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# Solar-powered wireless sensor

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#### lickmyeyeball

##### Newbie level 2
I am starting to design my very first hobby-project, a solar-powered wireless sensor.

Being entirely new to this, I was looking for some rules-of-thumb or heuristics from you guys, as you are much wiser and capable than me.

Specifically, I was wondering what rules you follow when sizing solar arrays with respect to expected load. There are few clear sunny days, especially where I live. I expect to have to underestimate what power I will actually get.

Have you had best results when building arrays with 125% capacity relative to load? 150%? Even more?

As an illustration of what I mean, say I have a 60W load. Neglecting the power electronics likely needed between the solar array and load, I would design my solar array with a capacity of 90W if I overcompensated with 150%.

As well, do you think it is worth putting in the effort to simulate the system? I have a copy of matlab, but I feel that simulation would require a good deal of effort to learn; both about the program itself and the elements I am trying to model.

Also, I would really appreciate any general tips you might have regarding design in general. I am sort of winging it and don't know where things will take me.

I have learned some stuff in preparation for this project of mine: a bit of electronics, a bit on computer communications, a lot of control systems (not nearly as helpful as I was hoping), power electronics, and what little I could find regarding solar PV systems. I will still have to learn everything about microcontrollers, sensors, and wireless. Those things are still complete mysteries to me.

Thanks so much for your help and input. :-o First post.

Yes, you must factor in the proportion of cloudy days versus sunny ones.

The PV panel produces power during a few hours of daylight. Do you plan to store that power in batteries, for use later?

If so then a 90W panel could theoretically push, say, 90 / 12 = 7.5A, into a 12V battery.

If your battery has a capacity of 50 amp-hours then it would charge the battery in just over 7 hours. A bright sunny day might give enough charging time, when you take into account the sun being dimmer near sunrise and sunset.

On cloudy days it might take a few days to charge.

Home-Power is an excellent resource for alternative power:

www.homepower.com

Here in England the average output of a outdoor solar panel is typically 5% of the rated power of the panel. This is not sunny California.

If so then a 90W panel could theoretically push, say, 90 / 12 = 7.5A, into a 12V battery.

If your battery has a capacity of 50 amp-hours then it would charge the battery in just over 7 hours.

Battery charging is not 100% efficient. Lead acid battery charging is typically about 66% efficient.

The PV panel produces power during a few hours of daylight. Do you plan to store that power in batteries, for use later?

Home-Power is an excellent resource for alternative power:

www.homepower.com

I hope to store the energy in a relatively small battery. The system should cycle between a sleep mode and periodically awakening to check stored energy levels and if sufficient energy is present, make a reading and relay the data. I hope that sleep mode should draw something around 3nW while readings might average somewhere around 5mW.

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Here in England the average output of a outdoor solar panel is typically 5% of the rated power of the panel.

Wow, that yield is much smaller than I actually expected! It's really useful to know however, thank you.
I would probably expect some results similar to yours, as I am at a relatively high latitude.

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