inverter sizing

Options
northshore
northshore Registered Users Posts: 12
Hello-
I would like some guidence on inverter sizing for my proposed small off grid system. I did an actual measurement of the watts I used at my site and the largest consumer is ,of course, my coffee maker(rated 900w, used 7.5 minutes). The refridgerator is small, 4.3 cu. ft. and has a continous rating of 70-100w and it runs for >2.5 minutes and is off for 11 minutes with only slight variations.(Thats with no teenagers constantly opening and closing the door). Do I double the watts for the refer motor startup, ie 140-200w, or is there another method of determining this load. It has a 1/20th hp motor and I believe a static capacitor. I measured my loads with a stopwatch and reading ratings labels (crude-but works) My total usage is 900w's/day if I don't conserve to 350w's/day if I do, so 625w's/day average is what I'll start with. However, I would like to tie in my 4000w construction genset (old and loud but works fine) for bringing the batteries up quicker. Do I go with say, 300w inverter and tie the genset with an Iota type charger or would an inverter/charger combo be more economical in the long term.I originally planned on 24v system but 12v equipment is cheaper and my proposed system will be fairly small( less than 600w solar) Be gentile- I'm new at this. Terry

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Options
    Re: inverter sizing

    Step 1: get rid of the coffee maker. Boil the water on a propane stove or over a can of Sterno. Heating is an awful use (waste) of electricity.

    Step 2: Get a Kill-A-Watt meter and take some accurate, over-time readings of your devices. The nameplates on things are notoriously inaccurate. Sometimes the ratings are high, sometimes they are low. Refrigerators, for instance, are tricky because their run usage varies depending on how much work they have to do to compress the refrigerant at any given time. Hotter weather = more work = higher consumption. There's no way to precisely calculate the start-up surge either, and even Kill-A-Watt won't necessarily capture that info. In general, a 'small frame' motor such as it would have uses 3-5 times the run current at start-up. This also varies with Voltage; if there's a drop in V, then A goes up to make W.

    Step 3: to determine inverter size, you need to know both your biggest draw (probably the 'frige) and your total cumulative draw (everything that's likely to be running at the same time). From this you can calculate the total Watt hours per day, and thus determine the size of battery bank you need in Amp/hrs.

    Step 4: going by the battery bank size you can determine how much PV array & what sort of charge controller you need to recharge them. Having the 4kW generator is good; it's probably over-kill in terms of size and vicious on gas usage, but you've already got it so use it! For boosting batteries via generator, all you need is an ordinary battery charger (capable of supply the Amps needed for your bank size). The solar panels should be able to 'finish off' the charge on even a cloudy day.

    I'd suggest a target system of 1500 Watts might be what you're after. Take a look at my sig's description of my system. And know that it's only that large to run the blasted pumps necessary to make the water works work! The full-size refrigerator is only about 1.6 kW/hrs per day on average.
  • northshore
    northshore Registered Users Posts: 12
    Options
    Re: inverter sizing

    Hello thanks for the info- I realize that the coffee maker is a large draw on power, but it stays( used once a day for less than 8 minutes, then shut off) I'm only there maybe 4 days a month so while I'm gone the solar end of things should easily maintain a full charge on the batteries. I'm looking at 4 135w panels to a good mppt controler and a battery bank of about 400 to 440ah- 12v system.(4-6v 200-220ah batteries wired series/parallel) Did you( caribocoot) mean a 1500w inverter or system size? Also, I will be( or would like to) hardwire the inverter directly to my AC breakers (I am currently hooked to the grid) What size off the shelf battery charger would you consider for use with the generator. I've read not to oversize the inverter because of inefficiencies of both power consumption and money spent. The inverter is the last segment of my simple design and I'll have to live with the nameplate wattages possibly being off. This system will be OFF grid.
    PS The coffee maker is the only luxury item I have up there- no running water, indoor bathroom etc. so I won't give it up. I have posted a thread on this site before and have used the advice to get this far in the planning process, so thanks in advance for any suggestions. Terry
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Options
    Re: inverter sizing

    Since your largest peak wattage is the coffee maker, get a 12V one and wire direct ( fused of course ). This eliminates the large peak needs ( AC ) and you can use a smaller inverter.

    http://www.wholesalemarine.com/c/230001/12+Volt+Galley.html?gclid=CM67o-7I6KACFUmU7Qod_TUGHg. This is just the first hit on a Google search, not recommending this linked site

