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Do these solar panels look any good?
16 July 2017, 10:30,
Do these solar panels look any good?
Looking for two or three panels to charge a small 12v battery bank, I only want to generate 30 or 40 amps a day to charge iPads, phones etc.

Do these look ok?

Ideally I'll use it as a portable system, setting up only when I need it. Thanks for any input.
16 July 2017, 23:15,
RE: Do these solar panels look any good?
Without knowing the size of your battery bank, I can't evaluate whether the ones in your link are adequate.

This link may help:


If the battery bank is oversized, you risk not being able to keep it fully charged; if the battery bank is sized too small, you won't be able to run your intended loads for as long as you'd planned.

The energy stored in the deep cycle batteries can then be used directly to power DC loads or it can be inverted to power loads. The batteries recommended for RE systems are deep cycle batteries. To ensure you have enough reserve capacity to provide the electricity you need (without running additional generators), invest the time to size your deep cycle battery bank properly. Because of the various conditions affecting battery bank sizing, this process may be one of the more challenging calculations you’ll have to do when planning your RE system.

Before tackling the calculations, start by identifying a few key pieces of information:

Watt/hours of electricity usage per day
Number of Days of Autonomy
Depth of Discharge limit
Ambient temperature at battery bank


The first thing you’ll need to know is the amount of energy you’ll be consuming per day. Measure starting and running loads
of pumps, refrigeration compressors, electronics, etc. and for what lengths of time. Track this information on a loads list; as you will need it for sizing other components as well. Your final tally should be expressed in Watt-hours (Wh) per day. If you know the kilowatt hours (kWh) per day just multiply that number by 1,000 to determine the Watt-hours per day. (Example: 1.2 kWh = 1,200 Wh)


Next, you must determine the number of days of battery back-up that you want to have on hand. In other words, if you are unable to charge your deep cycle batteries by any means, and you still need to draw power, you must provide this additional storage by increasing the size of your battery bank. For solar panel powered systems (PV), Days of Autonomy represents the number of cloudy days in a row that might occur and for which you intend to store energy. Consult a weather website, local meteorologist or even long-term area residents.

If you conclude that you need more then five days of battery backup, you may want to explore multiple sources of electricity generation or backup generator options (like a fossil-fueled generator). If your primary electricity source is wind power, determine the number of days when there is little or no wind. This information can be found in the data you’ve collected using your data-logging anemometer. Hydroelectric turbine systems are unique because they usually operate continuously, and therefore do not require extensive storage. If you’re sizing a battery bank to be used in conjunction with an on-demand fuel-powered generator, the number of days of backup will represent the number of days you wish to go without using your generator.


Another factor to consider is the planned (DoD) of your deep cycle battery bank. Flooded lead acid batteries (FLA), sealed AGM batteries and sealed gel batteries are all rated in terms of charge cycles. A single cycle takes a battery from its fully charged state, through discharge (use), then back to full charge via recharging. The depth of discharge is the limit of energy withdrawal to which you will subject the deep cycle battery (or battery bank). DoD is expressed as a percent of total capacity. The further you discharge a battery, the fewer cycles that battery will be capable of completing. Simply stated, deeper discharge shortens battery life.

It’s recommended that you never discharge a deep cycle battery below 50% of its capacity ; however, many battery manufacturers recommend even shallower DoDs. For off-grid applications, a 25% DoD will extend battery life significantly. On the other hand, if you’re only using the batteries occasionally, as a backup system, you can factor in a DoD of 50% or perhaps more.


Deep cycle battery life and capacity are affected by temperature. Unlike PV modules, batteries perform best in moderate temperatures. In fact, the temperature standard for most battery ratings is 77° F. Cold temperatures tend to reduce battery capacity while high temperatures tend to shorten battery life. For this reason, people in colder climates will often find a place to keep their batteries indoors rather then leaving them subject to outside temperatures. FLA batteries can be destroyed in freezing temperatures. While sealed deep cycle batteries can operate in sub-freezing temperatures, their reserve capacities will be reduced substantially. Identify the lowest temperatures that the batteries will be exposed to and factor this into the calculation using the temperature table (below).


By this point, you will have identified your system voltage. This is typically 12V, 24V, or 48V.


