Solar Panel Wattage for Two L16's ..?

Johnboy777
Johnboy777 Registered Users Posts: 19
First ... hi, all.

I have two L16 Trojans (6V/390 AHr--bought new and cheap) and wanted to use them as a simple basic back up for the fridge, TV and computer...my 'Emergency Preparedness System'.

What wattage of solar panels should I be looking at? Also, I want to be able to charge on cloudy days as I live in Northeast Ohio.

And with regard to an inverter - if ,1000 watts will do, is there any advantage to going with a 2,000 or 2,500 watt unit - as perhaps it will work less hard, procuce less heat, future expansion options, etc.

Thank you in advance for the input.

John H.

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    The two L16's would be connected in series for a 12V system, thus the Amp/hr rating remains the same; only the Voltage goes up.
    So you have 390 Amp/hrs @ 12 Volts (nominal) and would be trying for a charging current of approximately 10% of the Amp/hr rating: 39 Amps. In the simplest form: 39 A * 14.2 V (charge Voltage) = 553.8 Watts. Factor in the typical panel efficiency and you get 692.25 Watts of panels - or about 700 (curiously similar to my own set-up :D ).

    For cloudy days you're probably better off using a small generator to be sure your batteries are up, as PV's are expensive and having enough to cover bad weather circumstances leaves you with a surplus of power on sunny days - but a very small bank account.

    Inverter size is dependant on your maximum total loads. It is very important to calculate your potential loads, both in terms of peak and daily Watt hour usage. 12V systems really shouldn't go above that 2 kW mark, as the DC current gets uncomfortably high. You will find that 1 kW is a lot if you can conserve and don't use electric for any heating!

    Of what you mention, refrigerator is probably the biggest over-all user. It will also want pure sine wave. On the bright side I run an electric 'frige off my 700 Watt system - so it is doable.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar Panel Wattage for Two L16's ..?
    Johnboy777 wrote: »
    I have two L16 Trojans (6V/390 AHr--bought new and cheap) and wanted to use them as a simple basic back up for the fridge, TV and computer...my 'Emergency Preparedness System'.

    What wattage of solar panels should I be looking at? Also, I want to be able to charge on cloudy days as I live in Northeast Ohio.
    Since solar PV power (and especially off-grid solar power) is very expensive, it is usually a good idea to measure the power usage of your loads, and to replace any older/energy hog appliances/devices with Energy Star / high efficiency units.

    For small appliances (120 VAC, 15 amp plug-in), a Kill-a-Watt meter is a great way to measure peak wattage and average power used over a day. Do conservation first, then plan/build your solar PV system.
    And with regard to an inverter - if ,1000 watts will do, is there any advantage to going with a 2,000 or 2,500 watt unit - as perhaps it will work less hard, produce less heat, future expansion options, etc.

    For the inverter, you don't want to use too large of one. Larger inverters are expensive (especially if you use a True Sine Wave, TSW, Inverter). And large inverters also can have a lot of "standby power" (24 hours * standby power can equal a substantial load on your battery bank/solar panels).

    Regarding cloudy weather--Sometimes you will see ads that talk about a company's solar panel being better for use in cloudy weather/etc... In the end, most common commercially available solar panels are all very similar in performance. Any shading, shadows from pipes/trees/wires, or clouds will cut down output--potentially by a whole bunch.

    For example, my 3.5 kW grid tied system will generate between roughly 10-20 kWH per day in sunny weather (depending on season). For light cloudy weather, my system will generate maybe 1/2 of the power for a clear day... With stormy weather/heavy dark clouds, 1/10 or even less.

    Normally, people just plan for cloudy weather by cutting loads and/or using a backup genset. Depending on your personal situation, that may be a gasoline powered small genset, natural gas, propane, or even diesel. In general, for a system your size, you are best off using a very small generator (1-2 kW peak) for fuel efficiency--specially if you have to run on stored fuel vs natural gas.

