if you can into schematics and datasheets, look up this regulator chip, or similar: AP34063

otherwise, ready made circuits can be found by googling. one easy way is to use a Blackberry car adapter (charger) with takes in 10-25V and puts out 5V. $3 at the thrift store. other brands may work, but are not specifically rated for 25V.

>>928027
It's not 'milliampere-hours', it's just milliamps, and it's not '0.28mA', that would be 280 *microamps*, it's 280 *milliamps*, or 0.28A.

Now, then.. what you need is a step-down (or 'buck') switching power supply. Since you need 5V (not 'around' 5V, it's *5V*) you can get a module that does this off-the-shelf -- which in your case is a good thing, since I don't have much faith at this point in your being able to actually construct one.

>>928027
..oh, and since you're confused about it: 'milliampere-hours' is a unit of measure you'd use for a *battery*, not a solar panel of any kind; it's a unit of measure with regards to *storage capacity*.

You have 5W, you can do whatever you want with it.

5W / 5V = 1A

You want a 5Ω resistor.

>>928027
Break it apart. Find out what the output of the (separate) solar panel(s) is/are. Go from there. There are no solar panels that directly output 18V, so it is probably in series to increase voltage. Typical output is about 0.5V per piece. This is the best way to do it without losing efficiency by using adapters etc.