# Charging and usage

Registered Users Posts: 5
Dear All,
What is the Panel Wattage should be to charge 105Ah Deep Cycle Battery in few hours? Would 2 Panels of 60W each would recharge the battery fast in average weather conditions? the estimated usage here is 200W/h. This is used to light on 3-4 economy lamps and a TV.

Same as above, but for charging Deep Cycle Battery of 130Ah, for 8hours usage too.

• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Charging and usage

The quick and easy rule of thumb is.

Take the name plate rating of the PV, divide that number by two to account for all system loses, then multiply that number by 4 to represent the average number of hours of good sun one might reasonably expect over the course of the year.

So your 120 watts of PV might look like this: 120/2=60*4=240 That would be 240 watt/hours per day, on average.

As for a 105 ah battery. Ideally it should charge between 5-13% of ah capacity, or somewhere between 5-13 amps. Your 120 watt panels, under IDEAL conditions might put out ~7-8 amps into 12 vdc, so you are in the ball park.

Finally, recharging a battery "fast" is a relative term. If you are able to put 7 amps in, and you have drawn out ~ 20 ah (~200 wh) it will take nearly the entire 4 hours I mentioned above. If however, you are drawing loads on the battery, while you are charging, you might never get there.

For example, if you are drawing 3 amps while you are charging at 7, you are really only putting 4 into the battery. 4amps *4 hours= 16 ah. If you have drawn down 20, you are going to get further behind every day.

I suggest that you accurately log your loading, and then come back a design a system that is well matched between the batteries and the PV to carry that loading.

Welcome to the forum, good luck and keep in touch,

Tony
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charging and usage

from icarus,
"the average number of hours of good sun one might reasonably expect over the course of the year."

solar's in trouble if that's the case. substitute day for year.

zelmayor,
these are lead acid batteries and most don't do a fast charge in terms like you may be thinking and they don't like to be drawn down below 50% of capacity. now 200wh is 200wh/12v=16.67ah and that represents 16.67ah/105ah=15.9% dod so there's still 84.1% state of charge (soc) left. if you want it all in an hour you will boil the electrolyte very hard and need more maintenance more often. go about a 10-13% rate of charge after losses and most of it will be back in the first hour, but the remainder needs to charge and do note that the absorb stage takes a bit of time possibly being 2 or 3 hours, but at a much lower rate towards the end as it tapers the current downward. a 120w pv will most likely fall short, but the more common wattage value out there now for a 12v pv is about 135w and it may fit your requirements a bit better. http://www.solar-electric.com/kykc130wasop.html even this one has a 7.63a current rating and may be a bit lower when losses are factored in. if you'd want even faster charging you'd either need more 12v pvs or obtain a higher voltage pv with better wattage specs and a more exspensive mppt downconverting controller. btw, get a battery temp sensor for the controller.
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Charging and usage

Thank you Neil, That would be 4 hours per day over the course of an average year! Remember,, your mileage will vary so to speak! Many places more in the summer, lot's of places less in the winter etc.

Tony
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charging and usage

sorry tony as i forgot to put this at the end of my comments to you. :p:p:p
• Registered Users Posts: 5
Re: Charging and usage

Happy New year folks, and really thank you for sharing the knowledge with me.

I have went through many times your replies, and always I have answers and questions comes out from them

The system was mounted to reply to the need of a client, and it is working smooth, they have adapted to it, and the system is friendly and people are treating it properly.

I would like to inform you that the system is handling the 120W and the recharge is ok, client is able to get his need from TV and light and he is happy.
He is more satisfied in getting that much of what, while he had to pay a bigger amount for neighborhood generator supply.

The formula is ok, can be understood, or even I can take it as granted, but honestly speaking, I need to understand this once for all.

Btw, that system, is generating enough +120W for around 6 hours, and client is not complaining from shortage, even it is winter here, which it means cloudy days and short days.

Getting fast recharge might be wrong terminology, and easy to confuse, but getting steady recharge against usage, during sunny or cloudy day, that might be more comprehensive, and hope now I made my point.

now, I am always getting a problem when negotiating with a client about quoting for him a Solar Electricity Generator System, I don't have scientific formula(s) to calculate the needed Solar Panels, along with needed Batteries to form the Bank. Is there is calculator for my country Lebanon, Middle east, that I can get and use?
Beside, is there is a formula that can be used to calculate the Batteries needed to form a Bank? and what are the criteria to be taken into consideration to make the equation/formula?

Hope I was able to ask the question properly, and that I am able to get loser to these situations.

Another request please, where I can get material that I can read to understand how PV systems work, I am not an engineer, but educated and worked with them, so I really need material to read that comes with formulas and along with identification of formula criteria.

Last thing, I am willing to use sun tracker, so how should be the impact on the above system when using a suntracker?

Thanks a lot for everything
Re: Charging and usage

You have several ways to proceed...
1. measure/estimate the loads (Amps*Hours @ battery voltage);
2. given a certain size battery what size solar panels and inverter
3. given a panel size what battery and inverter would work...
Which way would you like to use? We can discuss all three--but I don't want to confuse you by making a big/complex post full of stuff you don't really need.

Regarding a tracking solar array, you can use a program like PV Watts to calculate the amount of solar power you will get by month--for fixed or tracking arrays.

As an example, pick a location near you with similar weather patterns... I will try Cairo Egypt. Next, how large of solar panels... Use 1 kW (1,000 watts) as this is the smallest the program accepts. Next, use 0.52 for system derating for off-grid system efficiency. Lastly, you can choose fixed or tracking (1 or 2 axis array). So here are a couple examples. First is a fixed 1kW array and the second is the same thing but 2 axis tracking:
"Station Identification"
"City:","Cairo"
"State:","EGY"
"Lat (deg N):", 30.13
"Long (deg W):", 31.40
"Elev (m): ", 74
"Weather Data:","IWEC"

"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 30.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:","-99.0 pound/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (pound)"
1, 3.93, 59, "N/A"
2, 5.03, 69, "N/A"
3, 5.57, 83, "N/A"
4, 6.18, 85, "N/A"
5, 6.59, 92, "N/A"
6, 6.87, 91, "N/A"
7, 6.86, 94, "N/A"
8, 6.67, 92, "N/A"
9, 6.65, 90, "N/A"
10, 5.08, 71, "N/A"
11, 4.35, 62, "N/A"
12, 4.14, 61, "N/A"
"Year", 5.66, 947, "N/A",
"Station Identification"
"City:","Cairo"
"State:","EGY"
"Lat (deg N):", 30.13
"Long (deg W):", 31.40
"Elev (m): ", 74
"Weather Data:","IWEC"

"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: 2-Axis Tracking"
"Array Tilt:","N/A"
"Array Azimuth:","N/A"

"Energy Specifications"
"Cost of Electricity:","-99.0 pound/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (pound)"
1, 4.73, 72, "N/A"
2, 6.14, 85, "N/A"
3, 6.82, 102, "N/A"
4, 7.90, 111, "N/A"
5, 8.79, 126, "N/A"
6, 9.51, 129, "N/A"
7, 9.30, 130, "N/A"
8, 8.40, 118, "N/A"
9, 8.04, 110, "N/A"
10, 5.95, 84, "N/A"
11, 5.14, 73, "N/A"
12, 5.08, 76, "N/A"
"Year", 7.15, 1216, "N/A",

So, for a 1 kW Fixed array:
• 3.93 hours of full sun per day / 59 kWH per month for January
• 6.87 hours of full sun per day / 91 kWH per month for June
And for a 1 kW 2-axis tracking:
• 4.73 Hours / 72 kWH per month for January
• 9.51 Hours / 129 kWH per month for June
If you had a 200 watt array, then just multiply the above numbers by 0.2 (1/5) for the output of a smaller array.

For example, a fixed array, with 120 Watt panel would produce per day:
• 59,000 WH / month * 1/30 days per month * 0.120 kW of panels = 236 WH of useful AC power per day
• 91,000 WH / month * 1/30 days per month * 0.120 kW of panels = 364 WH of useful AC power per day
If you only had "hours of sun per day" for a fixed array from your local weather department, you can calculate the same thing by:
• 3.93 Hours of full sun per day * 120 watts of panels * 0.52 derating = 245 Watt*Hours per day
Note, the above numbers are averages and in real life, most people can only pull (on average) around 50 to 75% of the "rated" power per day...

There is a limited amount of power storage (typically 1-3 days in a battery bank) and variations in weather that reduce output at times.

Do not plan on using 100% of the predicted power every day. Your customers would run the risk of never fully recharging the battery (at least every few days or once per week for long battery life).

When using a battery, ideally we recommend not using any more than ~50% on a daily basis of the battery's Amp*Hour / Watt*Hour capacity for long life. And never discharge the battery below 20% state of charge or run the risk of permanently damaging the battery bank.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 5
Re: Charging and usage

Dear Gentlemen,

Thanks a lot for Your replies!

Check this example I found made for Thailand:

Beginning
"How to Design Solar PV System

What is solar PV system?

Solar photovoltaic system or Solar power system is one of renewable energy system which uses PV modules to convert sunlight into electricity. The electricity generated can be either stored or used directly, fed back into grid line or combined with one or more other electricity generators or more renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc.

Major system components

Solar PV system includes different components that should be selected according to your system type, site location and applications. The major components for solar PV system are solar charge controller, inverter, battery bank, auxiliary energy sources and loads (appliances).
• PV module – converts sunlight into DC electricity.
• Solar charge controller – regulates the voltage and current coming from the PV panels going to
battery and prevents battery overcharging and prolongs the battery life.
• Inverter – converts DC output of PV panels or wind turbine into a clean AC current for AC
appliances or fed back into grid line.
• Battery – stores energy for supplying to electrical appliances when there is a demand.
• Load – is electrical appliances that connected to solar PV system such as lights, radio, TV, computer,
refrigerator, etc.
• Auxiliary energy sources - is diesel generator or other renewable energy sources.

Solar PV system sizing

1. Determine power consumption demands
The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:
1.1 Calculate total Watt-hours per day for each appliance used.
Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which
must be delivered to the appliances.

1.2 Calculate total Watt-hours per day needed from the PV modules.
Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get
the total Watt-hours per day which must be provided by the panels.

2. Size the PV modules
Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider “panel generation factor” which is different in each site location. For Thailand, the panel generation factor is 3.43. To determine the sizing of PV modules, calculate as follows:
2.1 Calculate the total Watt-peak rating needed for PV modules
Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get
the total Watt-peak rating needed for the PV panels needed to operate the appliances.

2.2 Calculate the number of PV panels for the system
Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available
to you. Increase any fractional part of result to the next highest full number and that will be the
number of PV modules required.

Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.
3. Inverter sizing
An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery.
For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting.
For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation.

4. Battery sizing
The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:
4.1 Calculate total Watt-hours per day used by appliances.
4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.
4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.
4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.
4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you
need the system to operate when there is no power produced by PV panels) to get the required
Ampere-hour capacity of deep-cycle battery.

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy
(0.85 x 0.6 x nominal battery voltage)

5. Solar charge controller sizing
The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application. Make sure that solar charge controller has enough capacity to handle the current from PV array.
For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration).
According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array, and multiply it by 1.3
Solar charge controller rating = Total short circuit current of PV array x 1.3
Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge Controller)

Example: A house has the following electrical appliance usage:

One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.
One 60 Watt fan used for 2 hours per day.
One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.
The system will be powered by 12 Vdc, 110 Wp PV module.

1. Determine power consumption demands

Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed = 1,092 x 1.3
= 1,419.6 Wh/day.

2. Size the PV panel

2.1 Total Wp of PV panel capacity
needed = 1,419.6 / 3.4
= 413.9 Wp
2.2 Number of PV panels needed = 413.9 / 110
= 3.76 modules

Actual requirement = 4 modules
So this system should be powered by at least 4 modules of 110 Wp PV module.

3. Inverter sizing
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.

4. Battery sizing
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days

Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3
(0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah
So the battery should be rated 12 V 600 Ah for 3 day autonomy.

5. Solar charge controller sizing
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40 A at 12 V or greater."

End

My questions:
a) I need the Panel Generation Factor for my country (Lebanon), can you please specify where I can find it in your example? (is it there but under another name maybe?) if not there, how to get it?
a.1) It's a standard, right? So whenever I find it, it will be a constant to use in any system calculation in my country, isn't it?
b) The total Wp will be then the Total usage divided by a) answer. In my real case, when I read the technical data sheet of the 60W panel, it was mentioned on it "Nominal peak power [Pmax] ±3% 60Wp". So my Wp of my system is 120Wp. so it's the total Solar Watts needed for a system where the total usage is identified?
c) in 2.2, where 110 came from? what it stands for? can you identify it?
d) derating factor 0.52 was hard to understand. I think if I find a), this won't matter after that, true? But can it be explained to me?

In my system mentioned earlier, the following was identified:
Item Appliance Watts hours Wh
1 TV 110 8 880
2 Bulb 30 8 240
3 Bulb 30 4 120
4 Bulb 22 2 44

Total usage Watts 1,284

Loss Factor 1.30

Total PV Panels energy needed: 1,669.20

Panel Generation Factor (PGF-Lebanon) "?"
Total Wp of PV panel capacity needed= 1669.20 multiplied by "?"

need to identify then the question mark "?"
How that?

Then to calculate the number of Panels, this will be depending on which Panel size to use, right? Total Wp needed divided by that Panel Peak Watt.

Will stop here and will wait for your feedback gentlemen.

Real Thanks

Romeo
• Solar Expert Posts: 1,571 ✭✭
Re: Charging and usage

Seems ok at a quick glance. Instead of the "panel generation factor" you could use this online tool to work out the Wh of production for a given panel and inclination for a given month:

http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php?map=africa

You can select Lebanon in the map and then enter the details for the array. So after step 1.2 you'll have an answer for how many Wh per day you need. Then through trial and error enter different array sizes in the online tool until it gives you the correct Wh per day you need for the given month.
If this is for all year round use, you may want to size the array for the worst month - or alternatively size it for the 3rd or 4th worst and then use a generator or other backup power source for the months where production isn't enough.
Re: Charging and usage
zelmayer wrote: »
Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy
(0.85 x 0.6 x nominal battery voltage)
I believe the above equation should be:
• Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy * 1/(0.85 x 0.6 x nominal battery voltage)
Let me try the same example, but based on the way I do the calculations (I am getting a bit lost in the "fudge factors" -- the various 1.x times and such). They may be perfectly OK...

Example: A house has the following electrical appliance usage:
One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.
One 60 Watt fan used for 2 hours per day.
One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.
The system will be powered by 12 Vdc, 110 Wp PV module.

1. Determine power consumption demands

Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed = 1,092 x 1.3
= 1,419.6 Wh/day.
The above 1.3 factor does not make too much sense to me... I would do it this way (assuming energy is stored during the day and used at night--if you use energy during the day, then you have less battery losses).

Energy needed from battery bank:
• 1,092 WH per day * 1/0.85 inverter eff = 1,285 WH from Battery Bank per day
Choosing Battery Bank Voltage... Roughly, I like to limit 12 volt systems to 1,200 watts peak power, 24 volt systems to 2,400 watts peak, and 48 volt systems for anything over 2,400 watts.

The above system would seem to be OK at 12 volts--But for a refrigerator, they typically take upwards of 5x or more starting loads. At least in the US, for a typical 120 watt motor and with 500 watts of defrost/ice maker heaters, a 1,200 to 1,500 watt inverter is recommended. That is right on the cusp of using a 24 volt battery bank.

Because Solar Charge Controller outputs are based on current--a 12/24/48 volt capable battery charger will handle 2x the wattage of solar panels at 24 volts vs 12 volts (power=voltage*current)... So, I will assume a 24 volt battery bank here.

Sizing the battery bank (for various reasons, recommend around 1-3 days of autonomy with 2 days being a good cost effective point--More than 3 days will drive system costs up because of the large battery bank):
• 1,285 WH from battery bank * 1/24 volt bank * 1/0.50 maximum discharge * 1 day of autonomy = 107 AH battery bank @ 24 volts
• 1,285 WH from battery bank * 1/24 volt bank * 1/0.50 maximum discharge * 2 day of autonomy = 214 AH battery bank @ 24 volts
• 1,285 WH from battery bank * 1/24 volt bank * 1/0.50 maximum discharge * 3 day of autonomy = 321 AH battery bank @ 24 volts
So, the recommended battery bank is 24 volts (although 12 volts would be OK too in this case--choose based on the best 12 or 24 volt inverter available for your region).

Personally, I prefer 1 big battery (or, for example, 6 volt batteries in series) rather than several batteries in parallel... If you choose to parallel batteries, I would suggestion no more than 2-3 parallel battery strings.

At this point, I will throw in now how to define the optimum battery wiring/fuse/breaker ratings.

Say 1,500 watt 24 vdc inverter (based on US NEC national ectrical codes that use a 1.25 safety factor for wiring and protective devices--If you use 1.00x factor, you may have fuse/breaker trips at full load. Battery cutoff voltage is based on 10.5 volts minimum for a 12 volt battery bank and wiring voltage drop):
• 1,500 watts * 1/21 volts battery cutoff * 1/0.85 inverter eff * 1.25 safety factor = 105 Amps minimum for inverter fuse/wiring
Next sizing the PV panel. There are two ways you should be sizing the solar array... First based on battery charging current requirements and second based on available sun/loads. Note that many times, a large battery bank needs a larger solar array to provide optimum charging current for a battery bank than is needed for actually powering the loads.

For batteries, we recommend around 5-13% rate of charge rule of thumb (based on 20 Hour Rating for batteries). Using the 2 day battery sizing of 214 AH @ 24 volts and ~10% rate of charge as a good sized solar array:
• 214 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 806 watts of solar panels
Using stephendv link for Lebanon and 40% losses (I aim for ~52% efficient system or ~48% losses--Stephen's PVGIS link does the calculations differently so I have to play with my assumptions to get them to fit) and 1kW of panels (round number):
Latitude:    33°45'39" North,
Longitude:    35°51'33" East
Nominal power of the PV system:    1kWp
Inclination of modules:    29deg.
Orientation (azimuth)  of modules:    0deg.

Fixed angle
Month        Ed        Em        Hd        Hm
1        1.87        57.9        3.38        105
2        2.14        59.9        3.93        110
3        2.68        83.1        5.05        157
4        3.15        94.5        6.10        183
5        3.45        107        6.80        211
6        3.71        111        7.43        223
7        3.69        114        7.44        231
8        3.60        112        7.27        225
9        3.44        103        6.87        206
10        2.92        90.4        5.68        176
11        2.28        68.4        4.27        128
12        1.88        58.1        3.42        106
Year        2.90        88.3        5.65        172

Ed: Average daily electricity production from the given system  (kWh)
Em: Average monthly electricity production from the given system  (kWh)
Hd: Average daily sum of global irradiation per square meter received by the modules of the given system  (kWh/m2)
Hm: Average sum of global irradiation per square meter received by the modules of the given system  (kWh/m2)

The above "Hours of Sun" are assuming fixed array and all system losses (unlike PV Watts which does not include electrical losses). 4th "darkest" month is 10 (October) 2.92 Hours of Sun per day. So, your "may need generator" panel sizing based on 1,092 WH per day:
• 1,092 WH per day / 2.92 Hours of sun per day = 374 Watts of solar panels
So based on Battery Bank rate of charge of 10% and 2 days of autonomy, you would need ~806 watts of solar panels. Based on 5% rate of charge, 403 watts of solar panels.

Based on October sun shine, 374 watts of solar panels minimum. So we get roughly the same minimum array sizing, but from a completely different path. And I would still suggest roughly 806 watts of solar panels for long battery life (especially if "tall" flooded cell batteries).

Notice that sizing solar panels is the last step rather than one of the first steps in the way we do the calculations here. This will usually result in longer battery life and happier end users (but not necessarily a cheaper system).
2. Size the PV panel

2.1 Total Wp of PV panel capacity
needed = 1,419.6 / 3.4
= 413.9 Wp
3. Inverter sizing
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.
Does not take into account the 5x starting loads of the typical refrigerator. There may be newer pump designs with lower surge currents--but I would want to measure/test on my own setup before I recommended a small inverter for customer systems.

