Sizing the system...

SystemSystem Posts: 2,511 admin
Good Afternoon.

My wife and I will be beginning an owner/builder Straw Bale project within the next year or two. We are doing all of our research now to determine total costs.

I found a website that shows 'typical' useage of various electric appliances and based on that I calculated that we consume 19 KwH of electricity per day (worst case) while realistically we would (on a typical day) use much less. If I remember my formulae correctly, this comes out to 160 amp hours (with rounding) and I believe that it is the amp hours I have to size my system to. (Correct?)

I'm planning on building an off-grid solar system and acquiring as much as possible via wholesale and other cost saving routes. We plan to use Solar Water heating, radiant floor heating for our home, Propane gas for our fireplace, cooking, and hot water backup.

My basic questions are as follows:
From reading posts and such the 19KwH sounds like it is quite high for just my wife and I. Did I screw up my calculations? I computed out an estimate of how many hours per day each appliance would run and compute that out to total watt hours per day.

Now that I have an amp hours number that defines the batteries and also the solar panel requirements. Correct?

So I compute the amp hours per day * 4 for the battery size (4 days of sunless support). Then multiply that amp hour size by 4 for the solar grid. Correct?

Any further help in these computations would be appreciated.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Sizing the system...

    19kWhrs per day is pretty high--Your best bet is to start by looking at your current utility bill (last year or two)... If you have electric heat/hot water and/or A/C, that would really push up the usage...

    Mine, for a family of four, work/educate at home, is less than 1/2 that in an old stick frame home (upgraded with insulation, energy star appliances, no A/C, natural gas for all the normal stuff).

    If you are off grid, you should be aiming for probably closer to 3-6kWhrs per day instead.

    Amp*Hours is probably not the correct unit for your caluclations... Amp*Hours is simple Watt*Hours/voltage--typically used for DC installations and sizing batteries batteries.

    For example, a 19kWhr system with 4 days of storage at 48volts (and not discharging the batteries below 50%):

    19,000 Whrs * 4 days * 1/50% maxdischarge * 1/48vdc bank = 3,167 Amp*Hour battery bank... (about the equivalent of 124 car batteries)...

    For a grid tie system, you would just stay in kWhrs--and add up all your usage for 1 year and offset that against how much a solar gird tied system would generate in your location... For example, NY, NY:

    19kWhr*365days/year * 1kW of panels / 1,218 kWhrs per kW of solar panels = 5.7kW of solar panels

    That would be, very roughly a $50,000 system installed (retail).

    In the end, solar power is not cheap, and conservation and alternative fuels when possible (natural gas, propane) will reduce the costs dramatically (spending on conservation is typically cheaper than spending it on a larger solar system).

    For the smaller appliances, a kill-a-watt meter is a good item to have.

    For larger hardwired appliances, checking your utility meter (or buying an old one you can wire in for the larger appliances) is useful too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Sizing the system...

    As usual, Bill says it perfectly.

    Conservation is your very best solar dollar far and away. For most PV systems consider eliminating all heating loads. Consider a very high efficiency fridge or consider propane fridges (my choice). Use gas ranges with standing pilots rather than hot surface ignition. Read all you can about conservation and implement all you can and it will make any system cheaper.

    If you have the option of going grid tie, consider doing so. It is much cheaper long term to grid tie rather than using batteries. The up front cost of the grid tie is likely to be expensive, but the ability to sell back your excess capacity, plus the life cycle cost of a large battery bank is large.

    We live in a small house and use about 30 amp hours (12vdc) per day, but we are very frugal.

    Good luck,

    Icarus
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Sizing the system...

    Just on a rough average for the US, 19kwh would come out needing around 5kw of panels, and that can vary a lot.

    "..'typical' useage of various electric appliances .."

    You can probably get by with much less, I question the numbers from most of those charts, as many have not been updated for 30 years.
  • System2System2 Posts: 6,290 admin
    Re: Sizing the system...

    Thanks for the responses.

    So how do I compute my house load when I do not live in the house yet? My current electric bills put us around an average of 1500 per month -- but that is with an all electric home with an electric water heater, electric forced air heater, heat pump, and electric stove. All of those will be replaced by propane gas appliances and radiant heat pumps with solar water heaters.

    Since the two envrironment are drastically different and I've never lived in an the environment I'm building, I'm stuck as to how I get a good picture of what I think my electric load will be.

