suspicious of solar calculators; load sizing

kabubakabuba Registered Users Posts: 3
hi all,
I'm a newbie when it comes to solar but have a passion for it. i want to put a solar hybrid solution for a load of 6700Wh. this is the load that would need to be supported in a 24-hour period and when there's no insolation, i can switch to mains. the various solar calculators i have been using have been giving me a 12*200Wp-panel array, another gave me a 8*200Wp-panel array. my question is, why the discrepancy and does anybody out there have a better way of deciding the inverter size, battery bank and most importantly, the number of panels that i need to use?
i was planning on using a 3.6kVA numeric inverter (has a solar option). i live in nairobi, which has an elevation of 1800m above sea level and has an average insolation of 5.62 hours in a day.
replies would be highly appreciated!!!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: suspicious of solar calculators; load sizing

    Welcome to the forum Kabuba.

    I assume that you have done the first several steps for solar--You have done as much conservation as possible (low power/efficient appliances, turns stuff off when not used, etc.) and that you have measured your loads so you know how much power per day you need (estimating power usage from equipment name plate ratings is not usually very accurate--many times, it will over estimate your loads).

    To the math... Loads drive battery bank size. Normally, we recommend 1-3 days of "no sun" and 50% maximum discharge (for longer battery life). Picking 2 days and a 48 volt battery bank, I would suggest:
    • 6,700 Watt*Hours * 1/48 volts * 1/0.85 inverter efficiency * 2 days of no sun * 1/0.50 max discharge = 558 AH @ 48 volts

    Then to size the solar array... Two ways of sizing, first is based on the charge rate of the battery bank (5% to 13% recommended) and second is based on load and hours of sun per day.

    Based on rate of charge:
    • 558 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 2,138 Watt array minimum
    • 558 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 4,276 Watt array nominal
    • 558 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 5,558 Watt array "cost effective maximum"

    Next is based on the load and hours of sun per day. I like to use PV watts to estimate sun. I see that there are three locations for Narobi Kenya, I will pick Nairobi/Dagoretti, fixed array tilted to latitude 1.3 degrees south--Note that for arrays to be "self cleaning" from rain, you might want a minimum of 5 degree tilt:
    Month   Solar Radiation (kWh/m2/day)
    1      5.99 
    2      6.53 
    3      5.81 
    4      4.91 
    5      4.37 
    6      4.10 
    7      4.04 
    8      4.76 
    9      5.35 
    10      5.19 
    11      4.70 
    12      5.31
    Year      5.08
    

    Normally, I would "toss" the lowest three months and assume a generator for backup power during winter... In your case, I will use 4.04 hours of sun (you have good sun all year long). Note that you should never plan on using 100% of rated power every day--But only about 66% to 75% of your rated power every day, on average, to ensure that you fully recharge the battery bank several times a week and allow for those days when you have more load.

    So, based on 4.04 hours of sun (for July), the suggested minimum array would be:
    • 6,700 WH * 1/0.52 off grid system efficiency * 1/4.04 hours of sun per day = 3,289 Watt array minimum

    The recommended array would be around 3,289 Watts to 5,558 Watts (note, no solar calculations are accurate to 4 places--I am just using a bunch of digits so you can reproduce my math and follow where I am getting my numbers from--Within 10% is about as close as you will get).

    Assuming you pick the "nominal" array of 4,276 watts, your average July production will be:
    • 4,276 watts * 4.04 Hours of Sun * 0.52 off grid system efficiency = 8,983 Watt*Hours per day (average July)

    Note that this is not a "small system"... So there are still a lot of design and equipment selection choices to be made--But this is the basic system as I understand your needs.

    A 2,400 watt array, on average would produce:
    • 2,400 watts * 4.04 hours of sun * 0.52 system derating = 5,042 WH per day (July)
    • 2,400 watts * 4.76 hours of sun * 0.52 system derating = 5,940 WH per day (August)

    Neither will produce 6.7 kWH per day of your required power during part of the year. So, you would have to use less power / more generator power during these periods.

    Note that I have made many assumptions. Your basic system (flooded cell battery bank and inverter efficiency), that you collect power during the day and use it at night (batteries are ~80-90% efficient) and such.

    There are ways you can save money. If your loads are during daylight hours, you could use a slightly smaller array. You could use a smaller battery bank if you do not have large surge current/usage at night (although, if you have periods of cloudy weather--you would have to cut back on power usage/use genset more).

