How to size battery and solar panel for garden lights
- Thread starter DMP
- Start date
svetz
Works in theory! Practice? That's something else
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Depends on how long you want them on for (to convert from watts to watt-hours) and the location (to understand how much solar power is available seasonally. These links might help you get started:
Depends on how long you want them on for (to convert from watts to watt-hours) and the location (to understand how much solar power is available seasonally. These links might help you get started:
DMP
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Thanks, Svetz but I've a very small application and my hope was to have someone "do the math" and speak in terms of "you need a xxW solar panel, and xxAA batteries" to power (7) 1.5W bulbs. Pretty small area but somethings not right with a 10w panel and 11 AA batteries. They work, but go out pretty quickly. Do I need a bigger solar panel or more batteries, or both? Thanks if you can...
stienman
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Well, without knowing how much sunlight the area gets, the placement of the solar, and how long you want them to last at night, there are too many unknowns to just give you a number.
If I make assumptions based on my location, placement, and how long I want them to run, then here's what I come up with:
10.5w consumption when lights are on (7 * 1.5)
5 hours of LED light after dark (based on winter sunset around 5pm, and wanting the lighting available until 10pm)
4 hours of useful day available, on average, in the winter months
80% efficiency all around
Batteries:
This means I need 13w per hour of lighting (20% loss on 10.5w due to converter or simply voltage mismatch losses). To be conservative, let's round that up to 15w per hour.
5 hours at 15 watts is 75WH each evening.
If the battery is needed to last for years, it'll drop to about 80% of its capacity over that period of time, so let's increase the battery to 93WH.
AA NiMH cells are 1.2v nominal, and 2AH, so each cell is 2.4WH.
You need 39 AA NiMH cells to power this setup with the above parameters.
Solar:
You need 75WH per day.
The simple/stupid charging in small systems like this does not operate at maximum solar panel power.
Like the batteries, solar panels degrade, and if you want it to last several years you need to plan for this degradation.
What we do know is that there are only about 4 usable hours, on average, per day where the solar panel could be producing most of its rated output.
So in a perfect world we would only need a 19W solar panel.
However, charging the batteries directly will probably only provide maybe 50% efficiency from the panels, and after a few years the panels will degrade 20%.
This suggests a 50W panel should be sufficient.
Rubber hits the road:
I don't know the voltage of your lamps, nor what the parameters are for the photocell switch. So the following is assuming you have a way to use 12v to light the lamps.
At this size, I'd suggest standardizing on a 12v system, because parts for it are readily available and inexpensive. A cheap gel-cell battery would work fine, and to make it last a very long time we double the capacity. This cell: https://www.amazon.com/Mighty-Max-Battery-Brand-Product/dp/B01JNYUD3C should last for awhile.
A 50W solar panel with a charge controller will cost about $90, such as this unit: https://www.amazon.com/SOLPERK-Controller，Suitable-Automotive-Motorcycle-Powersports/dp/B0899YNTX7/
You can buy a 12v 50w panel and connect it directly, which will save you $10-$15 at the cost of efficiency.
However you're powering your lights, make sure they turn off once the voltage goes below 12.2v. This will prevent the cell from being deeply discharged, giving a much longer useful life for the battery. A $10 module such as this one will do this for you: https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ
If I make assumptions based on my location, placement, and how long I want them to run, then here's what I come up with:
10.5w consumption when lights are on (7 * 1.5)
5 hours of LED light after dark (based on winter sunset around 5pm, and wanting the lighting available until 10pm)
4 hours of useful day available, on average, in the winter months
80% efficiency all around
Batteries:
This means I need 13w per hour of lighting (20% loss on 10.5w due to converter or simply voltage mismatch losses). To be conservative, let's round that up to 15w per hour.
5 hours at 15 watts is 75WH each evening.
If the battery is needed to last for years, it'll drop to about 80% of its capacity over that period of time, so let's increase the battery to 93WH.
AA NiMH cells are 1.2v nominal, and 2AH, so each cell is 2.4WH.
You need 39 AA NiMH cells to power this setup with the above parameters.
Solar:
You need 75WH per day.
The simple/stupid charging in small systems like this does not operate at maximum solar panel power.
Like the batteries, solar panels degrade, and if you want it to last several years you need to plan for this degradation.
What we do know is that there are only about 4 usable hours, on average, per day where the solar panel could be producing most of its rated output.
So in a perfect world we would only need a 19W solar panel.
However, charging the batteries directly will probably only provide maybe 50% efficiency from the panels, and after a few years the panels will degrade 20%.
This suggests a 50W panel should be sufficient.
Rubber hits the road:
I don't know the voltage of your lamps, nor what the parameters are for the photocell switch. So the following is assuming you have a way to use 12v to light the lamps.
At this size, I'd suggest standardizing on a 12v system, because parts for it are readily available and inexpensive. A cheap gel-cell battery would work fine, and to make it last a very long time we double the capacity. This cell: https://www.amazon.com/Mighty-Max-Battery-Brand-Product/dp/B01JNYUD3C should last for awhile.
A 50W solar panel with a charge controller will cost about $90, such as this unit: https://www.amazon.com/SOLPERK-Controller，Suitable-Automotive-Motorcycle-Powersports/dp/B0899YNTX7/
You can buy a 12v 50w panel and connect it directly, which will save you $10-$15 at the cost of efficiency.
However you're powering your lights, make sure they turn off once the voltage goes below 12.2v. This will prevent the cell from being deeply discharged, giving a much longer useful life for the battery. A $10 module such as this one will do this for you: https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ
