one more crazy newbie in Maine, looking for free advice

OK.  I have read many postings on this forum. 
I talked with a local solar guy here in Maine.
I think I am ready to just try my hand at the calculations for my system requirements here in Garland, ME.
This will be a system to run a fairly low-key house with no grid connection, nor other sources (generator, wind turbine, etc).

Assuming a load of 2.5 kWh / day or ~ 208.3 Ah / day (all of this based on 12Vdc).
Inverter efficiency of 85% gives me 208.3 / 0.85 = ~ 245 Ah / day
I like AGM batteries from what I have read for the lack of gassing and width of acceptable voltage on recharges.
So... if I go with decent AGM with 98% efficiency for recharges I get 245 Ah / 0.98 = ~ 250 Ah / day.
For my area, I think in the dead of December, 2.7 hours is all I get of good sun (assuming no boost from piles of flashy white snow).
So... that should be 250 Ah / 2.7 = ~ 92.6 A that I would need in panels and what the controller could handle as input.
If I went with all 12V panels (not sure how much benefit I would get from using 24V and changing down to 12V ??)...
Then I would need around 13-14 KE125 a 7.1 A output each.  (That seems like so many panels).  *sigh*
If you include the 3 days without sun issue... and allowing no greater than 50% discharge on the battery bank...
I would need around 250 Ah / day @ 12Vdc * 3 days / 0.50 = ~ 1500 Ah capacity on the batteries.
This could almost be done with 6 Deka or MK (8A8D AGM).  ~1470 Ah capacity.
Even with the 92.6 Ah panel output, and roughly 5 Ah load during those hours...
That should give me a good rating for the  5-10% ratio of charge current (92.6 Ah - 5 Ah) / 1500 Ah capacity =  0.0584 or 5.84%.

This seems to be what I would need... just seems like a lot, to newbie-me.  Any thoughts?


  • System2
    System2 Posts: 6,290 admin
    Re: one more crazy newbie in Maine, looking for free advice

    P.S. How much worse is it if I use an inverter to go to a nominal 115 Vac?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: one more crazy newbie in Maine, looking for free advice

    welcome aboard and to the realities of photovoltaics. it may be more efficient for you to use a higher voltage system, but the higher voltage pvs still cost big $. you can't get around that sad fact of solar life at this time. do know that anything that can run direct from a battery will run more efficiently than through an inverter, but there are some things that are 115vac that aren't common or cost efficient for dc use. this means a high initial investment to run the dc appliances if available as opposed to readily available 115vac models. of course if these appliances have motors that would mean a sinewave inverter.
    in general your calculations seem ok, but i'll work it from another angle here. you need 2.5kwhrs per day. dividing by 2.7hrs sun per day requires 926watts of pv. let's say the wire losses, controller losses, and charge losses total about 15%(if done well). this brings the pv power needed to 1065w. now with an inverter again there are wire and operational losses and let's say this is another 15%. now you're up to 1225w in pv needed. this shouldn't be the stc rating, but the more realistic ptc rating. this is alot in pvs.
    now in a 12v battery system this would yield a current of about 102amps. upping to a 24v battery system has the wiring handling 1/2 of that or 51amps for the same wattage in pvs. you could go one more step to 48v, but there's less dc appliances available at that voltage and a converter to 12v or 24v would be needed for some things to be dc run and this conversion leads to more inefficiency just as an inverter would lead to more inefficiency.
    if you have good wind or even semi-good winds you could go for a wind generator to help keeping the batteries up. in the winter when there's the least sun is when you'll probably benefit from this the most. it may in fact be better to try and save for more pvs and use a large wind generator(1kw or better) or buy a gas generator to supplement and keep the large battery bank you'd have (compared to the available charge current) topped off until you can assemble the system you need.
  • halfcrazy
    halfcrazy Solar Expert Posts: 720 ✭✭✭
    Re: one more crazy newbie in Maine, looking for free advice

  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Re: one more crazy newbie in Maine, looking for free advice

    Another couple of questions for you:
    • The AGM batteries cost quite a bit me than the price of flooded cell batteries. How are you for maintenance? Will you refill the batteries every few months and clean them and the connections?
    • When sizing your inverter needs, you need to also know the peak demand (a 1.5 kW microwave and a well pump may require a 4+ kW sized inverter. Large loads would be much easier to wire if you use a higher voltage. Also, if you are wiring DC around your home, higher voltages allow the use of smaller gauge wires in your walls.
    • Make sure whoever installs and maintains the system is very familiar with safety requirements. Low voltages and large battery banks are vary dangerous if done incorrectly. Higher voltage AC (via inverter(s) distribution and appliances can actually be much safer and may even be cheaper overall to implement. What kind of loads to you plan to power?
    • The 2.8 kWh/day--is that an absolute minimum value or can you use much less during periods of bad weather. If you use it for lights/TV and such--you can choose to save power. If it is for refrigerator, then you cannot live with solar's variable production--Which points to a gas/diesel generator/more batteries/more panels--Which means more costs, more maintenance.
    • Are you off grid because of location, or by choice? If you can be grid connected, a grid tied solar system (assuming Maine supports Net Metering) is probably the most cost effective/lowest maintenance choice.
    • Compare the added costs of batteries vs the costs of bringing in electrical service (and remember that batteries need to be replaced every ~7-15 years). Poor maintenance, over charging, and/or deep discharges can very quickly destroy a battery bank.
    • If you need backup power for storms (few days to a couple of weeks of outages are not unusual for your area), a small generator/battery-inverter and/or solar system for critical loads may be enough. A 2kW Honda eu2000i would use ~1-2 gallons per day to supply your load--a weeks worth of fuel would only be 2-3 five gallon cans. It can cost you less than $1,000 to buy the Honda eu2000i.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: one more crazy newbie in Maine, looking for free advice

    Thanks all, for such thoughtful responses. And I will be looking you up halfcrazy!
    My max load will not be huge. No microwave, Sunfrost or comparable frig.
    Masonry heater, no moving parts for heat/cool.
    Propane stove, dryer, water heater (may change that after meeting halfcrazy).
    The 2.8 Kh/day could, and likely will be less, often.
    That is more of a weekend figure... when my wife and daughter would be in the house.
    During the week... very little usage. So... in a way the figures are worst case scenario.
    Also... I know myself. I want the lowest maintenance style batteries I can swing.
    I'll wash the dishes by hand, but I don't want to be tied to the house if I want to visit folks down south for a while.