Also, remember if the inverters run 24 hours per day, they have their own switching losses ranging from 6 watts to 30 watts or more--If 24 hour per day inverter operation, you need to take these losses into account (30 watts * 24 hours = 720 WH per day--which makes your daily loads 50% greater than first calculated--unless you get an inverter with "search mode" function which looks for >6 watts AC loads before turning on 100%).
4. Battery sizing
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days

Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3
(0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah
So the battery should be rated 12 V 600 Ah for 3 day autonomy.
Did not include inverter losses (85% efficiency, and nothing about switching losses for 24 hour per day operation).
5. Solar charge controller sizing
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40 A at 12 V or greater."
OK calculation, but note that if you use a 24 volt controller, the rated current would only be 20 amps instead (smaller/less expensive controller needed for higher voltage battery bank).

Also, if using an MPPT charge controller, you can size it smaller because they can controller their output maximum current to safe levels (switching power supply)... So, if I took 440 watts of solar panels, I would rate the output current as:
• 440 watts * 1/29 volts charging * 0.77 panel+controller derating = 11.7 amps
Now you can use a "tiny" MPPT charge controller like the MorningStar 15 amp MPPT version. Or the Rogue 12/24 30 amp MPPT controller.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Charging and usage

Romeo,
My questions:
a) I need the Panel Generation Factor for my country (Lebanon), can you please specify where I can find it in your example? (is it there but under another name maybe?) if not there, how to get it?

You can make several charts from Stephen's link... For example, the version I made assumes 29 degree (from horizontal) fixed array, ~0.52 end to end efficiency:
• 0.77 panels+charge controller eff * 0.80 flooded cell battery efficiency * 0.85 inverter eff = 0.52 overall efficiency
Note that new batteries and AGM batteries can run from 90-98% efficiency... But old flooded cell run less efficiency can get down towards 80%--I like to be conservative and have the system work until the batteries fail--instead of working well for 1 year then having problems as components age/panels get dirty/telling the customer they are not using the system "right".
a.1) It's a standard, right? So whenever I find it, it will be a constant to use in any system calculation in my country, isn't it?
You have to know your tools. PV Watts and PVGIS seem to be very close in how they operate and output their data--However, when you look at the details, they have very different assumptions and how they calculate their displayed output.

So, you could create a couple of these "charts" for your area (flooded cell vs AGM batteries, with or without AC inverter, etc... Then you can put the data into a spread sheet to quickly give what-if answers to your customer's questions.
b) The total Wp will be then the Total usage divided by a) answer. In my real case, when I read the technical data sheet of the 60W panel, it was mentioned on it "Nominal peak power [Pmax] ±3% 60Wp". So my Wp of my system is 120Wp. so it's the total Solar Watts needed for a system where the total usage is identified?
Roughly, I like to assume from Solar Panel Rating to Charge Controller Output as 0.77 efficiency. For very cold regions/weather, you may do better in winter (especially with a MPPT type charge controller).
c) in 2.2, where 110 came from? what it stands for? can you identify it?

110 Watts rated Solar Panel.

Typically, panels >100 watts tend to be cheaper than <100 watt panels (\$\$/Watt)... Large panels can be ~\$1-\$3 per watt; small panels ~\$5-\$10+ per watt.

Several issues with choosing solar panels... In general, panels <100 watts tend to be designed for use with PWM controllers (and MPPT) and 12/24/48 volt battery banks... I.e., a "12 volt" solar panel has Vmp~17.5 volts -- This allows for hot weather (Vmp drops as temperatures increase), wiring voltage drops, and charge controller voltage drop.

Panels >100 watts tend to be designed for Grid Tied systems with Vmp-array in the range of 200-600 VDC. They have no reason to design the panels for multiples of Vmp=17.5 volts.

This usually means that >100 watt panels will need a MPPT type charge controller (PWM is not efficient with Vmp>>17.5 volts on a 12 volt battery bank).

Also, there is no "standard" Vmp for >100 watt solar panels... There have been cases where somebody needs to replace a broken panels or wants to add to the array and they can no longer purchase the same make/model of solar panels and they have to mix and match with the best they can--And in some cases, they have to add a second MPPT charge controller to support the Vmp-array of the new panels (or even remove the old array and install all new panels).
d) derating factor 0.52 was hard to understand. I think if I find a), this won't matter after that, true? But can it be explained to me?

