in is achievable.
If the only assist needed is for hill climbing, consider geared motors. But if going uphill is important, downhill issues should be considered as well; issues like not smoking your brakes, and possibly loosing them. Going over the high-side at terminal velocity is problematic. A small parachute would help as an aerobrake, but non-geared hub motors can do regenerative braking. The motor can do the braking and, as a plus, recharge your battery a bit. So while a mid-drive system is more efficient at hill climbing, that is all it is really best for. A non-geared hub motor is not as efficient assisting in hill climbing, but is just the thing at all other times, and may be essential going down a long, steep hill. If efficient hill climbing is paramount, use both a mid-drive (or geared hub motor) and a direct-drive hub motor system that does regenerative braking.
A reasonable alternative to a mid-drive system for hill climbing is to use a quiet direct-drive hub motor most of the time and add a geared hub motor for additional assist when climbing. Climbing requires a lot of power and direct-drive hub motors hauling a heavy load (cargo or person) up a steep slope may overheat on long 7% and steeper slopes. A geared hub motor will not and will operate more efficiently going up slopes greater than about 3% (so on the more typical lesser slopes, direct-drive is more efficient). Geared hub motors freewheel and so cause no drag when not in use. If used only for hill climbing or to power into a high headwind, a geared hub motor would be effective—not quiet as efficient as a mid-drive system, but much simpler, quieter, less expensive, and reliable. A relatively quiet geared hub motor, like the eZee, has nylon gears that are replaceable if ever they wear out. If the geared hub motor is a separate system, it can also serve as backup should the direct-drive system fail. If the direct-drive system powers the rear wheel, a geared hub system could power the front wheel or, if pulling a trailer a trailer wheel could be powered turning the trailer into a pusher when needed. Pulling a trailer or loading a bike down with cargo is where direct-drive hub motors can fail, so adding the geared motor to the trailer, for use only when needed, is a sensible solution. The trailer and extra motor system would be there if needed and left behind when not hauling a load.
For direct-drive hub motor, consider Bionx and Falco eMotor. For mid-drive, consider Ecospeed (hear the whine in YouTube video) or Optibike (hear the whine at 5:10 on YouTube video). An Optibike climbed Pikes Peak, over 7,500 feet in 24.5 miles, going over 22 mph (36 km/h) uphill in just over an hour, but it was a stunt on an ultra-light motorcycle that looks like an e-bike so their "unmatched" accomplishment could be hyped in ads. There are many mid-drive Bosch videos but all seem to have heavy music overlays for some reason. The Bosch is preparing to take over the e-bike market globally, but say it's not so — Bosch - ignore abnormal click, but hear the whine. The Bafang mid-drive system is said to be quieter than some direct-drive hub motors, and may be a contender. For a geared hub motor, consider eZee. If an e-bike is an assisted bike for the sub-athletic, should it whine in passing even if the rider has gotten used to it?
Human-power ultimately derives from solar power, but so, ultimately, do fossil fuels which are not renewable. The average food the average person in an industrialized society eats is about 1 part embedded sunshine (recent), and 10 parts industrial power (mostly coal, oil, natural gas with a bit of hydroelectric, nuclear, and other sources tossed in—virtually none of which are sustainable). So SAD (Standard American Diet), too bad, our food is only about 10% solar. So human-power is 90% unsustainable. Until the Fossil Fuel Culture came along, humans lived 100% by sunshine. So we have a choice: stay on the meteoric bandwagon of exponential fossil fuel driven growth (while it lasts), or hitch your bicycle-based technology wagon to Sol who has been at it, slow and mostly steady, for 4.5 billion years.
To be healthy requires a moderate level of activity. A level of human-power input into our transportation system needed to meet our activity requirements is good. Considering that much of our food is made from fossil fuels, for going beyond that level, sunshine is better than overexertion. Even if all food were produced, processed, and transported using only renewable energy, photovoltaics producing energy as needed is more efficient. Someday super-capacitors may store electricity for later use, but for now we have chemical batteries to sip on and relish.
One vision of a human/solar powered transportation system envisions vehicles as giant batteries with wheels. Batteries are best not fast-charged, so maybe you could pull into an exchange station and exchange them for recharged ones? If exchange stations were surrounded by solar farms, maybe such a scheme would make sense. Maybe some day, but for now you could live in a solar home that recharged your vehicle and never go further away from home than you were willing to push or pedal your vehicle back to.
Another option would be to put solar cells on the vehicle and not go further than the power provided allowed. If the vehicle spent most of its time sitting in the sun and was only occasionally used, that would work. For extended functionality, the vehicle could pull a lightweight solar array on wheels as a trailer. This would allow unlimited range for the price of pulling over when the batteries were low, deploying the array, and sunning up. Back in the day, you could ride a horse to get where you needed to be, but the horse needed grazing time. So too would a solar vehicle need sun grazing time.
A nuclear powered vehicle would not suffer such a limitation, but demanding no limitations is maladaptive and in the long run the maladaptive pass away. Perhaps sitting in a Go-go-go-24-7 vehicle would not be that good a thing. Perhaps reading a book, meditating, or talking with fellow travelers as they sunned up would not be a bad thing. Traveling across vast land masses in multiple bounds is better than not being able to travel far at all.
So for power, consider solar panels. When the sun isn't shining, you're tired, and batteries are low, don't go anywhere. Where you are may be just as good as where you plan to be.
We end up, then, with a lightweight, low-power, solar electric, 250W - 500W hub motor assisted recumbent tri-cycle with weather exclosure. Envision one and make one, or if you demand one, others will offered them.