# Thread: How big solar panels do i need?

1. Registered Guest
Join Date
Jan 2012
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## How big solar panels do i need?

hi
I have a 105aH 12Volt lead acid battery, that i would like to charge in one day.
The solar vendors in my region says we have 6 hours effective sun light.

I also understand that it is best not to discharge the battery below 50% DOD.
So is it correct if i say the following.
50% DOD is 105/2=52.5ah i need from the panels in 6 hours. So i look at 52.5/6=8.75 amps an hour from the panels.
So if a look at 12V PV panels, i would need 8.75*12=105 watt panels. Is this correct?

If i get 2x50Watt 12V PV panels and hook them up in serie, do i have then 100Watt 24V input to the batteries?
This will give then 100/24=4.17 amps an hour, is this correct?
Why is it less than 1x105watt 12Volt PV panel, which gives 8.75amps an hour?

I am new to the world of solar, so please forgive me if the maths is wrong.

Any answer would be appreciated, thank you.

2. ## Re: How big solar panels do i need?

Your on the right track, but on the wrong train. I can't write a lot this morning, but here a Couple of crucial points. First, watts are measure of power. 100 watts i00 watts (and 100 WH is 100 WH) regardless of system voltage, Volts*Amps=Watts.

Change the voltage or the curreent and the number changes. 12v *10 amps=120 watts, Or 120v*1amp=120 watts.

Next, solar panels rarely if ever it it thier rtes it put. A basic derate of 20% is a good starting place. So a 100 watt panel , one could use ~80 watts as a calc. Base line.

Finally , it takes ~20% more power to recharge a. Battery than that battery can deliver, So a 100 WH draw will take _~120 WH to recharge. Gotta go. I'm sure 'coot and bill will keep the conversation going.

Tony

3. ## Re: How big solar panels do i need?

You need to place your 12V (18v on the spec sheet : V power max) panels in parallel. If you place in series to generate 24V, you will "waste" have your watts from voltage mis-match of 24V PV : 12V battery. Unless you use a MPPT controller.

about 18V is needed to recharge (force power back into a 12v battery).

4. ## Re: How big solar panels do i need?

Like Tony said, the panels won't actually produce 50 Watts. More like 38 Watts.
Like Mike said, they need to be wired in parallel not series.

Also, although you need to replace 52.5 Amp hours it will take more than 52.5 Amp hours to do it. The charging rate slows as the battery charges, requiring more time to "put back" the capacity. Expect about a 20% inefficiency here.

I wouldn't count on 6 hours of equivalent good sun either. Even where you have long days you won't get this unless you can constantly reposition the panel so that it stays directly pointed at the sun. You'd be amazed at the power drop-off you get from a little change in angle.

Given that, you're looking for 52.5 / 0.80 or 67 Amp hours worth of power. Probably in 5 hours, so an average charge rate of 13.5 Amps @ a nominal Voltage rating of 12 = 162 Watts. If all factors are good and you don't loose too much efficiency in the whole design you need to add at least 1 more 50 Watt panel to the mix to get a practical amount for recharging (this follows with the "10%" rule of thumb too: 10 Amps * 12 Volts = 120 Watts / 77% efficiency over-all = 155 Watt array).

With three panels in parallel, each should have a small fuse (about 5 Amp) on it just in case something goes wrong. You will also need a charge controller capable of about 10 to 15 Amps. Considering the cost of small panels, unless you already have them or get them cheap used, you might want to just buy a bigger panel instead. Per Watt they tend to be far less expensive.

5. BB.
Just some guy
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## Re: How big solar panels do i need?

To try and help you with less confusion, it would be easier if you gave us your rough requirements (XXX watts of load for YY hours per day, etc.)... There are lots of tradeoffs to be made when designing the complete system and it would make more sense to you if we did the entire design in one pass from beginning to end...

Anyway, first question about panels in series... Yes, you can place place the in series for higher voltage. However, you do have to match panels to battery voltage.

First, "12 volt" panels are around 17.5 to 18.5 volt Vmp (voltage maximum power). This allows for battery voltage while charging (upwards of 15 volts) and wiring voltage drop. Plus solar panels, as they get hot, the Vmp voltage falls.

If you put two 12 volt panels in series for a 24 volt rating (really ~35 Vmp-array), you should do that for a 24 volt battery bank and not a 12 volt battery bank. Solar panels are current sources and a "24 volt array" will actually only be around 50% efficient charging a 12 volt battery bank (if you are using "inexpensive" PWM--Pulse Width Modulation--Solar Charge Controllers).

You can use (and should) use a MPPT type (maximum power point tracking) charge controller if you have a Vmp-array at higher voltage than the battery bank. MPPT controllers can efficiently take high voltage/low current array power and down convert it to low voltage / high current needed to recharge the battery bank. On the down side, MPPT controllers are about 3x or more expensive than a similar PWM type charge controller.

For small systems, PWM controllers are usually OK (under ~200-400 watt array). For larger systems (over ~800 watts) or installations with longer wiring run from the array to the battery bank/charge controller, a MPPT can be much easier to use (keep wire sizes smaller for >5-10 meter wire runs between array and charge controller). More or less, MPPT controllers are like a DC version of a variable transformer (technically, they are usually a form of Buck Mode DC power supply if you are familiar with power supplies).

Also, note that for figuring out the maximum output current from a panel/array, use panel Imp (maximum power) and Isc (short circuit) current ratings... For example a 100 watt "12 volt" panel will output Imp current as:
• 100 watt rating / 17.5 volts Vmp = 5.71 amps Imp

And not:
• 100 watts / 12 volt battery = 8.3 amps (not the way solar panels work)

Solar panels are, for the most part, current sources and will maintain Imp in full sun at any output voltage below Vmp rating.

-Bill

PS: By the way, the poster is from South Africa (I am guessing) and there are regions were he could have 6+ hours of average sun.

6. Registered Guest
Join Date
Jan 2012
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## Re: How big solar panels do i need?

Thank you for all the answers, and the offer for designing the system.
And your guess is correct i am in Delmas, South Africa. 28.680861° Long -26.146580° Lat

The system is actually for my parents, to reduce the electricity bill.
The requirements as follows
1 x TV, 200Watt for 1.5 hours = 300Watts
2 x CFL, 20Watt for 5 hours = 200Watts
2 x Radio, 18Watt for 8 hours = 288Watts
Total watts for the day is 788 Watts
Add the inverter efficiency at 90% total watts is 788*1.1=866.8 watts
Add the inverter usage 866.8 + 20Watts = 886.8Watt need for the day.

I figured 2 x 105ah, 12Volt, deep cycle lead acid batteries should be ok.
Considering 50% DOD, i have 2x52.5ah=105ah , which gives 105x12Volt = 1260Watt hours.

Problem is how to put the 105aH back into the batteries, from PV solar panels.
According to previous posts, i am looking at putting back actually 105x1.2=126ah.

Now this is where i need help with the size of the solar PV panels.
The solar panels vendors say we are looking at 6 hours sunlight.

I am planning to use 2 x 105ah batteries
1 x 10A MPPT charge controller, is this ok?
1 x 1500W 12V inverter
? x ?W solar PV panels

For days when the sunlight is not enough, like heavily overcast or rain, we can always go back on the grid

I presume i should have the batteries in parallel, and the panels also in parallel.

7. ## Re: How big solar panels do i need?

First, if at all possible (and I realize it probably isn't), get a Kill-A-Watt meter and measure the loads for real. Problem there is that even if you get one they are for 120 VAC 60 Hz which may be incompatible with the power there?

Second, assuming the 788 Watt hours per day is correct: 788 / 0.90 = 875 Watt hours. The inverter drawing 20 Watts per hour * 24 hours is 480 Watt hours: total 1355 Watt hours per day, not 886. It may be worthwhile to shut the inverter down when not in use or better, since the loads all together add up to 276 Watts, buy the Morningstar 300 which only uses about 6 Watts running and a tiny amount when in "standby" mode. That reduces the inverter draw to 144 Watt hours, bringing the daily total down to about 1 kW hour per day.

To get that on 12 VDC you need less than 100 Amp hours. This makes two 105 Amp hour batteries in parallel. Better yet is to use two 6 Volt 220 Amp hour batteries (golf cart) in series. Either way you'll want to try for around 20 Amps peak charging current @ 12 Volts, or 240 Watts. Given panel inefficiencies (especially in hot climates) you'll want a total array around 312 Watts. Using the Icarus formula that looks like this:
312 Watts * 6 hours equivalent good sun * 50% over-all system efficiency = 936 AC Watt hours.
So you may want to increase the array size to about 340 Watts. It is always better to have more charging capacity rather than less. The sun does not always shine brightly.