    You maybe able to use the MorningStar Sure sine for whats left ( 300watt/600 peak ), depends what the start-up surge on the little fridge is
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    Options
    Re: inverter sizing
    PS The coffee maker is the only luxury item I have up there
    Terry, I agree .. keep the coffee maker .. luxury it's not, necessity it is. Coot is a tea drinker from up north, doesn't understand that " Americas" dependence on foreign oil is second to or dependence on foreign coffee.;)
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Options
    Re: inverter sizing

    Ditch the coffee maker and use this instead: http://www.coleman.com/coleman/colemancom/detail.asp?product_Id=2000000719

    Since that is your biggest load, you can get away with a much smaller inverter. The only difference between the coleman and a plug in is that you can't use a timer to auto start it,, and you have to pour the coffee off into a thermal carafe' after it has brewed. (which you should do anyway!)

    We have been using one for years, works just as well as any plug in, takes less than ten minutes to brew a pot, and it uses the burner on ~1/2 on so the propane usage is pretty small. Works fine on a camp stove as well.

    Bottom line, why use up ~2-400 wh/day when a good reliable alternative exists. Now my propane toaster is an even better deal,,,,
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    Options
    Re: inverter sizing
    Now my propane toaster is an even better deal,,,,
    OK so share .. one who flew to close to the sun. Inquiring minds want to know!
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Options
    Re: inverter sizing
    icarus wrote: »

    Since that is your biggest load, you can get away with a much smaller inverter. The only difference between the coleman and a plug in is that you can't use a timer to auto start it,, and you have to pour the coffee off into a thermal carafe' after it has brewed. (which you should do anyway!)

    We have been using one for years, works just as well as any plug in, takes less than ten minutes to brew a pot, and it uses the burner on ~1/2 on so the propane usage is pretty small. Works fine on a camp stove as well.

    Amazon has these coleman coffee makers for $38.95, shipping included. Got one a couple weeks ago and like it really well. Got a caraffe coming from amazon this week. Will post how it works after I get it. I got a coffee press and it was a mistake. Coffee press doesn,t keep the coffee hot for very long S:Dlarvic
  • northshore
    northshore Registered Users Posts: 12
    Options
    Re: inverter sizing

    Well recieved- thank you
    Like my first thread(cold weather batteries), I seem to have encountered another underground network of salesmen- this time for coleman 12v coffee makers. Very good advice but lets say, for the sake of argument, that the big draw is not a coffee maker- but my wife's hair dryer. Now anyone who's married knows that the system in question has to be designed around said hair dryer. With that being the case, I am looking for the smallest inverter/charger combo( with auto transfer switch) to charge the batteries from my genset. I do know that they are more expensive, but as of this moment, I am still under budget and according to the owner of the hair dryer, "if I don't use it, I'm going to lose it". I am even open to used equipment . PS I have not purchased anything as of this time- everything is still on paper. Thanks Terry
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Options
    Re: inverter sizing

    Northshore,,

    Now if you wish me to send you some links to butane powered hair dryers I can,,,,!

    Seriously, it is just like anything else, size for expected loads with some head room, but look seriously at what you really need vs what you think you need vs what you want.

    Additionally, there are whole fleets of RV/trucker stuff, that runs on 12vdc. So the reality is such that you can do a lot with 12 vdc instead of invertering. I do like the Suresine 300. Covers my peaks loads quite well,, and in my case that is ~150 watts (the bed warmer! Can't find a propane powered bed warmer yet, although we do heat flax bags on the wood stove,,, bed warmer is much "cooler"!) We usually run about 75 watts max.

    Tony

    PS How do you like this early spring?
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    Options
    Re: inverter sizing

    Terry, back to the point, "hair dryer" aka coffee maker @ 900 watts + refer startup @ 200 watts = 1100 watts. I would at least double that. You haven't listed lights, TV's, radios or anything else.

    The way to size an inverter is to add everything that might be on or start at one time. Thus far, the wife is blow drying her hair in the dark and you are sitting there waiting for her to finish so you can make coffee because, god forbid, if the refer starts ... she will never get her hair dry!

    Based upon what you have told us, I would recommend http://store.solar-electric.com/fx2012t.html . I like Outback others like others. You can stack these, add more, and have a built in charger.

    I like your original thoughts of a 24 v system but, It's your choice. BTW are you aware that a 60 amp charge controller can only handle 720 watts of panels @ 12 v but can handle 1440 watts @ 24v and 2880 watts @ 48v?

