help with solar post light build, please

working

What wattage is the bulb? Will the fitting take a low energy CFL bulb? How many hours a night must it run? What's the climate like in winter?

we

and mostly they aren't very bright.

Likewise, but they probably won't be big enough.

working

What wattage is the bulb? I HAVE THREE 40 WATT BULBS IN THERE NOW, BUT ONE

40 WATT WOULD BE ENOUGH, I BELIEVE Will the fitting take a low energy CFL bulb? I WILL CHANGE OUT THE FITTING TO ACCOMODATE ANY REASONABLE SOLUTION How many hours a night must it run? FOUR TO SIX WOULD BE IDEAL, FEWER HOURS WOULD BE OK What's the climate like in winter? SUNNY ALL YEAR LONG. THIS IS SEMI-TROPICAL COUNTRY. WE WILL, OF COURSE, HAVE A WEEK OR TWO OF "GREY" BUT ..IT WOULD NOT BE A NOT A PROBLEM IF THE LIGHTS DID NOT WORK 52 WEEKS OUT OF THE YEAR.

we really don't like the shape and appearance of the new lights.

and mostly they aren't very bright. NO DOUBT YOU ARE CORRECT..THAT IS WHY I AM ASKING HERE.

Likewise, but they probably won't be big enough. I DON'T KNOW THE FORMULAS TO MEASURE "ENOUGH"...BUT...IF I KNEW THE OUT PUT; PRICE AND SOURCE OF A SMALL 4 X 4 INCH READILY AVAILABLE SOLAR PANEL, THEN I BELIEVE I WOULD BE STARTED IN THE RIGHT DIRECTION.

Thanks for any further help...Rich

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for

FORMULAS

Here is how I would look at the numbers...

Ok so lets stick with the requirement to power a 40W bulb for six hours. In theory that means the battery needs to supply 240WH. In practice you need to allow more capacity to take care of things like:

1) losses in the step up inverter (used to convert battery voltage to mains) 2) low temperature performance of the battery in winter 3) loss of capacity due to aging etc. 4) need to avoid deep discharging the battery (eg if lead acid)

It wouldn't be unreasonable to double the calculated capacity. Others may disagree but lets call it 2 x 250W = 500 WH.

If you use a 12V battery it would need to have a capacity of 500/12 = 42 AH If you use a 6V battery it would need to have a capacity of 500/6 = 84 AH

Now lets look at the solar panel...

That needs to put as much energy into the battery during the day as the lamp takes out at night plus any losses in the charging process. Good batteries are 80% efficient so lets say we need to put in 240 *100/80 = 300WH

Lets say that a winters day is 8 hours and the panel produces 12V to make charging easier..

The current required is nominally 300/(12 x 8) = 3.1Amps

So a panel that produces 3.1A at 12V in winter is required.

12V x 3.1A = 37.2W

So you need a panel the produces 40W at 12V on a cloudy winters day.

Here in the UK typical summer to winter derating is a factor of 4 so that's

4 x 40 = 160W. A panel that size as a kit with a charge controller might cost £600 or $1200. Probably $600 in the USA as we get ripped off over here.

If you go for a low energy bulb you might be able to reduce most of the numbers by say 2/3rds except cost which isn't linear. Also low energy bulbs don't allways work well at low temperatures (problems starting).

There is a lot of room for error when working the numbers and it wouldn't be hard for to be out by a factor or 2 and need to double up on the solar panels or the battery. You still need to be reasonably rich to play around with solar power.

You would be best off to buy a complete solar lamp. These use LED technolgy with low power solar panels and chargeable batteries that can work under severe temperature and weather conditions. With these solar lamps, you can get a very cheap one, or a very good quality high cost one. You are mainly paying for reliability, lifespan, and lumination strength when going for an expensive unit.

If you want to run a regular mains light bulb with solar power, this would be a very expensive and much more complicated project.

Jerry G. ======