Look at homepower.com
Solar Electricity is NOT currently economically viable in the US. From what I have read, in Germany there are so many Government kickbacks to those who install the panels that you can come out ahead. Taking money from one taxpayer and giving it to another does not make it viable in my opinion. There are some Government incentives in the US that make a photovoltaic system on your house pay itself off in about 15-20 years, IF you forget about the fact that you could have invested the money spent in the stock market and earned a pretty good return. Without the incentives, most would never pay themselves off, once you factor in the occasional repair/replace/maintenance costs.
I am a fan and an advocate of solar and other alternative energies. Unfortunately there is a lot of hype about them. You made a comment: "Sustainable means it works as a business investment, not as a donation." that shows Photovoltaics to be weak. BUT, if there were a true level playing field, where there were no government incentives/donations to oil, coal and gas companies and everything worked on the same business competition rules, solar might not look so bad. It would probably still be cheaper to use grid power, but it might not be quite so much.
In the US, grid power is cheap. If you wanted to provide all your power with solar, wanted to live like a typical American, and you did not want to be hooked up to the grid, you would need a large, expensive setup with lots of batteries etc. THAT would be an economic disaster. What most people who have access to the grid do is use their array to sell power to the grid when they have excess and buy it back when they need more. Many are able to provide all their electrical power for the year. It still is more expensive, but it makes them feel good so that is fine. Right now, to provide the huge amount of electricity that most households use would require a $20,000 system (or more, depending where you live).
If you do not have access to the grid, or just really want to do it without being attached, you really need to cut down your usage. Most who do this use a propane refridgerator and super efficient everything and just plain do not use much electricity. I do not have solar. Most people I know have electric bills well over $100. My electric bill is $30. $7.60 of that is just a hook up fee. I measured my fridge and found it took more than half my electricity. It cost about $19/month to run. Just getting rid of that would save a ton, but I still want to keep my food cold. I can get a REALLY efficient fridge, but the cost is such that it would take 10-20 years or more to pay itself off.
All over the third world the NGO's and others have put in Solar electric systems. Usually they come in, spend many thousands of dollars putting in a barely adequate system, then leave. No training on how to operate it or to maintain it. A few years later it is toast, or stolen. There are some organizations that do it right, but most would be better off just giving the poeple the money and let them buy beer.
Currently, solar panels are running roughly around $10/peak watt installed. Assuming an equivilent of 4 hours average sunlight and your quest for 50 homes using 50 watts for around 5 hours per day = 12,500 watt hours. You would need 3,125 peak watts so you are talking around $31,125 for a system that could do what you like. I don't think that even includes the batteries, but the numbers stated above are for home sized systems, for a big system you could probably get teh equipment for cheaper prices etc so maybe the above price could include the batteries etc. You will see estimates that are MUCH lower, but from Homepower.com I do not see that being a reality.
I have looked at small villages for years. The one house electric solution is very expensive. To do the whole village seems great, however the corruption will stop this, where there is a need, then corruption is big. Catch 22
The solution for me it to connect to the grid and power up, then do some trickle re-charge.
Electicity is a luxury, highly over-rated.
The whole problem is corruption, not the creation of electicity for small villages in Africa. If there was not corruption, they would be on the grid already.
I quizzed a girl here in Togo, she wants a fan, not lights, not tv, but a fan.
I want a fan, but the simple solution would be to sleep outside.
LED lights in Nepal are great, I have yet learn how to buy.
I think a 10 Dollar lighting system that would give the light of 4 bulbs of 25 watts for 5 hours would sell if powered by sun.
A completed package with the wires to reach four rooms.
I started out writing a short post, but it grew like a fungus.
My 15 watt compact Flourescent Bulbs use about 11 watts when warmed up and
are the equivilent to a 60 watt incandescent if I remember right. Running
4 for 5 hours per day = 220 watt hours.
A typical car battery can supply, with 80 % discharge, about 2,112 watt
hours. A typical bike batter (Kawasaki KLR 650) 115 WHrs @ 80% discharge.