I've never seen a 10 Amp MPPT charge controller, but no matter. Your looking for one that can handle at least 20 Amps, like the Rogue: http://www.roguepowertech.com/products/mpt3024.htm

Panels? What can you get that meets the specs for a reasonable price? Three Kyocera 135 Watts?
As to series or parallel on the panels, the Rogue 3024 would handle the three Kyoceras in series in that climate. This will reduce problems with Voltage drop between array and controller.

This stuff isn't cheap. How bad are the electric rates there? You may save money on the utility bill, but that doesn't necessarily mean you will save money over-all.

8. BB.
Just some guy
Join Date
Mar 2006
Location
SF Bay Area (California)
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## Re: How big solar panels do i need?

First, we always try to push conservation first... Is there any way they can get a more efficient TV that will meet their needs? A 15" laptop computer + TV tuner is only 30-50 watts or so... You should be able to get a TV down under 100 watts if you look around (maybe?) that could meet their needs.

Next, your power needs are already pretty low--so you are at a good starting point for solar. A question about power costs... In the US, we are looking at \$0.10 to \$0.30 per kWH for most people. And off grid solar (even with our "cheap" equipment costs here) runs around \$1-\$2+ per kWH by the time you take equipment life (~10 years or so) and battery life (5-8 years or so) into account.

Some folks have been working on conservation here and found that their utilities are beginning to raise their "fixed" charges (\$25 per month or more), so that off-grid solar for weekend cabins/small homes becomes a possible money saving option.

First, just a minor Units Fix:

Originally Posted by theuns
Thank you for all the answers, and the offer for designing the system.
And your guess is correct i am in Delmas, South Africa. 28.680861° Long -26.146580° Lat

The system is actually for my parents, to reduce the electricity bill.
The requirements as follows
1 x TV, 200Watt for 1.5 hours = 300Watts*Hours
2 x CFL, 20Watt for 5 hours = 200Watts*Hours
2 x Radio, 18Watt for 8 hours = 288Watts*Hours
Total watts for the day is 788 Watts*Hours
Add the inverter efficiency at 90% total watts is 788*1.1=866.8 watt*Hours
Add the inverter usage 866.8 Watt*Hours + 20Watts (WH, but Watts, you need X hours*watts of inverter "on time" to add to equation) = 886.8Watt*Hours need for the day.

I figured 2 x 105ah, 12Volt, deep cycle lead acid batteries should be ok.
Considering 50% DOD, i have 2x52.5ah=105ah , which gives 105x12Volt = 1260Watt*hours.

Problem is how to put the 105aH back into the batteries, from PV solar panels.
According to previous posts, i am looking at putting back actually 105x1.2=126ah.

Now this is where i need help with the size of the solar PV panels.
The solar panels vendors say we are looking at 6 hours sunlight.

I am planning to use 2 x 105ah batteries
1 x 10A MPPT charge controller, is this ok?
1 x 1500W 12V inverter
? x ?W solar PV panels

For days when the sunlight is not enough, like heavily overcast or rain, we can always go back on the grid

I presume i should have the batteries in parallel, and the panels also in parallel.

• 867 Watt*Hours of load per day (winter/summer)
• 5 Hours of Sun minimum per day (need power in "winter" too), usually more time/lights in home during bad weather
• 0.52 system derating (panel+controller+flooded cell battery+inverter losses)
• Losses are assumed: 0.52 = 0.77 panel+charger deratings * 0.80 Battery Eff * 0.85 Inverter Eff
• Battery storage should be ~1-3 days of "no sun" and 50% maximum discharge--Assume 2 days no sun + 50% max discharge
• Charging Current for a battery bank should be around 5% to 13% of rated AH capacity
• When discussing battery AH capacity, we generally use the 20 Hour Rate--Usually matches charging/discharging for off grid home close enough
• You should never plan on using 100% of available power every day... Variable weather conditions and power usage, plus extra things like cell phone and battery chargers aways add up
• Review power usage--Conservation is almost aways a better place to spend the money vs adding more solar power
• Backup Power question--Will they keep their Utility Power or will there need to be a genset? Does solar need to supply 100% of power or only 9 months of power during the year with generator/utility backup during bad weather?