    OK, playing with some numbers. Assume you have a good Energy Star Refrigerator that runs on 1.2 kWhrs per day. Usually the recommended inverter would be around 1,500 watts (and run a few lights plus a small TV/laptop computer). Say your TV runs on 35 watts for 4 hours per day:

    Power requirement:
    • 35 watts * 4 hours per day = 140 Watt*Hours per day
    • 4 CFLs * 13 watts * 6 hours = 312 WH per day
    • Refrigerator = 1.2 kWH per day = 1,200 WH per day
    • Loads = 1,652 WH per day
    Battery requirements... A good starting rule of thumb for a battery bank is 3 days of no sun (carry through a couple days of no sun) and 50% maximum discharge (for long life). 1 day of no-sun would probably be minimum battery requirement. For a 12 volt battery bank:
    • 1 day * 1/0.50 max discharge * 1,652 kWH per day * 1/12 battery bank = 275 Amp*Hour @ 12 volts minimum
    • 3 day * 1/0.50 max discharge * 1,652 kWH per day * 1/12 battery bank = 826 Amp*Hour @ 12 volts recommended maximum
    Solar panel requirements... Solar power is very location specific--You should have south facing exposure with NO Shading (at least between 9am and 3pm). Using a program such as PV Watts which uses 20 year average solar radiation for predicting typical energy collection (Akron; 1kW of panel, even number; 0.52 system derating/efficiency; fixed array):
    "Station Identification"
    "City:","Akron"
    "State:","Ohio"
    "Lat (deg N):", 40.92
    "Long (deg W):", 81.43
    "Elev (m): ", 377
    "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:"," 40.9"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.5 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 2.40, 39, 3.31
    2, 3.38, 49, 4.17
    3, 4.21, 66, 5.61
    4, 4.76, 70, 5.95
    5, 5.33, 78, 6.63
    6, 5.41, 74, 6.29
    7, 5.53, 78, 6.63
    8, 5.40, 76, 6.46
    9, 5.15, 72, 6.12
    10, 4.08, 62, 5.27
    11, 2.48, 36, 3.06
    12, 2.03, 31, 2.64
    "Year", 4.18, 730, 62.05

    Now, typically, we plan for 100% of power to come from solar panels 9 months of the year and use the generator to carry through during bad weather.

    So, pick February for 49 kWHrs per month or:
    • 49 kWH per month / 30 days = 1.633 kWH per day or 1,633 WH per day
    To power your 1,652 WH per day load:
    • 1,652 WH * 1/1,633 WH per day per 1,000 watts of panels = 1,011 Watts of solar panels
    If you had a 862 AH 12 volt battery bank, then the recommended solar array to power your loads based on the above assumptions would be ~1,000 to 2,000 watts of solar panels.

    For your smaller 390 AH battery bank, the 1,000 watt array is probably the rough maximum size of array practical for your battery bank (you could go larger, but your battery bank would have a difficult time absorbing all of the current on a sunny day).

    For the AC backup battery charger, another rule of thumb for charging is 5% to 13% of the battery's 20 Hour rated capacity:
    • 390 AH * 0.05 = 19.5 amps minimum
    • 390 AH * 0.13 = 50.7 amps maximum
    A 20 amp 12 volt battery charger would require a ~1,000 watt genset, and 40 amps would require a ~2kW genset.
    • 20 amps * 14.5 volts charging * 1/0.80 charger efficiency = 363 watts
    • 40 amps * 14.5 volts charging * 1/0.80 charger efficiency = 725 watts
    Gensets typically run must efficient if they have 50% minimum electrical load... Putting a "small battery charger" on a large/high quality 1,800 RPM genset is typically a huge waste of fuel.

    So, I will recommend the Honda eu1000i or eu2000i as a backup for your system (if you will be using gasoline--with a fuel stabilizer, fuel changed once per year). Both are very quiet and fuel efficient at lower loads (even down to 25% or less electrical loading).

    For a battery charger, Xantrex makes both a 20 and 40 amp 12 volt version (20 amp model here). The charger is programmable, has Power Factor Correction (PFC--great for small gensets) and a remote battery temperature sensor. Iota also make high quality battery chargers too.

    Anyway, I will stop here--Lots to think about...

    In the end, 1. conserve, 2. know your loads, 3. Make requirements for power, 4 plan the system...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Johnboy777
    Johnboy777 Registered Users Posts: 19
    Re: Solar Panel Wattage for Two L16's ..?

    Good Grief!

    I am going to need a 700W solar array just to run a fridge, computer and TV?

