Target

Power a 12V/2A (24W) system for 24h/24h, using an autonomous solar system in the Netherlands (read: not very sunny).

Initial thought

24W * 24h =~ 0.6kW and 2A * 24h = 48.0Ah , so given ourself a 100 Ah battery (AGM, deep-cycle), should last us to roughly 2 days of operation.

After first measurements my 20W Solar panel is preforming 50% less if I put it a bit into the shadow (balcony fences). Putting it into fully into the sun makes it generate roughly 1.2A/12V which is indeed ​as advertised. The input should run at 17.5V to make it take the full 20W.

The change controller ​Steca Solsum 6.6F seems to regulate the input to 12V, leaving me wondering if the panel will be fully utilized. Cause 20W/14W * 100% = 70% e.g. loosing 30% seems a bit to much.

Taking this numbers into account we need at bare minimum at least 48Ah * (100% / 70%) =~ 69Ah . Giving the fact that effectively we have 4 hours of sun in the Netherlands will this make 69Ah / 4 = 13Ah which correspond to a 160W panel.

Estimated cost for bare minimum will be:

Panel 150W             : 600 EUR
Accu 100Ah             : 200 EUR
Charge Controller 10A  :  50 EUR
Connectors and cables  :  50 EUR
Measurement Electronics: 100 EUR
--------------------------------
Total                   1000 EUR

This setup is geared towards most optional numbers. Now take more realistic numbers into a account:

• The sun in the Netherlands will do roughly ​1800 hour, but ​75(!) days of the year I will not see any sun at all. If this period last for more than 2 days my battery will be depleted, so I properly need to add extra battery-pack to my system (+200 EUR).
• The panels are not always running at 100%, I expect average case to be 50% and worst case 25% (if sun is shying during the winter). Consequences will be that I need between 1 to 4 panels of 120W (which gives me +2400 EUR).

Experiments

• Using a dummy load (20W, I like to see if the panel is able to generate the full 20W).
• The Current Controller needs to be build/bought to be able to generate statistics on long running experiments.

Idea/Brainstorm

• Use ​mirrors to increase efficiency.
• Measure DC current with an ​LT1495 and Arduino.
• ​Solar calculator suggests; Power 25W/24h using 2 * 120W and 100Ah accu. (giving 5h of direct sunlight).

Verbs

Vpmx  : Voltage at rated power.
Voc   : Open Circuit Voltage. 
Lpmax : Current at rated power.
Isc   : Short Circuit Current.