Basically:
• 0.77 is the solar panel derating (PTC ~ 81% of STC panel rating and a couple percent for dust on panels) and 5% charge controller losses (0.81*0.95=0.77)
• 0.85 efficient AC inverter
• 0.90 efficient for AGM (sealed) batteries (can be 98% eff when new)
• 0.80 efficient for Flooded Cell batteries (can be >90% when new)
So, the generic factor becomes:
• 0.77 * 0.85 * 0.80 = 0.52 end to end efficiency...
And that is pretty much what people find--Roughly 1/2 the solar panel rating * Hours of Full Sun Equivalent per day = harvested power.
Will stop here and will wait for your feedback gentlemen.

Real Thanks

Romeo

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 5
Re: Charging and usage

Hi Bill,

Your reply gave me few good stuff and I am working on building my calculation sheets. Your explanation about the loss percentage were very good. Thnks.

now, I should sort things again, to narrow the questions and start getting the equations for designing some parts of the PV system.

Let us start with the battery bank size, I have came out with that formula:
Power Consumption Needed by user:
PCN=SUM(Unit consumption power per Watts x Daily working hours) / 0.85 (which is the inverter efficiency)

Then,
Bank size:
PC/0.80 (which is the maximum of Lead Acid Batteries consumption)= Efficient Power to Generate=EPG

to take a real example of a system I am testing:
Appliances connected:
Saving Lamp 30W x 8hours Max.= 240W
Saving Lamp 22W x 4hours Max.= 88W
Saving Lamp 22W x 2hours Max.= 44W
TV 32" LCD 110W x 8hours Max.= 880W

Total Power to Consume= 1,252Wh

PCN=1,252/0.85=1,473Wh
EPG= 1,473/0.80=1,842Wh

For calculating the Panel size, here I got some confusion.
Power to generate should be equal to EPG above with addition to the losses in the system right?

EPG=SWs (means Solar Watts) x 0.52 x 4hours (average sunny hours per day)

SWs= 1,842Wh / (0.52 x 4) = 1,842 / 2.08 = 886 Solar Watts needed to generate above EPG.

For the days wehre there is no sun, I will recommend to use a small battery charger to backup during such days. in my country we have around 300 sunny days from 365.

I think now |I have narrowed things in a real system I am test and combined ur information with my conclusions from my researches.

Waiting your valuable feedback folks, you were really great

Romeo
• Solar Expert Posts: 1,571 ✭✭
Re: Charging and usage

The calcs look good. A few notes:
- Check your inverter specs, but 85% is not very good, the good brands should be between 90-95% depending on the load.

- To improve the battery charging factor ( 0.8 ) you could consider using AGMs instead of flooded cells.

- Basing everything on the average sun hours per day means that you're only supplying enough power for half the year. Given the number of sun days you have, it could be worthwhile using the lowest sun hours in the worst part of the year so that you cover 90-100%

BTW, have you considered these guys for the PV panels? http://www.philadelphia-solar.com/ they're just next door in Jordan
Re: Charging and usage

The 85% efficiency is on the conservative side...

However, if you use a higher percentage, remember that you have other loses. most inverters have around 6 to 30+ watt idling loses and/or inverters are less than 90% efficient at very low/very high power levels.

The less conservative you are, the more you have to take all losses into account and the more accurate your load predictions have to be.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Charging and usage

To clarify the inverter collections...

A 300 watt inverter ruining 24 hours per day:

o 6 watts idling x 24 hours = 144 wh

Vs a 1,500 watt inverter which may be:

o 30 watts x 24 hours per day = 720 wh per day

A large inverter running 24 hours per day uses almost as much power as a refrigerator.

So picking the correct inverter and perhaps paying more for one with a standby function to cut power usage for the 50% of the time the compressor is not running.

Some people have figured out how to turn the inverter off when the refrigerator pump is not running to save power.

Or using a small inverter to power 24x7 loads and a big inverter that is only turned on when needed to power big water pump (for example).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 5
Re: Charging and usage

As I can see, that the concern was focused on the inverter loss. I am using a small inverter DC/AC 500W , MAx efficency >89%.

Regarding Solar Watts needed, my calculations showed the need of 886W which almost 6 Panels of 150W each.