A typical 12 volt deep discharge battery from Wallmart might give you
1,152 useful watt hours ($250). Bringing a regular battery to 80% discharge
repeatedly will kill it VERY soon. Bringing a typical deep discharge
battery to 80% frequently will kill it soon enough, but it will last longer
than a car battery.
Assuming you will be using this a lot, you would get a deep discharge battery and try to never bring it below 50%. If we got the cheaper Sears marine battery at $200 with 860 WHrs you would get 430 WHrs, which would allow you to run your lights for about 2 nights, in case one day there is no sun or there is a problem with the system that you did not notice until you got home (those damned kids)
To generate the electricity with a photovoltaic, you have to plan for the worst part of the year. Lets assume you are not far from the equator so you are getting about 6 hours equivilent of useful sunlight. I don't know how much you would really get where you want to use it, but this number could be high. That means you have to have a 37 watt panel. A panel that small may be tough to come by, but if we use the estimate of $5/watt purchase price it would be $185. It would be a good idea to have some sort of charge controller and if you want to use AC lights rather than DC, you need an inverter. So a rough guess, for battery, panel, electronics and bulbs is pushing $600. That's if you did the work yourself. You could get away with making things cheaper, by using less electricity, buying cheaper quality stuff etc. You might want nickel-iron batteries if using the system with less knowledgable people as these can tolerate more abuse and will last longer, although they are less efficient.
You could also go with LED household bulbs. They are a bit more efficient than flourescent but they are directional so you need less light for tasks, reading and such. You could easily get away with a 1.5 to 3 watt model for most work. But they are also a lot more expensive (ten times) and impossible to find in remote areas.
All of a sudden oil lamps and candles are looking pretty good. Besides, you could make a fairly cheap system that could charge the batteries up from a bicycle to provide that much power. two or three hours of bicycling will give you that much.
I just noticed you said provide the LIGHT of 4 - 25 watt bulbs. lets call it one 100 watt bulb, which would be about a 25 compact flourescent. Now we are talking 125 watt hours, so the system, presumably would cost half as much.
As you said, most people don't need TV, lights, electric water pumps etc for personal use, only a fan. Businesses are different.
Now, my ceiling fan uses 40 watts. A 20" window fan uses 144 watts. A table top fan uses around 50 watts. Even if you only used the ceiling fan to slee, you are talking mybe 8 hours at 40 watts = 320 watt hours. That's less than $.05 worth of electricity from the US grid, yet we are talking $400 to $800 for a system. A little creativity and scrounging and you might get the price down to a couple of hundred, but there must be a better way. If you don't have to store the electricity for night time the system becomes much cheaper (no batteries) but that requires hooking to a grid. One solution as you have said is a localized shared system, but you would have to have someone to run it, some way to charge people etc. Generators are used worldwide for a reason, they make more sense most of the time (right now anyway). I would love to see the price come down as you requested. Using the 125 WHr number your $10 = $.40/watt installed, with wiring etc. THAT will probably never happen.
BUT. Think of it this way. A company could make a cheap quality LED task light powered by AA batteries for a couple of dollars cost. If these use 3 watts (quite a bit of light really, you could get by with 1 watt or less), and are used for 5 hours, it would be 15 watt-hrs. now you could easily create the power to run these with a hand crank or foot power of some sort. If you wanted to use rechargable batteries, you could charge them up in 5 hours outside using a 3 to 5 watt panel. You can buy one of these for about $80 so cost must be more like $40, then add some batteries for around $5 and you have a system. Of course, you could just buy $80 worth of AA batteries. AA NiMh batteries can hold 2-3 watt-hrs each. You would want 3 or more so that you do not excessively discharge them and so your electronics would be simple.
Still, the best would probably be a good quality LED run by some mechanical generation means (hand/foot crank) or just by charging a battery in a car/off the grid. Maybe some sort of thing where you buy a battery and pay someone to charge it every week or two. Some day large scale solar will take off, as will various other methods of producing power. The technology is here to do it, but not to do it economically. What people fail to realize is there are plenty of fossil fuels. Just not cheap fossil fuels. The market is an amazing thing, if we stop subsidizing any type of power generation and force the market to decide what is best, we will get what is best. Maybe best will be to just stop using so much energy.