Best Practice Guide for off-grid lighting product development

Few products that we review in the laboratory or test with end-users in the field are likely to succeed commercially. Either they have design flaws that render them unappealing to the target market or deficiencies in their manufacture will result in premature failure. Often both are the case. Following are some recommended principals to follow in the design of successful off-grid lighting products.

Best Practice Notes
Design with the ultimate user (and use) in mind. There are a wide variety of potential users, and uses. "Multipurpose" lights can end up serving no single purpose well. Existing form factors (e.g., "lantern") are not at all necessarily the best for the intended task.
Match illumination to end-user need. Very different illumination levels and light delivery are required for tasks such as reading, medical examinations, vending, or social interaction. Variable optics and/or light output can be an effective solution as long as these features are not vulnerable to failure and do not add excessive cost.
Offer a grid-charging option (even if grid-free is preferred). Assuming market-based pricing, first-cost-sensitive buyers will typically opt for a grid-charged option over a solar option.  Provide for this when grid-charging infrastructure is available (e.g., via cell-phone charging shops).
Size batteries with total cost of ownership--and convenience--in mind. Smaller batteries need to be charged more often, which is an inconvenience, and translates into a higher cost of ownership if grid-charged (where fees are largely independent of battery size).
Don't let batteries stagnate in the supply chain. Particularly in the case of lead-acid batteries, an old battery can be a dead battery.
Manage power properly. Incorrect power management can lead to unacceptable reductions of light output as the batteries discharge during use, undercharging of batteries, etc. Always run the LEDs below the recommended maximum current listed on the LED datasheet.
Conduct demanding quality assurance during component sourcing and manufacture. Virtually every component of off-grid lighting products (LEDs, charging, switches, batteries, moving parts, housing) are susceptible to serious quality problems. These can trace to non-adherance to design specifications, product/material specifications, or poor manufacturing practices. [See Lighting Africa recommended testing procedures for lumen depreciation]
Consider solid waste streams associated with the product. Poorly designed products (or those where batteries cannot be accessed for replacement and are thus discarded when the batteries fail) create excessive solid waste. Discarded batteries represent a very large waste stream in developing countries. 
Design for locally-available replacement batteries. Exotic or specialized batteries are rarely available to end-users in developing countries.
Provide a charge indicator. If users must wait until the batteries are drained before recharging, they may instead recharge prematurely. A recharge indicator can help maximize battery life and minimize fee-based grid-charging fees.
Provide icon-based instructions not requiring literacy or command of a specific language. Correct use should not be assumed. Many target users cannot read, and thousands of languages are spoken among the diverse target markets globally.
Design for harsh environments. Products will experience severe conditions, including exposure to water, humidity, wide temperature fluctuations, dirt, and hard use. Failure of a single component often means failure of the entire product.
Achieve a realistic price point. Price build-ups between factory and end-user can easily be a factor of two or three, depending on duties, value chain, and profit margins. A $50 light may garner some interest from charity groups or others with "deep pockets" but will be unable to achieve a meaningful market share without subsidy or other form of unsustainable market intervention.
Warranty is a must. An unwarranted product is an un-trustworthy product.