I know this has probably been asked already...

St8koutSt8kout Registered Users Posts: 33 ✭✭

I've tried some online solar calculators but they always want to include cloudy days. I'm in Vegas and cloudy days are more rare than an honest politician. On the .00001% chance it gets cloudy, I'll just turn everything off :)

For an extended emergency setup to power a few things, on a basic level do I just match battery bank watt-hours (divided by 2 for 50% discharge), to solar panel watt-hours, (ignoring losses to charge controllers and inverters)?

I'm looking more at 'cost vs reward' to get some idea if it's worth pursuing. Naturally the calculations will need to be fine tuned before actually buying anything.

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Welcome to the forum,

    Usually we figure out the loads, then figure out what battery can handle those loads, then figure out how much solar array we need to handle daytime loads while also charging the battery.

    We really need to know your loads and when they occur.  For example if you only need air conditioning on sunny afternoons, you don't need to size your battery to handle the air conditioning.  I only do laundry on sunny days or when I have the generator running... there's no need to store laundry energy in a battery.  Other folks only pump water when the sun shines.

    Is it your intention to do a 50% discharge on a daily basis? 

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Desert RatDesert Rat Solar Expert Posts: 122 ✭✭✭
    To elaborate on the good advice given by vtMaps, here are a couple of basic formulas for calculating your system size:

    Determine battery bank size given total loads
    Watt-hours total daily load/battery voltage X 1.18 (85% inverter efficiency) X days of storage X 2 (50% maximum battery discharge =battery bank amp-hours
    In your case, just enter 0 for days of storage. You can add a bit to the battery bank size if you want to figure less than 50% discharge.

    Determine array size given battery bank size
    Battery bank amp-hours X volts charging X 1/.77 (panel & charger deratings) X .10 rate of charge = Desired solar array watts

    AS vtMaps says, a conserving lifestyle is a must: Don't do laundry, run power tools, weld, etc. at night when you're not producing any power.
  • Desert RatDesert Rat Solar Expert Posts: 122 ✭✭✭
    Edit:  I mistakenly said to enter 0 for days of storage; that won't work! If days of storage is not a concern, enter 1 or just leave it out of the formula.
  • St8koutSt8kout Registered Users Posts: 33 ✭✭
    edited April 2016 #5

    Determine array size given battery bank size
    Battery bank amp-hours X volts charging X 1/.77 (panel & charger deratings) X .10 rate of charge = Desired solar array watts

    Could you clarify "volts charging." Do you mean the rated voltage of the battery bank or the voltage coming from the charge controller (and since it varies, use bulk charge?)?

    And shouldn't hours of sunshine be factored in?




  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    St8kout said:
    Could you clarify "volts charging." Do you mean the rated voltage of the battery bank or the voltage coming from the charge controller (and since it varies, use bulk charge?)?
    Use bulk charge voltage.  Remember, these formulas are used in rules-of-thumb... they are used to get you close to a workable design.  If your batteries are cold, the charging voltage (with temp compensation) will be higher than your bulk setting.  Also, if you are equalizing. 

    Early in bulk, the charge rate may be greater than 10% because the voltage is below bulk voltage.

    Also, the 1/.77 derating is a wild guess... on clear and cold winter days I often have sustained periods with no derating.  When it's hot and hazy my derating is more like 1/.60.

    The rule of thumb formula you used is for a 10% rate of charge.  Remember, that assumes no loads. 
    St8kout said:
    And shouldn't hours of sunshine be factored in?
    Factored in for what?  You used a formula that tells you how many watts of solar panel will give a 10% charge rate to a battery of an amphour capacity that you can specify.   The formula works for 1 minute of sunshine or 4 hours of sunshine.

    It is a rule-of-thumb that an array that can deliver a 10% charge rate will satisfy many full-time off grid households during most of the year.  Whether that rule-of-thumb will satisfy YOUR household depends on many other factors such as your loads, the time of day of your loads, your climate, and hours of sunshine.

    As for how many hours of sunshine per day...  of course, that is an important consideration when designing a system.  It's important in many ways... for example some batteries take more hours to charge than there are hours of sunshine in a day... a larger array does not make absorb stage go faster.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,706 admin
    Yes, hours of sun also matter. For example, a fixed array in Las Vegas will see "average noon time hours of sun per day" of:
    http://solarelectricityhandbook.com/solar-irradiance.html

    Las Vegas
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 54° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    4.63
     
    5.11
     
    6.32
     
    6.70
     
    6.74
     
    6.63
     
    Jul Aug Sep Oct Nov Dec
    6.27
     
    6.06
     
    6.15
     
    5.85
     
    4.99
     
    4.58
     
    In general, we use 5% as the minimum rate of charge for a flooded cell lead acid battery system (generally emergency/backup, weekend, seasonal usage). And 10% to 13% or so for a full time off grid system (more solar panels makes managing your battery bank state of charge easier). If you have a 10%+ rate of charge solar array, for many folks where there is lots of sun, you will get plenty of power (if you sized your battery bank for your loads).