So, first the battery bank sizing based on loads:
• 867 WH per day * 1/0.85 inverter eff * 2 days no sun * 1/0.50 max discharge * 1/12 volt battery bank = 340 Amp*Hour 12 volt battery bank

Next, solar panels required... Two calculations. One based on 5-13% recharging rate of battery bank. Second based on 5 hours of sun average minimum per day:
• 340 AH * 14.5 volts batt charge * 0/0.77 panel+charger deratings * 0.05 rate of charge = 384 Watts of solar panels minimum
• 340 AH * 14.5 volts batt charge * 0/0.77 panel+charger deratings * 0.10 rate of charge = 768 Watts of solar panels "nominal"
• 340 AH * 14.5 volts batt charge * 0/0.77 panel+charger deratings * 0.13 rate of charge = 99 Watts of solar panels "cost effective maximum"

And based on 5 hours of sun per day:
• 867 Watt*Hours of power * 1/0.52 system derating * 1/5 hours of sun = 333 Watts of solar panel to meet load requirement

So, in your case, assuming lots of sun and a nominal 12 volt battery bank, we are looking at ~384 to 768 watts to both meet the load requirements and proper battery charging. Obviously a system with 768 watts of solar panel would generate way more power (on an average day) than your parents would use... Would need less "monitoring" of system health/state of charge if they used a bit more power than planned.

Even the minimum 384 watt of solar panels should work--assuming somebody monitors the battery bank state of charge and watches their power usage.

So, type/size of charge controller... Assuming you can install the solar array fairly close to the charge controller/battery bank, you can get away with a good PWM charge controller. The size of the controller would be (always check specifications/manual--Different manufacturers have different assumptions about safety factors):
• 384 watt array / 17.5 Vmp estimated for panels = 21.9 amps

So, you would need a 25 amp, or larger, 12 volt PWM charge controller.

If you picked a larger array (closer to 800+ watts), you may want to look at a MPPT type charge controller:
• 768 watt array * 0.77 derating * 1/14.5 volt batt charging = 40.7 amps

MPPT controller manage the maximum current through them and you can use a larger array and assume, on average, the losses of the system will allow you to use a bit smaller controller and still collect most of the available energy from the array.

I don't know what inverter you have chosen--But if you can justify it, take a look at the MorningStar 300 Watt TSW 12 volt inverter (is available in 230 VAC 50Hz). It has both a "search mode" (uses very little power when no AC loads; and a remote On/Off input (simple wall switch can turn whole inverter on/off instead of needing large DC switch/Relay). So far, people have found them to be very reliable and efficient.

All of the above assumes a 12 volt battery bank... If your system gets larger and needs more AC power--You may need to think about a 24 (or even 4 Volt system... That reduces the DC current by 1/2 to 1/4 (less wiring costs). And Solar Charge controllers are rated in output current... So if you install on a 24 volt battery bank, the same controller can support a 2x larger array (depending on controller specifications--some controllers can be configured to work on a 12/24/48 volt battery bank).

Also, think about monitoring your battery bank. Measuring voltage is not very accurate. A hydrometer is needed for flooded cell batteries. And a Battery Monitor (Victron is another one) is really easy to use--Will display battery state of charge in 0-100%. Some of the battery monitors even have a output contact that can be programmed to turn on an alarm or even shut down the Inverter if the battery bank goes below XX% state of charge.

Your whole design should be based on the battery bank and ensuring that the batteries are properly maintained. Also, safety is very important too (proper size wire, fuses/breakers, venting of hydrogen gas, wearing safety glasses when working with electrolyte, insulated tools, etc.).

Feel free to ask questions and criticize my work... Ain't perfect and I have made many assumptions which may not be correct.

-Bill

9. ## Re: How big solar panels do i need?

One critique, Bill; you've put in an allowance for 2 days with no sun. Since the grid will be available, this doesn't factor in to it. It just doubles the size of battery bank and panels.

Even when in a "no grid" condition it's better to calculate on 25% DOD, know that you've got 25% more for a dull day (some charging nearly always occurs), and be sure you have a generator for when the sun fails to materialize on day 3.
Of course you have to tailor to your expected local weather conditions too, but there's no sense in buying a lot of extra capacity that most of the time won't be used.

I still doubt this proposed system will actually save any money, btw.

10. ## Re: How big solar panels do i need?

It could be a long shot, but we should mention that it may be worthwhile looking into whether the utility allows grid-tie solar. If so, that could be the most practical way of using solar to reduce the electric bill.

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