    As to on-grid vs. off-grid, our hill is in near the Coconino Forest and we have to go off-road for two miles to reach our 'mountain' drive. I also have about a 200 degree clear view to the south so my generation potential is high...especially with a passive tracker. But I'd like to try and get a good computation of my energy needs before I decide to throw in the towel on off-grid. Especially since any excess power I give APS at yearend becomes a "grant" for which I get nothing.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Sizing the system...

    purchase a meter like the kill-a-watt and determine your rough power usage per month on each of those items you listed. you might try a week or even 2 of measurements to shorten the time needed in each determination and just multiply it out for the needed timeperiod usually 1 month, but some figure yearly). for 2 weeks multiply roughly by 2.14 and if it's a one week measurement then multiply by 4.28 base on 30 days per month. yearly figures adjusted accordingly too. add the calculated or measured draw of all of these items and add them together. these will be deducted from your bill and you should have a good working estimate of what you might need in the other place.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Sizing the system...

    Home power use is pretty personal and location dependent... Two families in the same home can have dramatically different usage...

    In my home, the previous family of 5 paid something like $200 per month for electricity (just the one bill I saw, no A/C). My highest bill for a family of 4 would be around $25 per month (pretty much same major energy efficient appliances, home, etc.).

    For my family, I would plan around 200-250 kWhrs per month (gas appliances, CFL lighting). I have central heat, no A/C, gas stove and drier with glow bars, Fridge and Freezer, microwave, toaster/convection oven, etc. If I try real hard, I can get down to around 175 kWhrs per month... Don't use many "motors" (fans, compressors, shop tools, etc.--ceiling fans for occasional hot weather use).

    Moving from a smaller house (only a few miles away) with less insulation, older appliances (including an electric drier but no central heat, older fridge, no freezer, gas stove/water heater/space heater) and somewhat less attention paid to conserving power--probably 200-300kWhrs per month (about 50 kWhrs per month more).

    So--power usage is as much a state of mind as well as a "state of appliances".

    If you are doing this to "save money"--take a close look at your electric power costs... Mine, around $0.12/kWhr for 300 kWhrs/month or less. Around $0.35 per kWhr for more than 1,000 kWhrs per month.

    I thing your area is around $0.09 per kWhrs (I don't know if you have tiered pricing or not). And, you don't have lines already at your building site--It may cost you an arm and a leg to run the lines.

    You can figure out some basic costs per kWhr if figureout how many kWhrs you will need for 25 years and you divide by the costs; assuming your solar panels will last 25 years, your inverter 12.5 years, your batteries 5-10 years (cheap-expensive batteries), add in your power line costs (assume 25 year "life"--free after). You can also add costs for generator fuel and other power systems (wind, etc.).

    It will only take you 20 minutes to do the assumptions and calculations to see what makes sense or is way off the reasonableness scale.

    Lastly, look at fixed (or fixed with 2-4 tilt adjustments per year--track sun) vs a real one or two axis tracking system... Generally, solar panels are getting cheap enough now that tracking systems are not always worth the price (just buy more panels)... Also, tracking system generally require a fair amount of maintenance and replacement parts to keep them working over the years (most of us here would recommend staying away from a tracker and just build tilting frames you adjust a couple times per year--must be stable to withstand wind/snow loads. Tilting up near vertical some feet off the ground makes the panels pretty much self clearing when it snows--plus you get the sunlight reflection off of the snow).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TelcoTelco Solar Expert Posts: 201 ✭✭✭✭✭
    Re: Sizing the system...

    How far along are you? If you haven't begun yet, I'd like you to consider the following links.

    dry stack concrete block building Use to build a house or a water storage tank

    [url=http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/collectors.htm[/url]solar shed[/url] Catch the sun, store it out back till ya need it.

    hydronic clothed dryer. Dry your clothes with hot water

    hydronic forced air heating system Heat the house with hot water

    The above links will show you a stronger house building method which won't cost much more than a straw house (and should eliminate the need for a tornado shelter) and several ways to take heating loads off both electricity and fossil fuel sources.
  • System2System2 Posts: 6,290 admin
    Re: Sizing the system...
    Telco wrote: »

    dry stack concrete block building Use to build a house or a water storage tank
    [\QUOTE]
    I am set on Straw Bale post and beam construction. It consumes very little energy while concreate consumes a lot to be made, and the finished home is beautiful.
    Telco wrote: »
    [url=http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/collectors.htm[/url]solar shed[/url] Catch the sun, store it out back till ya need it.
    [\QUOTE]
    I'll check it out.
    Telco wrote: »
    hydronic clothed dryer. Dry your clothes with hot water
    [\QUOTE]

    Is this any better than propane for clothes drying?
    Telco wrote: »

    hydronic forced air heating system Heat the house with hot water
    [\QUOTE]

    Radiant heat goes hand-in-hand with a straw bale design and I don't need anything but a small pump to move the water. And....I will be heating with hot water.

