# How big solar panels do i need?

theuns
Posts:

**7**Registered Users ✭
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.

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.

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## Comments

5,084Solar Expert ✭✭✭✭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

6,563Solar Expert ✭✭✭✭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).

|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar

gen: http://tinyurl.com/LMR-Lister ,

17,615Banned ✭✭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.

25,443Super Moderators adminTo 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:

And not:-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.

7Registered Users ✭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.

Thanks for everything, in advance.

17,615Banned ✭✭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.

25,443Super Moderators adminFirst, 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:

So, some assumptions (feel free to adjust to your needs):

0.05rate of charge = 384 Watts of solar panels minimum0.10rate of charge = 768 Watts of solar panels "nominal"0.13rate of charge = 99 Watts of solar panels "cost effective maximum"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):

If you picked a larger array (closer to 800+ watts), you may want to look at a MPPT type charge controller:

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 48) 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

17,615Banned ✭✭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.

17,615Banned ✭✭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.

25,443Super Moderators adminMarc,

I was assuming Off-Grid was the solution (high monthly connection fees for utility power--Going battery/local inverter will not save much money on a 1kW per day electrical connection. There is the advantage of battery+inverter+utility grid if the power is unreliable with afternoon blackouts).

The 25% depth of discharge is the same as 2 days of "no sun" and 50% maximum discharge. Either gives you 4x daily loads.

And "Theuns", here is another battery FAQ that is an interesting read:

www.batteryfaq.org

-Bill

25,443Super Moderators adminTheuns,

Another suggestion... With lots of sun, solar thermal/solar hot water can also be a big help in saving heating fuel costs. Also, solar thermal systems lend themselves to Do It Yourself projects very nicely.

Here is a thread that has lots of links and information about solar and other related projects:

Working Thread for Solar Beginner Post/FAQ

-Bill

291Solar Expert ✭If it were me

52 needed ahrs

6hrs a day

panel rate .75X6

1.25X52/6

This math = 11 amps

11 amps X 13.5 volts(mean working line voltage)

150 watts @ 12v

2.3kW [10] Enecsys SMI-240-60 micro inverters

10,311Solar Expert ✭✭✭✭that is overly optimistic and with some inaccuracies in it. for instance, few ever get 6 solid hours of full sun at 1000w/m^2. also batteries can go to 14.5v and more at times necessitating the pv to follow suit. how about v drop losses? the rough calculation by bb will serve the purpose more accurately even though there may be some wiggle room in it due to variables that could change that general outcome, but it won't be as low as your estimate.