Once you’ve pinpointed all these variables, it’s time to calculate the size of your battery bank! Let’s go through the steps below, using the following example system:

A system load of 6,000 Watt-hours per day
Three Days of Autonomy (back up) needed
Planned Depth of Discharge (DoD): 40%
Battery bank ambient average low temperature 60° F.
A 48V system


Identify total daily use in Watt-hours (Wh)
6,000 Wh/day

Identify Days of Autonomy (backup days); multiply Wh/day by this factor.
3 Days of Autonomy:
6,000 x 3 = 18,000 Wh

Identify Depth of Discharge (DoD) and convert to a decimal value. Divide result of Step 2 by this value.

40% DoD:
112,500 / 0.4 = 45,000 Wh

Derate battery bank for ambient temperature effect. Select the multiplier corresponding to the lowest average temperature your batteries will be exposed to. Multiply result from Step 3 by this factor. Result is minimum Wh capacity of battery bank:
Temp. in [degrees] F. Factor
80+ 1.00
70 1.04
60 1.11
50 1.19
40 1.30
30 1.40
20 1.59
60° F. = 1.11
45,000 x 1.11 = 49,950 Wh

Divide result from Step 4 by system voltage. Result is the minimum Amp-hour (Ah) capacity of your battery bank.
49,950 / 48 = 1,040 Ah

73 de KE4SKY
"Almost Heaven" West Virginia
16 July 2017, 23:20,
RE: Do these solar panels look any good?
Error step 3 should be
40% DoD:
18,000 / 0.4 = 45,000 Wh

the 18,000 carried over from step 2

Just looking to see if you were paying attention 8-)

73 de KE4SKY
"Almost Heaven" West Virginia
17 July 2017, 19:47,
RE: Do these solar panels look any good?
All that and no one noticed that they are USED panels.

No indication of the age, length of use, or for what shortcoming they might have been returned to the mfg for resale.

I think you can get a better value for your money.
17 July 2017, 22:45,
RE: Do these solar panels look any good?
"These panels are working perfectly. There were installed on a solar farm, half of which was damaged in a hail storm so the insurance company is paying to replace the whole installation with larger 260W panels. These are the panels which were not damaged in the storm, and are all working 100% and only been installed for a few years. The output on these panels is fine, but some of them have a cosmetic issue called "snail trails" where there are some silver trails across the cells. This does not affect their performance, its purely a cosmetic issue and we have tested them to confirm they are fine. "
18 July 2017, 11:14, (This post was last modified: 18 July 2017, 11:17 by Lightspeed.)
RE: Do these solar panels look any good?
Answering OP specific question

Calculate just 10% of solar cells' rated output in midwinter, although you'd still expect around 4 hours a day output at this time of year.

You'll also a need an invertor to convert 12v to the 240v that your mains chargers expect to see. Invertors frequently run at just 50% efficiency so you have to input twice as many watts of energy at 12v than you'll be getting out at 240v.( if that makes sense?)

Assuming a lap-top charger of 65w and 3 phones charging at 5w each, and all of them plugged in for three hours charging per day, by my calculation you'll need 7 x 175w rated panels to do the job in deepest winter, and conversely, these will of course be generating 90% over charge in the summer months so you'll need to decide what you'll be doing with that spare energy.

Another strategy would be to find a more efficient invertor and less device charging time to reduce the required generating capacity. With careful choice of components and non exhaustive demand from the lap-top and phones, you might get away with just three cells.

Don't forget backup batteries and charge regulation to complete the setup.
72 de



STATUS: Bugged-In at the Bug-Out
20 July 2017, 00:44,
RE: Do these solar panels look any good?
I avoid using inverters on my system whenever possible, and instead use dc to dc converters to avoid the massive power loss as LS has stated.

I use a 12v dc to 19v dc step up converter to charge laptops that are rated at 19v dc, which most full size ones are. Using this method there is only a minor power loss compared to using a 240v ac converter then stepping back down to dc using the device's local power supply.

For devices 12v dc and below I use a 12v dc to various dc voltage ratings, from 12v dc down to 1.5v dc, and the input connects to my battery via a standard male to female cigarette lighter plug and socket set up. The output connects directly to the device's dc input socket using the relevant universal plug adapter and switched to match the device's dc input rating.

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