    The house is pretty strong and I do not read much about tornadoes happening in Arizona.
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Sizing the system...
    Telco wrote: »
    How far along are you? If you haven't begun yet, I'd like you to consider the following links.

    These are sure handy, thanks
    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 ,

  • TelcoTelco Solar Expert Posts: 201 ✭✭✭✭✭
    Re: Sizing the system...

    Sorry, probably should have put in that I've done none of my suggestions, but the research I've done has shown that this is the way I'm going. Lots of tornadoes in Oklahoma, so I need something a bit stronger than hay since the Big Bad Wolf DOES hang out in my neck of the woods! That town in Oklahoma that was completely flattened is about an hour's drive from where I'm at now.

    I'm still trying to track down a suitable piece of land that won't require a 1hr commute, which is getting harder and harder to do every day. Ticks me off that just 10 years ago I could have bought 20 acres 5 minutes from work for 20 grand, and now I'm looking at 5 acres 45 minutes from work for 20+ grand. If only I'd known! :cry: Course, I really didn't expect to be staying in Oklahoma then.

    I definitely understand about being set on a method, to each their own. The SBC building method can also be useful for building a water storage tank for the hot water your radiant heating system uses. If it can hold a million gallons it can surely hold a couple thousand. There is a forum here dedicated to SBC building, but it's not very active so it may take a few days to get a response.

    As an example, 2000 gallons takes up 267 cubic ft of space, so a 300 cubic foot tank to allow for expansion would be ideal. This would be a box 5 foot tall by 6 foot wide by 10 foot long, or thereabouts. Using blocks 12in x 8 in x 16 in, you could expect to need 208 blocks, or about 625 dollars guesstimating high at 3 bucks a block. You would also need waterproof surface bonding cement rated to withstand 200 degrees continuous water temps to hold them together, and probably a sealer. This would give you an actual tank size of 378 cu ft, or the ability to hold 2600 gallons. Not sure what sort of bracing, if any, would be needed on the sidewalls to support the weight. Insulate to R40 all the way around, and you're good to go. Not bad considering that some folks are spending 500 to 1000 dollars for a used 1000 gallon propane tank, then are having to spend extra dough to get the mercaptan scrubbed out of it, then have to weld on the tank to put the fittings in, then build a tank base strong enough to hold the tank and water's weight, insulate, ect, and then they only have a 1000 gallon capacity. If you only wanted 1000 gallons, just make the tank 5 ft long instead of 10, and cut the block cost to just over 300 bucks.

    On the clothes dryer, this is something I'm not sure on. I'm planning on picking up a cheap used gas dryer sometime in the near to middle future to play with. Going with a gas dryer because they can be had in 110V. All I'd need then is a few junk heater cores, some copper tubing, a little wood, paint and acrylic to see if a solar powered dryer set up on the back porch is able to dry clothes, and how long it takes per load. Got a few other projects on the front burner right now though, but since they are selling these things I can't see why it wouldn't work. As to whether it would be better, dry clothes are dry clothes, and I'd rather be able to dry with sun power than something that has to be burned.
  • catkincatkin Registered Users Posts: 19
    Re: Sizing the system...
    I found a website that shows 'typical' useage of various electric appliances and based on that I calculated that we consume 19 KwH of electricity per day (worst case) while realistically we would (on a typical day) use much less.

    [snip]

    From reading posts and such the 19KwH sounds like it is quite high for just my wife and I. Did I screw up my calculations?
    One possible calculation error is that the website may have been listing typical peak power usage and that is different from typical average power usage. The first is useful for sizing the inverter; the second, in conjunction with hours-per-day usage, for sizing PV array and battery bank.

    For example a desktop computer may peak around 300 W but average consumption (depending on usage) may be less than 50 W. Washing machines, even with the heater disconnected, peak high (perhaps 2 kW) but average only 250 W per hour and that maybe only for an hour a day.
  • System2System2 Posts: 6,290 admin
    Re: Sizing the system...
    Telco wrote: »
    Lots of tornadoes in Oklahoma, so I need something a bit stronger than hay since the Big Bad Wolf DOES hang out in my neck of the woods! That town in Oklahoma that was completely flattened is about an hour's drive from where I'm at now.

    Well...I believe that Kansas is rated as a major part of Tornado Alley. Straw Bale homes built in the 19th century without foundation, pinning, and load bearing framing are still standing today. The cost is about 20% higher than a stick-built home, but the overall efficiency and strength of the home is outstanding.

    The straw bales are not hay. Hay would draw bug and vermin. Straw is highly compressed and then the bales are further compressed during construction. The bales wrap around the wall framing which is tied down to the foundation in the same fashion as frame houses. They are then covered with 3 inches of stucco plaster on each side of the wall.