    So, some quick calculations. A 1,000 Watt*Hour per day system is a good emergency backup system (will run most small stuff around the house--Use 3,300 WH per day system if you want to run a refrigerator):
    • 1,000 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 2 days storage * 1/0.50 maximum discharge = 392 AH @ 12 volt battery bank
    Next, a 5% to 13% array would be:
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller dratings * 0.05 rate of charge = 369 Watt array minimum
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller dratings * 0.10 rate of charge = 738 Watt array nominal
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller dratings * 0.13 rate of charge = 960 Watt array "cost effective" maximum
    Then there is how much power your array can supply based on hours of sun... Sizing the battery bank (above) assumes (more or less) you charge during the day and discharge at night. You can, of course, use power during the day+charge and use battery at night. Tossing the bottom three months (assuming you may use a backup genset during bad weather), for February with (for example) a 10% array, you would get:
    • 738 Watt array * 0.52 off grid AC system eff * 5.11 Hours of Sun = 1,961 Watt*Hours per day
    So--In Las Vegas, you can get quite a bit of power for use during the daytime too. It may be large enough to run an efficient Energy Star refrigerator (with a 1,200 to 1,500 Watt minimum AC inverter). Otherwise, if you are powering smaller loads, a 300 Watt AC inverter would be very nice.

    Lots of choices to make here... You can get a Kill-a-Watt type meter to measure your AC loads and figure out how large of backup solar system you would like.

    A 1 kWH (1,000 WH) system per day is a great backup system (run a genset for larger loads). A 3.3 kWH per day will run a small home/cabin year round just from solar (and some backup genset during winter) with a Refrigerator, washing machine, well pump, lights, TV, computer, etc....

    But solar power systems when strickly used as backup power (for less than 2 weeks per year) are a bit on the "expensive side". Using a Honda eu2000i genset + 10-20 gallons of gasoline (or propane conversion) can be more cost effective (use fuel stabilizer and change out the gasoline once or twice per year).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • St8koutSt8kout Registered Users Posts: 33 ✭✭
    edited April 2016 #8

    "for example some batteries take more hours to charge than there are hours of sunshine in a day... a larger array does not make absorb stage go faster."

    Ok, that's what I was missing. I had looked at some Goal Zero emergency systems and they come with only two 30 watt panels for their $2000 system, then says it takes 160 hours to recharge, lol.

    I'm trying to decide if it's worth trying to go big enough for a fridge (1.07 Kwh, 700 watt start up). Even my gas oven takes a lot of power with those glow plugs (some 600 watts on average). I'd have to unplug the fridge to run the oven.

    When our government gave out all those billions to 'green' companies, no wonder they still failed. Solar's expensive and most of the public is too broke to buy it.

  • BB.BB. Super Moderators, Administrators Posts: 29,706 admin
    edited April 2016 #9
    I am not sure--You may have missed my post just above yours (I posted a little before you).

    In general, running a 120 VAC energy star refrigerator/freezer will push you from a "small" to medium size solar power system (high surge current requires a larger battery bank and AC inverter).

    If you have stable utility power--Off Grid Solar Power is not cheap. Just run utility power and, perhaps, a smaller backup genset + fuel.

    If you have terribly unreliable power (lose power for many weeks/months at a time, or have afternoon brown outs--Then solar+battery bank+AC inverter can make sense.

    And in Nevada, last I heard, they have changed the billing plans for home based GT Solar power (cheapest/most efficient of solar power, can turn your meter backwards), the state PUC + utilities have removed the subsidies for solar power and pretty much have shut down the GT Solar industry in the state.

    You are doing the right thing--Researching first before you start buying hardware.

    No easy answers here... Conservation (if you have an older home/appliances) can help save money (insulation, new Energy Star rated appliances, new A/C or Heat Pump system, etc.) and be a better investment than solar electric power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    St8kout said:
    Even my gas oven takes a lot of power with those glow plugs (some 600 watts on average). I'd have to unplug the fridge to run the oven.
    Do research... there are a number of gas ovens without glow plugs. 
    St8kout said:
    I'm trying to decide if it's worth trying to go big enough for a fridge (1.07 Kwh, 700 watt start up).
    Yes, it's worth it.... if for no other reason than you won't have to breathe or deal with venting the exhaust.  

    Do research... you can have electric fridge / freezer for 0.5 kwh per day and NO start up surge.
    St8kout said:
    I'm looking more at 'cost vs reward' to get some idea if it's worth pursuing. Naturally the calculations will need to be fine tuned before actually buying anything.
    You are focusing on energy production and storage.  Your money would be better spent on conservation.  When you spend first on conservation (ultra efficient fridge, no glow bar oven, as examples), then energy production and storage become much more cost effective.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • PhotowhitPhotowhit Solar Expert Posts: 5,023 ✭✭✭✭

    I live off grid, about as cheaply as possible. I can't match the grid on a cost basis.

    ...but, what living off grid has made me do is be very conservative and aware of my energy choices. Only in that way am I saving money (unless grid energy hits @30 cents a Kwh).

    Listen to Bill and vtMaps! to save money look at where you are using energy. You might also look at the insulation in the home and water heater some large energy users even if powered by gas.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • St8koutSt8kout Registered Users Posts: 33 ✭✭
    edited April 2016 #12

    Thanks for all the responses.

    Yeah, the grid-tie industry was shut down here in Nevada, but I never liked the idea of having some $30,000 system without battery backup, (Tesla's Powerwall may solve that but they are still building their plant).

    A little backstory to that for anyone interested:

    Warren Buffet bought Nevada Electric last year and instantly raised our rates. People now realize that he knew he would get the rate increase even before signing the papers. Some casinos tried to switch to a cheaper utility but Buffet sent an army of lawyers after them threatening to have them penalized to the tune of hundreds of millions if they changed, claiming he owns the power infrastructure and this would upset it, or something like that.

    Then, when all the gird-tie homes started coming online Buffet saw his profits dropping so he demanded everyone else make up the difference with even higher rates. One of our Senators put a stop to that nonsense by changing the law, but it also killed the grid-tie business. So in reality, it was Buffet's greed that created this mess and killed solar for Nevada.

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