# How to choose a Solar Panel? [closed]

I have a project that consumes 2A and 5V. I plan on running the system 24 hours. So that brings me to 240Wh power consumption.

I need to know what type of solar panel and where I can find it.

I read some articles and it says that I need to take 60% of total sunlight exposure (40% transfer loss) which in my case I will assume 6 hours. That leaves me with 3.6 usable hours of sunlight. I need to supply 240Wh so that brings my solar panel specs to 66.66W

Can someone let me know if my math is correct?

Also, can you suggest how I go about getting solar panels (distributor suggestions) and do I need a battery or can I just connect my power adapter directly to the panels?

I have seen some 70W panels but they are 12V would that be an issue with a 5V device?

Any help would be appreciated.

## closed as off-topic by Eugene Sh., Leon Heller, Null, PeterJ, Daniel GrilloOct 2 '15 at 0:16

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• You need some sort of battery or your system will only run for those hours when it's getting sunshine. – DoxyLover Oct 1 '15 at 19:47
• So basically the panels just recharge the battery that powers the system? Do you know what wattage of solar panel would be ideal for this setup? Any suggestions on distributor? – presish Oct 1 '15 at 21:09

Your calculations are correct as far as they go. But.

First, the "normal" way to do this is to use a 12-volt battery to match your 12-volt array. The output of the battery would feed a 12-volt to 5-volt DC-DC converter.

Second, you have not factored in bad weather. If you don't want a cloudy day to kill your system, you must factor in a margin of safety, since after 1 day you'll have used up all of the previous day's energy, and if it's cloudy you won't get a full day's charge. How big this margin needs to be depends on exactly where you live and how much risk you're willing to take. Let's say you size the system to allow for 2 cloudy days in a row. Then you need to provide 3 times your base power, or about 200 watts from your PV cells. And frankly, I'd consider 60% system efficiency optimistic.

Third, your battery must be much larger than you probably realize. 240 Wh from a 12-volt battery is 20 Ahr. 3 days capacity is 60 Ahr, which is about the normal size of a car battery. However, if you're using lead-acid (they're cheap and the charger techniques are easy) and you want them to last a long time, you can't fully discharge them. In fact, for regular lead-acids, you should not discharge below 80% of charge. In this case, you need a battery 5 times larger than you thought, or 300 Ahr. If you're willing to shuck out the up-front costs, you can go with deep-discharge marine batteries, and you can live with a limit of 50% discharge, for a 120 Ahr system. But again, this assumes you're willing to live with a certain level of risk. Bigger battery = longer duration = higher price and larger size.

Finally, check the rules. This is not a product-recommendation site. Develop your Google-fu.

• So the battery should be of a significant capacity and rated at 12V but would the 70W solar panel be sufficient for recharging such a battery? – presish Oct 1 '15 at 23:02
• A wonderful, patient answer, @WhatRoughBeast. I see a problem though, I think you have under-emphasized your final point. Especially the entire final point. – MickLH Oct 1 '15 at 23:17
• @presish - See my second point. A 70 W panel will only provide 1 day's worth of recharge per day. It will not allow for stretches of bad weather - even 1 day! and will possibly wind up damaging your battery. If you don't use a battery the solar panel will only provide power when the sun is shining, and night-time operation will be impossible. – WhatRoughBeast Oct 2 '15 at 0:16