    The end result: you have a home that is very strong and has stood up to very high winds. In the 2004 fire rating test, they built a straw bale wall, then exposed it to 30 minutes of 240 degree fire. The opposite side of the wall had an array of temperature sensors -- and those sensors never went up above 100 degrees. This is the lowest temperature pass-through in the history of the fire testing labs...of all wall types tested.

    Also when you consider that concrete requires something like 4 times the concretes weight in carbon dioxide to make the stuff, you are really creating an environmental nightmare.

    YOU should research Straw Bale home construction.
  • System2System2 Posts: 6,290 admin
    Re: Sizing the system...

    Last night I went through my utility bills and took the energy usage for July. This is about the only month in the entire year that we don't have heating or air conditioning running here in Ohio. This means that my utility bill would reflect close to what we might spend in our new home.

    The end result? 500 KwH per day. This does not reflect all of the energy savings we instituted over the past few months (flourescent lights, unplugging devices) and it also includes our pond pump, aquarium pumps, electric dryer and range that won't exist in our new home. I figure we may actually move closer to 300 KwH per day.

    So for this size of a system, what is my next step in sizing an off-grid system?
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Sizing the system...
    The end result? 500 KwH per day. This does not reflect all of the energy savings we instituted over the past few months (flourescent lights, unplugging devices) and it also includes our pond pump, aquarium pumps, electric dryer and range that won't exist in our new home. I figure we may actually move closer to 300 KwH per day.

    Are you sure that is not 300kWhrs per month? (or ~10kWhrs per day)...

    Just go to the 2nd post in the thread (mine...), and sub out the 19kWhr per day with 10 kWhrs per day (or roughly 1/2)...

    5.7kW of panels * 10kWhr/19kWhr = 3kWhrs of solar panels

    Since they may degrade by about 20% (dirt and aging) over the next 35 years, I installed 3.5 kWatts of solar panels--the maximum for a 3kW grid tied inverter.

    I guess I am not sure where you are going to build this home (NY, Ohio, Kansas, or Arizona)... Use this link to figure out how much power you will get from 3.5 kWatts of panels were you will live. Then scale up or down to match your estimated power needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: Sizing the system...

    recheck your math, 300kWh/day is 9 Mega Wh month, or about 900 bucks a month at 10 cents kWhr.

    My 12kW name plate rated Gridtie PV makes about 60 kWh/day this time of year for comparison
  • TelcoTelco Solar Expert Posts: 201 ✭✭✭✭✭
    Re: Sizing the system...
    Well...I believe that Kansas is rated as a major part of Tornado Alley. Straw Bale homes built in the 19th century without foundation, pinning, and load bearing framing are still standing today. The cost is about 20% higher than a stick-built home, but the overall efficiency and strength of the home is outstanding.

    The straw bales are not hay. Hay would draw bug and vermin. Straw is highly compressed and then the bales are further compressed during construction. The bales wrap around the wall framing which is tied down to the foundation in the same fashion as frame houses. They are then covered with 3 inches of stucco plaster on each side of the wall.

    The end result: you have a home that is very strong and has stood up to very high winds. In the 2004 fire rating test, they built a straw bale wall, then exposed it to 30 minutes of 240 degree fire. The opposite side of the wall had an array of temperature sensors -- and those sensors never went up above 100 degrees. This is the lowest temperature pass-through in the history of the fire testing labs...of all wall types tested.

    Also when you consider that concrete requires something like 4 times the concretes weight in carbon dioxide to make the stuff, you are really creating an environmental nightmare.

    YOU should research Straw Bale home construction.


    I DID research straw houses and I didn't like what I saw. I'm also RESPECTING your decision with rejecting my suggestion and going with straw, please respect my decision to go with concrete. Notice I made a suggestion and didn't press the case after you said your mind was made up. My second post on the concrete dry stack blocks was not trying to get you to change your mind, I was suggesting that they be used for a hot water battery that the sun can charge. I realize that straw building is attacked regularly as a crappy way to build a house, but I was only making a suggestion, not slamming your chosen building method.

    So far as straw "surviving" a tornado just because there are 100 year old straw houses in Kansas, those houses may not have actually SEEN a tornado. Being in an area with a lot of tornadoes does not mean every square inch of land gets scrubbed with a tornado every year. And, not only have I never seen a tornado shelter made of straw, I have seen a great many concrete buildings designated as tornado shelters so long as you were out of range of flying glass.

    Cost along with the ability to build my entire house as a tornado shelter is why I chose what I did. If you want to build from straw, that's your business.
Sign In or Register to comment.