    Lastly, if a lot of your power usage were for water pumping--You could get a water pump that connects directly to a solar panel. You pump water during the day/sunny weather (irrigation or to a cistern and use a small DC or AC pump for pressure to the home/business). The cost of just a solar array to power a pump will be ~1/4 the cost of array+battery+charge controller+inverter system (plus no batteries to replace every ~5-8 years or so).

    Anyway, this is my guess/estimate of your needs. Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: suspicious of solar calculators; load sizing

    I should add--That building an off grid capable system when you have grid power is probably never going to save you money.

    However, if you have problems with reliable power (say you lose power many afternoons for 4-6 hours in the evening), I would be suggesting a smaller battery bank and an AC battery charger (or inverter/charger system) for you. A backup genset and solar panels can be added to support longer off grid power usage (a few days at a time with a genset, more with solar).

    The above numbers are based on assuming a 100% off grid existence. If you are only looking for backup power, the system could be much smaller (hopefully, you can shed loads during part of the outage to reduce over all system size/costs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kabubakabuba Registered Users Posts: 3
    Re: suspicious of solar calculators; load sizing

    thanks a lot BB! at least I'm getting some of the basics. i already have a grid connection. what i am looking for is to generate the energy i might use in a day (over a 24-hour period - 6700wh) but at the same time be connected to the grid, so that when there's sunlight, i can use solar and when there's cloud cover, i can switch to mains. this would, in theory, help me to save energy costs as i would be using free energy when there's solar and buying solar when there's no sun.
    how do i move ahead in planning the above system?
    i also didn't understand where the 59v charging voltage came from the rest i was able to follow.
    what is the system derating factor in a grid-connected system?
    i also have a 24V inverter and wanted to design a 24v system.
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: suspicious of solar calculators; load sizing

    Kabuba,

    It is difficult to save money with an off-grid or hybrid (battery backed Grid Tied/Off Grid) solar power system. In the US (with fairly cheap solar power products), it is not unusual to pay around $1-$2+ per kWH for electricity.

    So, as you cost out your system, I would suggest doing some return on investment math:

    (cost of parts+cost of install+replacement batteries every ~5-8 years+new electronics every ~10+ years) / (12 months * 6.7 kWH per month * 20 years) = estimated $/kWH price

    In the US, assuming you will live/work in a place for 20 years is a long shot for most people, and it is usually not possible to sell a solar equipped home and recover the cost of the installation.
    kabuba wrote: »
    thanks a lot BB! at least I'm getting some of the basics. i already have a grid connection. what i am looking for is to generate the energy i might use in a day (over a 24-hour period - 6700wh) but at the same time be connected to the grid, so that when there's sunlight, i can use solar and when there's cloud cover, i can switch to mains. this would, in theory, help me to save energy costs as i would be using free energy when there's solar and buying solar when there's no sun.

    The "best" solar power system is a "pure Grid Tied" setup. Basically solar panels + a Grid Tied Inverter connected to your main panel. At night, your meter turns "forward" and during the day the meter turns "backwards". You have less costs (no batteries, no battery replacement, no charge controllers, less losses from charging battery banks/converting voltages, etc.).

    But, I would guess, that your utility would not allow such a system (this does not make business sense for a utility). In the US, it took laws, regulations, and subsidies to the utilities to make "Net Metered" billing with GT solar power systems "legal".
    how do i move ahead in planning the above system?

    Know your loads. Your peak power (well pump, A/C, lighting, appliances) you plan on supporting (surge power to size inverter/battery bank). Your average load (say 837.5 Watts * 8 hours per day = 6,700 Watt*Hours per day). Will these loads be during the daytime, at night, or a combination.

    The above answers will then allow you to size your battery bank. Then the rest of the math from my previous post will let you size the balance of system.

    Then you can start looking at hardware to fit your needs.
    i also didn't understand where the 59v charging voltage came from the rest i was able to follow.
    what is the system derating factor in a grid-connected system?
    i also have a 24V inverter and wanted to design a 24v system.

    There are lots of losses and rules of thumb in designing a solar power system... A "12 volt battery" will be ~12.7 volts when "full and resting", around 11.5-12.5 volts when under load and discharging. And around 14.5 volts charging (24 volt system x 2, 48 volt system x4).

    So, 4x 14.5 volts = 58 volts charging (59 volts is used a lot too) for a 48 volt battery bank.

    In general, the larger the loads, the higher the battery bank voltage. Remember that Power=Voltage*Current ...

    Your vehicle uses around 100 amps to turn the starting motor (for short periods of time). You can see how thick those cables are to support that load.