    Off-grid systems have a habit of growing over time! It is cheaper and more efficient to grow them at a higher voltage than lower!
  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    Options
    Re: inverter sizing
    Terry, back to the point, "hair dryer"

    here is a gas hair drier that might work
    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=91033
  • dagr51
    dagr51 Solar Expert Posts: 72 ✭✭✭✭
    Options
    Re: inverter sizing

    I like that it heats to 3000 degrees.
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Options
    Re: inverter sizing

    northshore wrote: »
    Well recieved- thank you
    Like my first thread(cold weather batteries), I seem to have encountered another underground network of salesmen- this time for coleman 12v coffe maker.
    Coleman coffee maker doesn,t use electric!! I use mine on my gas range top burner!! S:Dlarvic
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Options
    Re: inverter sizing

    In recognition that this is a family forum and not a gathering place for guys whose wives divorced them because they did not listen to the off grid REQUIREMENTS...

    Get a Kill-a-Watt meter and measure the hair drier use... If it is for 10 minutes at a time, it may work out to as little as 200 Watt*Hours per day of use--not a terrible load for an off grid system...

    However, your battery bank and inverter(s) need to be sized to support that load.

    A battery bank, should be approximately 2.5-5x the minimum load in Amp*Hours (probably 8x for heavy power usage--such as a fork lift or pumping which goes on for long periods of time). Assume 1,200 watt hair drier on a 24 volt battery bank:
    • 1,200 watts * 1/0.80 inv eff * 1/12 volts * 2.5 = 313 AH @ 24 volts
    • 1,200 watts * 1/0.80 inv eff * 1/12 volts * 5 = 625 AH @ 24 volts
    • 1,200 watts * 1/0.80 inv eff * 1/12 volts * 8 = 1,000 AH @ 24 volts
    So, your battery bank sizing is pretty large to support a hair dryer or coffee maker, etc. surge currents--How do the rest of your loads look?

    To support the hair dryer, you could use a "cheap" MSW inverter, and use a smaller/more expensive TSW inverter to support the rest of your more sensitive loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • northshore
    northshore Registered Users Posts: 12
    Options
    Re: inverter sizing

    Hello again
    Sorry to create the confusion over the hairdryer, it was only hypothetical. I was under the assumtion that if the coffee maker was rated at 900w's and was used for 8 minutes it only used 120w's. Startup on refridgerator is unknown. My other loads are fairly trivial- undercounter led's @1.7w's for 3-4 hours and a couple of 13w cfl's for the same time. Usually no two lights are on at the same time. Also, no TV, radio has batteries. I liked the link from audredger on the Outback system (prewired solutions) but a lot of the specs I just do not comprehend but that is the route I'd like to go.( Thats why I thought I'd look for ideas on the inverter/charger/auto transfer switch) If a simple battery charger from Wal-mart would work(I know it won't) I would go that route. But to be clear, I do want to size this system for growth- I would like my usage to "catch up" to the system and not have to replace components because of inadequacies in design. I would like to eliminate the genset but like many who get into a solar PV system, I vastly overestimated the systems ability. Please, keep the ideas coming, I do analyze them all. MikeO- I love the hair dryer-I'll just have to get another 20lb tank to go with it! Tony in Quetico- warmest spring in history of recordkeeping for this area- simlpy amazing( I've been to your neck of the woods ice fishing and had to use a 48" extension on the ice auger to get through- beautiful country) Thanks Terry
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Options
    Re: inverter sizing

    it used 120wh not 120w. an inverter would still need to be capable of the high wattage of 900w continuously. what it consumes over time is in watt hours.
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    Options
    Re: inverter sizing

    Living off grid doesn't mean doing without ... I means doing more with less and making more informed decisions. (Well, maybe we do, do without, no Christmas lights hung all over the porch.)

    Loads do increase over time, old mothers move in for the winter and sleep with the TV on. Expected PV harvest don't materialize, bad weather and smoke from forest fires. If, you can afford it, it is better to have a system that is a little larger than necessary than one that is worked to death.

    Remember, what works for one may not work for another and, some like Fords others like Chevy's.
  • northshore
    northshore Registered Users Posts: 12
    Options
    Re: inverter sizing

    Thank you Neil Now I know why everyone is so persistant about ditching the coffee maker.(an ahhh-hhaa moment) If I keep the appliance, I'll go with an inverter to match the continuous ratings of these item(s). Most of the people who respond here are so far advanced on this topic that they may take for granted that the simple things are a given to everyone. I believe myself to be a slow-learner in regards to this subject(solar PV) but I'm smart enough to know that I'll make my mistakes on paper and with the help of this forum, my end result will be a small, well planned system. (ie a system designed by the members of this forum).
    Terry