    Let the food spoil - looks like I'll switch to freeze dried during an emergency.

    How about just a TV, computer, charge a few cell phones and a few 12V lights?

    Thank you for the reply, Cariboocoot, very helpful.

    John H.

    EDIT: Thanks BB - looks like we posted close to the same time.

    .
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    If this is just for back-up power when the grid goes down, the cheaper solution is to just buy a small gen for those occasions. If you want silent power, skip the PV's and use batteries recharged by gen/grid to run the inverter. How long does your power typically go out for? It makes a big difference when weighing up the costs.
    My set up, which is 24V and has a 3.5 kW inverter (to power water system, etc), cost nearly $10,000. But there are no power lines near, and running a gen 24/7 for 8 months of the year so to speak is impractical.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar Panel Wattage for Two L16's ..?

    Yea, solar is a very expensive backup power solution...

    I kept looking at off-grid or hybrid (grid tied + off grid) for my home--And the only thing that made sense was lots of conservation, a small grid tied system, and a generator+20 gallons of fuel for emergency power.

    In our area, we just don't get long power outages (at least so far). I have yet to start my generator(s) in the last 15 years (blackouts are 1-2 hours, once every year or two).

    In your area, ice storms and such can cut your power for a week or more (as I understand).

    Choosing a fuel that your home already uses (natural gas, propane) and a small quiet genset is usually a more cost effective solution.

    For me, I chose gasoline because my "stuff hits the fan" will be an earthquake--and I would not count on natural gas being available.

    Sizing your loads / genset / fuel storage is important.... Many people get a very nice 6-10 kW genset and find that they use 1/2-1 gallon per hour just to run a fridge and a few lights.

    The Honda eu2000i will run about 15 hours on 1.1 gallons of fuel while providing 400 watts of power... For me, that is 1-2 gallons per day of fuel usage and not too much of a pain to store/refresh once per year.

    A 10kW natural gas home emergency power genset, for me, would cost a minimum of ~$1.00 per hour for natural gas (with minimum/no load)... Just not worth it for me.

    A 1,600 watt genset provides enough peak power for my needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    It is a pretty simple equation. Do your load calcs and see what you really need.

    A simple, back of the napkin formulae for PV is this. Take the name plate rating of the PV, divide by 2 to account for all system loses, then multiply that number by 4 which represents the number of hours of good sunshine (average, as few people average more).

    So a 100 watt panel would look like this: 100/2*4=200wh/day

    Just for example, that is enough to run a 10 watt cfl for 20 hours, or a 100 watt tv for 2 hours or a 50 watt lap top for 4 hours etc.

    Your 2 L-16's will give you about 2 kwh of power if drawn to 50%. If you drew the battery down to that 50% the said same 100 watt panel would likely put ~ 6 amps into the battery, so 50% of the L-16 would take~ 33 hours to recharge. (In reality, way more).

    The bottom line, is your batteries might make a nice back up, but as has been suggested before, a honda Eu 1000 generator coupled to a 20 amp Iota or Xantrex charger would probably be a much cheaper way to use the batteries. (Using the generator directly would be the cheapest.)

    Tony
  • Johnboy777
    Johnboy777 Registered Users Posts: 19
    Re: Solar Panel Wattage for Two L16's ..?

    Thanks for all of the great information - I can see that I need to rethink what exactly it is that I am after.

    This all started for me when I started to look into Emergency Preparedness.

    I have no desire to go off grid - per se - but I want to be prepared if disaster strikes. I started with a 55 Gal. drum of drinking water that has a 5-year shelf life, and a wood/coal burning stove in the basement - I normally use natural gas.

    My goal is six weeks survival completely off-grid, if all infrastructure grinds to a halt - even in the middle of winter.

    Thanks again - I know I've got a long way to go before I get a decent dent in my plan.

    John H.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar Panel Wattage for Two L16's ..?

    I like to prioritize something like this:
    • 3 minutes without air--you are dead
    • 3 hours without shelter (ice/desert)--problems
    • 3 days without water
    • 3 weeks without food
    I plan on it being like camping... Anything else (heat, tv, etc.) is a luxury.

    And plan on common items with multiple uses... AA batteries for flashlights (LED), ratio, walkie talkie, water sterilizer, etc... Fuel--gasoline for genset and mutli-fuel camp stove. Propane is great if you have a large tank already.

    In the end, look at the minimum of what power you need (medications that need refrigeration?). In the end, water/food/fuel takes a lot of space for storage... If you have a well or ground water--then you may need to filter/sterilize (probably the biggest issue for sanitation--clean/safe water for your 6 week period). Etc...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Johnboy777
    Johnboy777 Registered Users Posts: 19
    Re: Solar Panel Wattage for Two L16's ..?

    Here in NE Ohio, we saw 0-deg. or below last winter for a week or two. Camping is out of the question, so I bought a back up wood/coal burner for the basement, and installed a few of those old fashioned brass floor grates/registers, so the basement heat would rise. I plan to get 1/2 ton of coal (stored in the garage) and some wood for it.

    I have 55 Gal. of water stored now, and I need to purchase about 25 10# cans of Mountain House Freeze Dried Food.

    If you're off-grid, completely, then information/communication is critical - so TV, laptops, cell phones and the like become critical vs. a luxury, IMHO. I have two walkie-talkies, too.

    I think the L-16's I have will work, I need to figure out the simplest way to add solar panels for the most basic of electrical needs during the 6-week period.

    I have my Go-Bag set up already as well - it's a large 6,500 ci dry bag/back pack...HERE: http://www.nrsweb.com/shop/product.asp?pfid=2900&utm_campaign=shop_comp&utm_source=froogle&utm_medium=cpc&utm_term=fr_2900

    Thanks, again for all the information - much appreciated.

    John.

    .
  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    If you decide to use battery power (however you charge it) I'll echo a warning made above about peak loads, with my own example. Be SURE whatever inverter you pick can actually handle the fridge starting! My own fridge - relatively new, bought it about 6 years ago - only draws about 110W while running. Some very tiny inverters can handle that.

    But when it *starts* there is an astounding 1100W surge current that lasts for several seconds! This is actually pretty typical for things like fridges. Making it worse, many inverters list their "surge" rating as double the normal rating, but in the fine print say it's only for 1/2 cycle or some other ridiculously small period of time. Nothing more annoying than buying something that *looks* like it'll work, only to find it will not in fact do the job!

    I wound up with a 1500W Samlex just to be sure it would start, even though my normal loads with fridge running would only be down around 200-300W. (That was my "first revision" system - running at 12V. I've since upgraded.)


    As for emergency preparedness, while a generator makes the most economic sense, my problem with that was I didn't want to have to leave it running. Overnight or when I'm away at work there may be things (like the fridge) I'd like to keep running but don't want to worry about the generator.

    The "large UPS" idea is a good one - install a subpanel for a few circuits, feed them via an inverter with built-in transfer switch, and anytime the power drops you're running on battery. If the power is out for a long time, recharge with the generator. The bonus here is your generator will stay loaded up, meaning it's running more efficiently than if you have just a couple lights on it.

    But in the end I did wind up with (at this point) 1080W solar panels, 220AH @ 48V battery bank (8 Trojan T-105s), and a full Outback system. Primarily because I find it a fun hobby to play with, in addition to being quite useful! I do also have a Honda eu2000i - excellent machine, very low fuel usage - but have so far never needed it. I just (try to) pull it out once a month and give it a test-run.

    Haven't had any major outages since I installed the system, but have had a few short ones. The "critical" stuff never even knew the power went out. I wouldn't have known, except some ceiling lights went out. Sweet! :) Especially when it's 4AM, and I'm trying to get ready for work - doubt the neighbors would have appreciated me firing up a generator at that time of morning!

    In the meantime, I switch my loads over to inverter each day and let the sun run them. I'll never "pay for" the system that way but it's neat to see in operation and does reduce the utility bill by 3-5kWh/day. (I once calculated the payback, just for fun - 66 years if *nothing* had to be replaced. Doubt I'm going to get that lucky! :p )
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    You might look at it this way.

    6 weeks of no grid: Use the l/16's for inverter lights and perhaps a radio/or laptop after hours. Get a smallish PV panel (~100 watts) to keep the battery on float between now and any prolonged outage. Use a Honda eu 1000 to top up the battery daily, as well as power/recharge other loads, through a Iota or Xantrex 120vac charger.

    4 hours of fuel in a Honda Eu 1000 would be ~ 1/4 of a gallon, .25*42=10.5 gallons of gas. If you had a energy star fridge that was WELL insulated you could probably run it for ~ 6 hours/day (2 on 6 off) and keep it cold as long as it was full and opened rarely. Probably need a Honda Eu 2000, and you might burn a bit more fuel net/net, but even at double that it would only be ~20 gallons. Not too much to store.

    Alternatively, get a LP fridge out of a wrecked RV, buy a 100# propane tank and regulator (20 gallons) and the fridge would run for several months. (You could also have hot water and cooking.

    There are tons of ways to work your scenario. My biggest question is what is it one is preparing for an why. If we think the end of the world as we know it is coming then that would lead to one set of solutions. If we are worried that our utility is going to be down for a while, that of course is a different set of solutions.

    Tony

    PS. Personally I can't spend a lot of time worrying about end of the world as we know it scenarios. I figure if things go that way, nothing much will matter anyway.
  • Johnboy777
    Johnboy777 Registered Users Posts: 19
    Re: Solar Panel Wattage for Two L16's ..?
    icarus wrote: »
    PS. Personally I can't spend a lot of time worrying about end of the world as we know it scenarios. I figure if things go that way, nothing much will matter anyway.

    Tony, I respectfully disagree - it always matters.

    Six weeks may be an arbitrary number on my part - but I think our infrastructure is a lot more fragile than what we may think. If something happens, it may take weeks to get help (Katrina) - better to be self sufficient, or at least as self-sufficient as possible.

    I like to plan for the worst case scenario, but I expect (hope) it’ll be unneeded

    John
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar Panel Wattage for Two L16's ..?

    If you decided that Inverter+Battery+Generator backup is for you--Then I would suggest you look at the Xantrex/Schieder XW system...

    It is a Hybrid Inverter (Grid Tied Inverter, Off Grid Inverter, and AC battery charger) all in one unit. Put a battery bank (600 amp*hour @ 48 volts minimum if you plan on using near 6,000 watts from the inverter) and you have a good size "home UPS" system.

    Add solar panels and MPPT Solar Charge controllers immediately, or later on as funds and circumstances permit.

    A Hybrid system is darn near the best solution from a cost/benefit point of few... Generally, much better than building a parallel off-grid only system + solar panels (assuming your utility allows Grid Tied solar + 1 year net metering).

    The XW Hybrid inverter (if you are looking for a 4-6kW sized system) is not a bad price.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?
    Johnboy777 wrote: »
    Tony, I respectfully disagree - it always matters.

    Six weeks may be an arbitrary number on my part - but I think our infrastructure is a lot more fragile than what we may think. If something happens, it may take weeks to get help (Katrina) - better to be self sufficient, or at least as self-sufficient as possible.

    I like to plan for the worst case scenario, but I expect (hope) it’ll be unneeded

    John

    Katrina, yes, and six weeks might be the right time horizon. Armageddon with the end of the political and social infrastructure we all depend on, all bets are off. That said, let's not argue about what ifs, we're here to address your issue(s)

    T
  • colderthan
    colderthan Registered Users Posts: 20
    Re: Solar Panel Wattage for Two L16's ..?

    John, as always you've gotten very good advice on this forum(these guys are awsome!)
    I think you are very wise planning for the worst and hoping for the best.
    On the electrical I can't add much, but on the freeze dried food(very expensive for something you will probably never use) why not just store more of what you normally use? Buy on sale and rotate your stock. Saves you money by not having to buy when prices are high and the insurance is free. If you want something that stores well just add a 25# bag of rice and/or beans.

    Good luck
    Paul
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    Tony,
    If we reach the end of the world scenario, you'll just be wondering why there's so little activity on the NAWS forum for a day or so :D.

    How dry is the North now? We get excited when the forecast calls for clouds down here, ready for rain dancing. Enough rain to have to run the generator would be nice (3 days?)

    Ralph
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Solar Panel Wattage for Two L16's ..?

    Ralph,

    Very dry, with what is shaping up to be record fire season. We haven't had any real rain/snow since mid February! Pretty scary,

    T