    100 amps * 12 volts = 1,200 watts (nominal)

    Cabling a DC power system to "move" ~100-200 amps is about the maximum continuous rated current that is "cost effective" in copper wiring, connectors, fuses/breakers, switches, etc...

    For example, if you have a 3,300 watt array (from my earlier calculations), the average peak current from such an array would be:
    • 3,300 watt array * 0.77 panel+controller derating * 1/14.5 volts = 175 amps @ 12 volt battery bank
    • 3,300 watt array * 0.77 panel+controller derating * 1/29.0 volts = 87.6 amps @ 24 volt battery bank
    • 3,300 watt array * 0.77 panel+controller derating * 1/58.0 volts = 43.8 amps @ 48 volt battery bank

    The typical larger MPPT charge controllers ($500-$800 each) are rated on their output current. I.e., 45 amps, 60 amps, 80 amps, 90 amps, etc... Whether at 12 or 24 or 48 volt battery bank.

    You can see that if you decide on a 60 amp charge controller, your 3.3 kW array will need 3x 60 amp MPPT charge controller at 12 volts, 2x at 24 volts, and 1x at 48 volts.

    So--With larger systems, you can save significant amounts of money by using a higher voltage battery bank--You need fewer/smaller charge controllers to manage a large solar array.

    Also, wire diameter can be smaller too (less money spent on copper). And higher voltages are easier to send longer distances (a 12 volt system should have no more than 1 volt wiring drop, a 24 volt system 2 volt drop, and a 48 volt system can have a 4 volt drop). High voltage drop waste energy and can overheat wiring. Plus, your loads see lower voltages and will fail/cut off at some minimum input voltage.

    You can use a 24 volt system--Just need more copper to wire it up. And possibly higher capacity (higher Amp*Hour rated) batteries to support the larger current flow (remember, power=V*I, so still same amount of power, just 1/2 voltage is 2x the current for same power).

    This is the problem of buying parts before you have designed the system. It does put you in a box--Do you design the system around your inverter (which forces you to make compromises on charge controllers, battery bank, wiring)... Or do you sell/dump the current inverter and work on an optimum (for you) system.

    If you can give me the watt rating of your inverter--We can design a system around it and see how it works out for you... Note that average loads are usually pretty small (your system would supply less than 1,000 watts over 7-8 hour period to meet your numbers). However, put a 1kW-2kW well pump on the system, and all of a sudden you are talking about needing to support 2-4kW of surge current (pump starting) through the inverter and battery bank... Again, know your loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: suspicious of solar calculators; load sizing

    use a grid tied system as in a nut shell it will stay connected to the grid at all times and during times of solar output you automatically consume the solar energy you produce with your loads providing you don't draw more than the solar produces. there's no need to switch to an off grid system. gt systems use the grid like a battery and you can export power to the grid during the sunny days and when the sun goes down or away you consume power back from the grid. if you consume 6700wh over a 24hr period then you would want a pv system to put it back over roughly 5hrs for 1360w, but due to inefficiencies the pvs would need to increase the watts by about 1.3x for 1767 stc watts minimally. for a gt system this would be assuming that your country allows the export of the grid tied solar to the grid.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: suspicious of solar calculators; load sizing

    Since he is in Nairobi, there is every possibility that the grid is as reliable as a politician's promise. In that case batteries will have to come in to the picture somewhere if he wants to maintain power when the grid goes down. Even so, the hybrid GT system will afford an advantage because of its ability to sell surplus power to the grid. If the utility does not allow sell-back then this is useless and so he may as well go with an off-grid system that uses the grid as a charge source. Seen a few of them get designed around here, eh? :D
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: suspicious of solar calculators; load sizing

    i agree coot that a hybrid gt system is good if he is allowed to sell power to the grid and experiences outages warranting the backup.
  • kabubakabuba Registered Users Posts: 3
    Re: suspicious of solar calculators; load sizing

    thanks for the replies!
    unfortunately, i don't get much time on the internet hence my poor response time.
    as cariboocoot said, the grid is even worse than a politicians promise and sadly, utility companies do not allow us to sell electricity to them, i am thus stuck with batteries.
    a summary of the loads i have and their approximate usage is below:

    Appliance Rating(w) Hours of use Quantity Watt-hour

    flourescent tube 36 5 1 180
    Flatscreen T.V 110 15 1 1650
    CRT T.V 110 3 1 330
    decoders 60 4 2 480
    Printer 450 0.5 1 225
    bulbs 14 6 20 1680
    Fridges 240 24 1 1200
    Computer 300 2 1 600
    Laptop 150 2 1 300
    Total Wh(24 hours) 6645
    as you can see, its basic house appliances
    @bill,
    i have already bought a 3600VA/2400W numeric digital inverter which a 24 volt system(what i can get in the local market). it also has a solar input and an internal charge controller which reduces my bills...i want to create a system around the above inverter....

    kind regards,
    kabuba.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: suspicious of solar calculators; load sizing

    i feel for your situation and digress to recommending an off grid backups type system so when the grid goes down it switches in the batteries and when up you can opt to charge via the grid and/or the solar.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: suspicious of solar calculators; load sizing

    Well the inverter is about the right size. I say this because mine is approximately the same and can run just about anything I connect to it. If this is not a sine wave inverter you may have problems.

    So the next step would be calculating battery bank size based on your daily loads. This can be a bit tricky as the numbers supplied by manufacturers as for power usage are not very accurate. Some of the loads are 'discretionary' so you can leave them off if need be. The other thing that comes into it is: how dependable is the grid? Would you say it is available 50% of the time or less? If you can count on grid power half the time or more you can use that for running non-critical loads and save yourself some money by sizing the battery bank just to maintain what must be kept running.

    Otherwise you're got 6700 Watt hours AC which is going to be about 7450 DC due to conversion efficiency. Then you have to power that inverter 24 hours per day (unless it has a standby feature) which will add about another 500 Watt hours (depending on how much it actually consumes). As such you'd be looking for 8kW hours DC, and on 24 Volts that's about 334 Amp hours, or a need for at least 668 Amp hour 24 Volt battery bank.

    Two observations on this: one, it shows a need for using a 48 Volt system (but you've already got the 24 Volt inverter) and two; it shows the need for knowing accurate power consumption figures and the amount of grid power you can rely on. If you can get that 'critical load' number down you will save yourself considerable money.

    After sizing the battery bank you can determine how much solar you'd need for full charging via panels. Again there can be some trade off here depending on how much you can count on the grid.
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: suspicious of solar calculators; load sizing

    Looking at your loads, these are the ones that stand out:

    Flatscreen T.V 110 15 1 1650
    bulbs 14 6 20 1680
    Fridges 240 24 1 1200
    Computer 300 2 1 600

    The refrigerator is typical, pretty efficient. The bulbs and flatscreen use a fair amount of power (more than the fridge--which is usually the heavy power users in a small/efficient home other than electric heating/hot water/air conditioning).

    So--if you can reduce their usage/replace these with more efficient versions--it will save you money (both in power costs, and backup power system costs). It is almost always cheaper to "conserve" than it is to generate/buy power.

    Otherwise, the total power (if everything is on) is ~1,796 watts total. So a 2,400 watt inverter should just about power everything (it is not unusual for motors to take 5x starting current--such as a fridge, and you may have another 500 watts or so if there are defrost heaters--automatic defrosting).

    So, several ways of calculating battery bank AH rating. Based on average Watts of power, peak power, and WH per day * days of "no sun" and 50% maximum discharge:
    • 2,400 Watt Inverter * 1/0.85 inverter efficiency * 1/24 volt battery * 8 hour maximum sustained batt discharge = 941 AH @ 24 volts based on average load
    • 6,700 WH per day * 1/0.85 inverter eff * 1 days backup * 50% maximum discharge * 1/24 volt battery bank = 657 AH @ 24 volts 1 day of storage
    • 6,700 WH per day * 1/0.85 inverter eff * 2 days backup * 50% maximum discharge * 1/24 volt battery bank = 1,314 AH @ 24 volts 2 days of storage
    • 6,700 WH per day * 1/0.85 inverter eff * 3 days backup * 50% maximum discharge * 1/24 volt battery bank = 1,971 AH @ 24 volts 3 days of storage
    • (2,400 watts+100 watt surge) * 1/0.85 inverter eff * 1/2.5 max surge flooded cell battery * 24 volts = 417 [email protected] 24 volts based on maximum power from battery bank

    So--Load drive sizing of the battery bank, which then drives the charging system...

    First, what size battery bank are you looking at give the above?

    If you are looking for random emergency power and 1 day is enough... Go with 657 AH @ 24 volts.

    If this is a system that will run multiple times a week and you want 2 days of battery backup, then go with 1,314 AH @ 24 volt battery bank.

    I would suggest that you not go over 1,971 AH--That is already a huge battery bank and you are looking at needing ~100-260 Amps of charging current (5% to 13% of rated battery bank AH capacity).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.