That isn't considered too much for charging 1 battery of 160Ah?

While, when I was discussing with a supplier, I showed interest in a system that will generate:
200W per hour for continuous 8hours
Total shall be:
200x8=1600Wh
1600/0.85= 1882Wh efficient

Bank for that was needed:
(200Ah x 12V) x 0.8 = 1920Wh

How we calculate from your side gentlemen the Solar Watts needed here?
As per that supplier advice, we need around 400 Solar Watts to recharge that Battery.

what do you think about that?

Romeo
Re: Charging and usage
zelmayer wrote: »
Let us start with the battery bank size, I have came out with that formula:

Power Consumption Needed by user:
PCN=SUM(Unit consumption power per Watts x Daily working hours) / 0.85 (which is the inverter efficiency)

Then,
Bank size:
PC/0.80 (which is the maximum of Lead Acid Batteries consumption)= Efficient Power to Generate=EPG

to take a real example of a system I am testing:
Appliances connected:
Saving Lamp 30W x 8hours Max.= 240W
Saving Lamp 22W x 4hours Max.= 88W
Saving Lamp 22W x 2hours Max.= 44W
TV 32" LCD 110W x 8hours Max.= 880W

Total Power to Consume= 1,252Wh

PCN=1,252/0.85=1,473Wh
EPG= 1,473/0.80=1,842Wh

That is how I would start...

You will need to go through your peak loads to size the inverter, look up its efficiency, and its "tare" losses -- Especially if the inverter runs 24x7.

For a small inverter, the Morning Star TSW 300 watt (True Sine Wave) is a great unit for 12 volt systems (there is a 220 VAC 50 Hz version for your area of the world). Read about its "search mode" (basically outputs AC power every x seconds until it sees > 6 watt load, then turns on 100%) and the DC Inhibit line (allows you turn off the inverter with a simple DC switch instead of a huge relay on the DC input power line).

Also, you can read about how to pick an inverter and the differences between TSW (true sine wave) and MSW (modified square wave) inverters... MSW inverters are ridiculously cheap but can cause problems with 10-20% of your loads (small electronics, wall mount transformers, electric motors, etc.):

Choosing an inverter for water pumping

Choosing the correct inverter choice is very important. And sometimes hard to justify the "high costs" of the TSW inverters to your customers.
For calculating the Panel size, here I got some confusion.
Power to generate should be equal to EPG above with addition to the losses in the system right?
There are two "panel sizes" you should calculate--One is the amount of solar panel needed to support your loads (as you have done below)... And second is the amount of solar panels to properly recharge your battery bank (large battery banks need larger solar array to properly recharge them).
EPG=SWs (means Solar Watts) x 0.52 x 4hours (average sunny hours per day)
This sort of double counting your "efficiencies/losses... I have already included the 0.85 inverter and the 0.80 flooded cell battery losses in the 0.52 derating factor.

As I understand your formulas, don't use EPG here, use PCN instead.

Also, be really careful about Watts (a rate like kilometers per hour) and Watt*Hours (an amount like kilometers driven)... Everyone wants to write Watts/Hour and Watts... There is no Watts/Hour--And when writing Watts, most people mean Watt*Hours.
SWs= 1,842Wh / (0.52 x 4) = 1,842 / 2.08 = 886 Solar Watts needed to generate above EPG you mean PCN???.
For the days where there is no sun, I will recommend to use a small battery charger to backup during such days. in my country we have around 300 sunny days from 365.
Is this power to be supplied from a fuel powered generator?

If so, it is actually pretty important to pick the correct size battery charger/genset combination so that you don't waste expensive fuel.

Here is a long thread where we discuss (and try to find) the optimum power supply to use with a Honda eu2000i (1,600 watt) generator and a 24 volt battery bank:

Question about battery charger selection with EU2000 generator

That thread goes into lots of details. Please feel free to ask questions here if it is over your head technically or language wise (your English seems better than mine--But I don't want to assume too much).

Regarding sizing solar panels also to the size of the battery bank, go back to my post #10 and re-read sizing the battery bank for 1-3 days of "no sun" with 50% maximum discharge... And then how to size the battery bank for 5% to 13% rate of charge.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset