Double Check My Numbers

MichiganOffGrid
MichiganOffGrid Registered Users Posts: 11
First of all, you have an awesome site here. I have been doing a lot of reading and I have learned a lot from you guys. Here is a little background on what I am trying to do. My wife and I currently rent a house. In six months when I get back from Iraq we are going to move out to a piece of land and live off-grid in a 400 square foot cabin. We have fully embraced conservation and are prepared to live without a lot of modern electrical appliances. We plan on heating the cabin with wood. We are going to try to get hot water in the winter from a heat exchanger off the woodstove flue(Fire Flue), and a solar hotwater collector in the summer. We are planning on having a low voltage DC well pump feeding into a 3000 gallon cistern with a booster pump providing pressure into a pressure tank.

I would like to have someone double check the numbers on the electrical items that I would like to have in the cabin. I've never looked at how much energy things use, so I dont know if these numbers are realistic.

AC
Refrigerator- 150 w/h per day. (Chest freezer converted to fridge.)<- Seems Low
Chest Freezer - 150 w/h per day. <- Seems Low
CFL Lights - 3 15 watt CFL lights on six hours a day. 270 w/h per day. (Too High?)
Microwave - 1000 watts on for 15 minutes a day. 150 w/h per day.
Laptop - 90 watts on for 2 hours a day. 180 w/h per day.
Ceiling Fan - 70 watts on for 3 hours a day. 210 w/h per day.
Radio/TV - 90 watts on for 3 hours a day. 270 w/h per day.

So that gives me around 1200 watt hours per day on AC.
I add 25% to compensate for inverter loss, so 1500 watt hours.
Then divide by 24 volts to get amp hours. So 62.5 amp hours per day on the AC side.

DC
Well Pump - 100 watts on for 5 hours a day. 500 watt hours per day.
Booster Pump - 100 watts for 1 hour a day. 100 watt hours per day.
So I have 600 watt hours per day from the DC loads. Divide by 24 volts, and I have 25 amp hours per day from the DC loads.

Does 87.5 amp hours per day sound right for the loads that I think that we will have? To me it sounds low.

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Double Check My Numbers

    welcome to the forum.
    do know that loads are usually underestimated by most people and it is better to overestimate a bit. for the freezers i would want to be sure the draw would be that low, especially during hot summer months where run times can go longer. for some of your items your math is a bit fuzzy on the light side of things like the microwave for 15 min at 1kw. this is 250wh and what happens if you use it more than the allotted 15 min? this is the case for many areas of power use as excursions from what you figured will be encountered. as time goes by many find they need/want even more power. conservation is great, but even the best at conserving here on the forum find their power needs creep upward so some fudge factoring is probably a good idea.
    of course all of this needs stored in batteries and you will likely want at least double the ah capacity that you'd normally use during a day so that you would not go beyond 50% dod as deeper cycling lessens battery life. many also like to have several days of backup battery power and this is good if you can properly charge the batteries.
  • NEOH
    NEOH Solar Expert Posts: 74 ✭✭
    Re: Double Check My Numbers

    I think you really under-estimated these two items ...

    Assuming 500 watts units (adjust per your nameplate value)

    1) 500 Watt Fridge x 5 mins / hr x 24 hours = 1,000 WH vs 150 WH
    2) 500 Watt Freezer x 5 mins / hr x 24 hours = 1,000 WH vs 150 WH
    And I am very low on Run-Time at 5 min / hour.
    Keep the door shut or this could easily double.

    Math error here ...
    3) Microwave 1200W for 15 Minutes = 250 WH vs 150 WH

    Therefore, 3,000 WH AC vs 1200 WH AC
    Add 25% for Inverter = 3,750 WH AC
    Add 600 WH DC = 4,350 WH per day

    4,350 WH / 24 Volts = 181 AH draw at batteries
    You need 4 Times that in Batteries to stay above 25% Discharge.
    181 AH x 4 = 724 AH 24V set of Batteries.
    4 @ 225AH 6V Batts in series and four of those 24V strings in parallel = 900AH.
    Best to slightly oversize for some reserve capacity.
    16 Trojan T105 Batts = $2,500 every 5 years or so.

    Possibly, consider a 48 Volt system to reduce amps, reduce wire size & reduce IR losses.

    What is charging the Batteries?
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Double Check My Numbers

    Personally, I think you are under estimating your loads in some cases. (Over estimating in others!) For example, I would use as a design factor, the ceiling fan 24/7, although you might consider that a typical three speed fan draws ~ 35 watts on low. It seems if it is warm (or cold) enough to need a fan, you might wish it to be on all day, and even all night. I would also suggest that the hours that you use the radio/tv and computer are too light on balance.

    With that said, using your load numbers, it looks like you are going to use ~ 2kwh/day.

    As you may well know, I like to use the 50%/2*4 rule of thumb

    So working backwards, knowing that you will likely use 2kwh/day you might need 1000 watts of PV to start. 1000/2*4=2000 wh.

    Just as an FYI, we live off grid in a house that is about the size that you are looking at. We routinely use ~6-800 Watt/hours/day. We have 400 watts of panel. On an ideal day, we can generate ~1200 wh/day.

    So for comparison, our system 400/2*4=800, so you can see that our system is fairly well balanced. Personally, I would give up the micro wave and the energy use in a heart beat as it is ~ 15% of your total daily use.

    If you consider adding a larger storage tank, you could consider doing bulk pumping either during full sun, or when you might run a generator to do some bulk charging early in the day, reducing your PV needs substantially.

    Good luck and keep in touch.

    Tony
  • MichiganOffGrid
    MichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    Thank you everyone for the recommendations so far. I admit that math is not my strong suit.
    I think you really under-estimated these two items ...

    Assuming 500 watts units (adjust per your nameplate value)

    1) 500 Watt Fridge x 5 mins / hr x 24 hours = 1,000 WH vs 150 WH
    2) 500 Watt Freezer x 5 mins / hr x 24 hours = 1,000 WH vs 150 WH
    And I am very low on Run-Time at 5 min / hour.
    Keep the door shut or this could easily double.

    I plan on converting a chest freezer into a fridge, and keeping them in the basement to keep the ambient temperature low. I got the 150wh estimate from a few different places. This website lists energy usage as .2kWh. This website lists usage as 0.15kWh to 0.18kWh.
    Possibly, consider a 48 Volt system to reduce amps, reduce wire size & reduce IR losses.

    I'm not sure if I should use a 24 volt system or a 48 volt. Originally I was going to go with 48v, but all of the DC water pumps that I have seen have been 12 or 24 volt. I'm not sure how I would wire a 24 volt water pump to a 48 volt system. I was also considering putting the well pump on an independant panel and let it pump only when the sun is out. I'm not sure if this is a better way to go.

    I was thinking about going with this battery: Crown 12-125-13 Industrial Fork Lift Battery 24 volt, 935 AH This will give me 935ah at 24v. I have not figured out how many panels to get yet. I was going to look at that once I was more confident on what kind of energy draw I was looking at. If you have any recommendations I am open to suggestions.
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Re: Double Check My Numbers

    MOG,
    Before you invest in that monster battery, realize it will take monster amounts of charging capacity. Someone will no doubt give you numbers, but you're looking at a lot of PV to charge that size of battery. 5% charge rate x 935ah = 46.75 amps. I have 2.1kw of pv and have never seen that high a charge rate.

    More will follow
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Double Check My Numbers
    ....

    I was thinking about going with this battery: Crown 12-125-13 Industrial Fork Lift Battery 24 volt, 935 AH This will give me 935ah at 24v. I have not figured out how many panels to get yet. I was going to look at that once I was more confident on what kind of energy draw I was looking at. If you have any recommendations I am open to suggestions.

    I'd skip the forklift battery. it's one giant cube, and you need a forklift or 2 to move it.


    Look into using 6V batteries to get 48V @ 400A.

    You charging should be "roughly" 10% of your battery capacity, more if you have lots of "cloud days" to recover from.

    48V, 400A battery , needs 60V @ 40A to properly charge, under best conditions. (single 60A charge controller)

    your 24V forklift @ 935 A, needs 30V @94A to charge. Some fat copper cables there! And 2 charge controllers, even the FM80's are undersized for your need @ 94A

    Either would need a solar array of about 3,200W of panels
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • MichiganOffGrid
    MichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    Ok. I am confused. NEOH recommended 4 @ 225AH 6V Batts in series and four of those 24V strings in parallel = 900AH.

    Thats what I was aiming for with that forktruck battery. I like the idea of lots of extra battery storage. Is it different with the batteries setup in multiple strings instead of in one huge brick?

    If I setup a 48 volt battery system, is there a controller to downstep the voltage to 24 volt for the water pumps?
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Double Check My Numbers

    There are down conversion 48 - 24 v converters, you loose a little in the conversion, but you save that in the copper costs for the main wires.

    But I didn't realize you already had 24VDC gear on hand, I was just thinking it was all 120VAC and would run off an inverter.

    4 strings parallel are tough. 1 or 2 in parallel are tricky enough to keep balanced, but 4 is rougher.

    Check out this articel about wiring batteries on the diagonal
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Double Check My Numbers

    Regarding the batteries...

    If you have the ability to move a one ton or larger battery at your place--Forklift batteries can be a nice solution.

    However, if you have to move them around by hand, then separate batteries may be a better solution.

    Also, forklift batteries tend to have a construction that support heavy discharge and charging currents... They tend to use more water and be less efficient than some of the other types of deep cycle battery (as I understand).

    Also, because the Forklift batteries are so tall, they tend to like 10-13% (C/10, C/8 charging rates to keep the electrolyte well mixed.

    For people that tend to use 5% (C/20) worth of solar panels, forklift batteries may not be a good idea.

    Lots of solar panels are expensive and many people find it difficult to justify enough to make a forklift battery happy.

    Remember too that batteries have performance degradation as they age--So a forklift battery with 5% solar panels may work well for quite a while--but in the second half of the life, they have higher self discharge and the 5% of solar panels begin to have serious issues just keeping the battery bank charged.

    I do like your idea of large cells in series as opposed to a much of smaller cells in series/parallel strings.

    Anyway--that is how I understand the choices--Perhaps somebody here with more experience can add to our knowledge.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MichiganOffGrid
    MichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    I don't have any gear on hand yet. I am still in the planning stages. So I am able to set this system up in whatever configuration is best. Now that I have a decent idea of my load requirements, I will start reading about batteries. You recommend one string of 6V batteries to give me around 400 AH at 48v?
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Double Check My Numbers

    Personally, I would shy away from forklift batteries as your "starter set". The sad truth is that most people have a pretty steep learning curve when it comes to PV and off grid battery use. The other reality is that most people over estimate solar harvest at the same time they are underestimating their loads.

    Closely matching the battery set to the expected loads to shoot for a daily discharge of ~20% is a nice goal. As Bill suggests, large batteries require large charging currents to remain healthy. I have seen many cases of too big batteries and to little charge capacity. The common scenario is to think, "I've got lots of battery, so I don't have to worry about the loads. By some L-16's or even some t-105s of the right total size for your loads. After a few years of really understanding your system, then you can decide what you really would use going forward.

    T
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Double Check My Numbers

    Others have more battery experience than I--But you can look at some of these interesting cells:

    Surrette 12CS-11PS 357 Amp Hour 12 Volt Battery (Surrettes are not cheap batteries)
    Surrette 4CS-17PS Deep Cycle Battery, 4 Volt, 546 Amp-Hour

    Trojan L16RE-B 370 AH Deep Cycle Battery (6 volt)

    Or, you can get some "golf cart" 6 volt @ 225 AH rated deep cycle batteries (per Tony's / Icarus' suggestion), and run two parallel strings... Not perfect but probably 1/2 the cost of other configurations.

    May only last 1/2 as long--But that will give you a chance to wring out your system and learn how to take care of the bank.

    When that first set goes (3-5 years?)--You will have a much better handle on what you will need for the long term.

    You will need a fuse/breaker for each parallel battery string. And I would suggest getting a DC Current Clamp Meter of some sort--It will allow you to check the current sharing between strings under heavy load/charging loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Double Check My Numbers

    Just to add an aside to my previous ideas.

    When we built our new place a few years ago, the conversation was do we go with 4 t-14's or 4 l-16s. We did a estimated calculation such that if the batteries had their expected life spans, it would be far cheaper net/net to by the smaller cheaper batteries, and replaced them more often.

    So I second my own suggestion, buy smaller starter batteries for your first set.

    8 t-105s would yield ~225 ah @ 48 vdc would yield ~ 10kwh of available power to 100% DOD. 25% DOD would give a good yield of ~ 2.2kwh,, just about where you need to be.
    8 t-105s might cost ~$100-150 each for a total of $800-1200.

    If you kill that in 3 years your per year cost would be ~$250/year

    Same 48vdc config with l-16s at ~$250 per would cost ~ $2000. If you killed them in 6 years it would be ~$333 per year. Of course you would have greater capacity (325*48=15.6 kwh) with the l-16s, but you have not demonstrated a need for that capacity, and indeed with so much more capacity, the chances of early death are pretty good due to too low charging currents, unless you are willing to build and buy a big PV system. The guess is which how long will either set last. My money was on the t-105s being cheaper net/net for me on an annual basis.

    Tony
  • MichiganOffGrid
    MichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    I like the idea of using the L-16s with the increased capacity. There has already been talk about getting a small washing machine and running a few loads when the sun is out. We may give up the microwave to help offset the extra load from the washer. We are just starting out on this property, so there is going to be a lot of construction going on. I’m sure tool batteries are going to need to be charged. I like the fudge factor that a slightly larger battery bank gives. Or do you feel that an L-16 battery bank is still too much for me if I add an effcient washing machine to the mix?

    So, if I go with the L-16s, I will have 15.6kwh capacity. If I use 2kwh per day, I will be using about 12% of my capacity per day. That gives me a few days of little to no sun, which is common in Michigan winters. You mentioned in an earlier post that I should be looking at around 1000 watts of panel. And to figure at least 50% bigger for possible expansion. I’m kind of confused on how panels should be wired up with a MPPT controller, and how watt values are calculated. Here is what I am thinking. Tell me if this is right:

    Three 205w 24v panels in series gives me 615w at 72v. Four strings will give me 2460w. Is this a good match for that size of battery bank, or is this too big for what I need? Would three strings be better?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Double Check My Numbers

    imo, 3 strings will probably do for the l16s. this is just under a 10% charge rate and maybe around 77% of that would be roughly counted on to be harvested. 615w x 3 = 1845w. efficiency may be about 77% of that for 1420w. do this for 4hrs a day and you get 5682wh. this is far more than you stated you initially needed, but imo you need more than what you think you do and maybe not enough to justify the large system you are contemplating here. this all would depend on the loads and how long they are used and i think even with more loads the 225ah batteries may suffice better. 225ah x 48v = 10,800wh battery capacity of which half is usable for 5,400wh.

    if you stay with a 24v battery setup then go with 4 l16s for 325ah x 24v = 7800wh of which half is usable for up to 3900wh. you can kick this around as you wish and have the pvs supply around 10% to these types of batteries while being sure the pvs can supply the power that not only is the correct range for the batteries to charge with, but that you cover all of your daily loads with.

    we can only generalize for you as there are many possible ways to go about this, but what your actual loads will be are still uncertain. you may not even know for sure until you actually are in the place using your appliances daily. you could go high with 48v batteries and many pvs as overkill (over fudge factoring?), but if it turns out the overkill is way overkill you might want to kill us as you spent quite a number of extra dollars on it. now you could expand on the pvs if you plan this all right should the loads be more than the pvs are providing. it is unfortunately very difficult for us to plan it for you given the variables and uncertainties involved and i think we got you thinking well enough to get a better feel for what you want to do and get because when you get right down to it, it's your decision. all we do is try to get you in the ballpark of where you want to be.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Double Check My Numbers

    Adding a washing machine to the mix, and what it's net effect on loads is largely matter of how often it is going to be used. (It is also a classic example of how loads grow!) Please also remember that with a washing machine come increased water pumping and water heating loads.

    As an alternative consider a gasoline powered wringer washing machine.

    http://www.lehmans.com/store/Home_Goods___Laundry___Washing___115V_Home_Queen_Wringer_Washer_with_Stainless_Tub___32901100?Args=
    Maytag Washers Available: We sell reconditioned Maytag square tub wringer washers with gasoline or electric engines. We also carry a complete line of Maytag parts. Call for pricing and details. Ask for our Service Department.

    These old style Maytag wringer machines are tanks, they run forever! There are several benefits of a wringer machine. The first is the clothes come out much dryer than with a spin machine. Second and perhaps most importantly is the amount of water you can save.

    When we do laundry, we wash one load of whites, wring them, then, using the same water we wash a load of (cleanish) darks, followed by a load of real grubby work clothes, all in the same water.

    Then we rinse it all in a fresh batch of rinse water, in the same order. If we still have more to do, we can then use that rinse water for the wash of the second batch. So we can get 6 loads of laundry done with 4 tubs of water.

    With Honda power (you can get these from Lehmans without motors of any sort) they are quiet (relatively) reliable, and they will be chugging away when your grand children are doing wash!

    Later on, if you have enough battery/solar, you can easily put a 1/4 hp electric motor on it.

    Tony
  • Peter_V
    Peter_V Solar Expert Posts: 226 ✭✭✭
    Re: Double Check My Numbers
    I like the idea of using the L-16s with the increased capacity. There has already been talk about getting a small washing machine and running a few loads when the sun is out. We may give up the microwave to help offset the extra load from the washer. We are just starting out on this property, so there is going to be a lot of construction going on. I’m sure tool batteries are going to need to be charged. I like the fudge factor that a slightly larger battery bank gives. Or do you feel that an L-16 battery bank is still too much for me if I add an effcient washing machine to the mix?

    Sounds like you're planning on having lots of money available (gov contractor?), but I'll still point out that if you undersize your setup you can always add on and expand it later (easier if you plan for expansion).

    If you oversize it, well all you've done is waste money.

    I'm more familiar with electric vehicles than off-grid setups (I'm grid tied) but it's a pretty common rule that newbies always murder their first set of batteries.

    If it were me, I'd opt for the cheaper batteries so it doesn't hurt as bad (financially) when you kill them early and have to replace them (you can always replace them with a different type).
    I'd probably reccomend T-105s for the first learner set and then switch to Rolles Surrettes (these can last 20 years with proper maintenance)

    Not sure if you've seen the PVwatts website, but I'd highly recommend you play around with it:
    http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

    You have a couple options here, you can use the above website to figure out when you'll produce the least amount of power and size your array to meet your needs then, in which case you'll waste some power during other times of the year, or you can size it to meet your needs most of the time and use a generator to fill in the low spots.

    The generator will be cheaper, but it's more of a hassle since it needs to be fed and cared for, plus it's not as "green" if that's a concern for you.
    Plus if you have hobbies that use lots of electricity (wood shop, etc.) you can always run those during the peak times so you're not wasting energy.

    Plus you can be more conservative during low times. This will probably be during the winter so you'll have the wood burning stove running, so you can use that to cook with instead of using the microwave. Wash clothes by hand during the winter, and use the machine during the summer, etc.
  • MichiganOffGrid
    MichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    Tony, have you been talking to my wife? We were just talking about the washing machine before you posted and she was trying to sell me on the same wringer from Lehmans. I thought that was awfully funny. We are going to dump the microwave. All it gets used for right now is warming up milk for the boys and thawing meat once and a while. Both of these things can be done differently if we plan ahead.

    After talking things over with my wife, I think we are going to go with the T105s. If 8 T105s end up being too small of a battery bank, then it is not a whole lot of money to add another string. I think I am going to go with three strings of 3 205w panels at 24v. If this ends up being too few, then I can add a fourth string pretty easily. Does this sound like a reasonable plan for the loads that I am anticipating? For a charge controller I am thinking I would need a 60 amp MPPT controller and an inverter rated for 1500-2000 watts. I still need to research the brands. Are there any other major components that I am forgetting?

    Thanks for the advise Peter. I think I am thinking along the same lines as you are. I think I am going to try to start out small and put some money away to upgrade a little later when I can see where the system needs help.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Double Check My Numbers

    Mich,

    If it were me, I would buy the used Maytag over the new SpeedQueen all things being equal.

    Lehmans doesn't normally ship the Maytags as they are used and they wish you to inspect them before you buy them since they all have dings on them. I was able to convince them to ship one to me last summer. It turned out I didn't need one as I gound one locally.

    T
  • NEOH
    NEOH Solar Expert Posts: 74 ✭✭
    Re: Double Check My Numbers

    MichiganOffGrid,

    FREEZER question:
    I am still puzzled as to how you can list the Zanussi 260 L Chest Freezer power useage at only 150WH per day when it is rated at 800WH per day? I understand that the Converted Fridge running at the higher temp of 42F will now use alot less power but how does your unconverted Chest Freezer which is still running at 0F now also only use 150WH per day just like the Converted Fridge?

    FRIDGE question:
    The one website stated that their Converted Fridge was in a location (outside garage?) that was only 50F degrees at night and less than 70F degrees during the day and it used between 200WH and 300WH (not 150WH). I suggest that every electric fridge would use alot less power under those colder than normal conditions. And they never opened it for 24 hours. And they didn't add any new warm Wine Bottles, either! Does that make 150WH per day realistic? Are those temperatures (50 at night and 68 during the day) realistic all year round for your basement?

    CAPACITY question:
    Is your basement really going to be 50F degrees at night? Are you planning on putting your batteries in the 50F degree basement? Whould that de-rate your Battery Capacity by as much as 20% ?