Array size and storage

Isolation · July 2014

I am wanting to hook up a 12v solar system for a camper (permanent location). I will be using a MS mppt charge controller. What is the maximum size solar array I can use if using an mppt controller to charge a 12v battery bank? I don't need to store a lot of power since at night the only thing running will be a chest freezer (propane fridge went out), a clock and charging laptop. Most of my loads will be ran during the day so I want to make sure I can get the batteries charged quickly and have excess power for other loads such as a fan, maybe watching a little tv. Loads such as my LED tv (32" vizio with blu-ray uses 80w per hour) will only be ran during the day for short periods of time so I didn't really want to have stored power for loads I will never use when the sun is down.

I am wanting to know the maximum solar array size I can have. I read a few threads that recommended going to 24v systems if you have over 1300 watts but I wasn't sure if that meant stored power or just what you consume hourly. I'll never consume anywhere near that per hour. Basically I want to avoid a lot of batteries since space is limited unless I put them in something outside the camper and also because I don't need a lot of storage. I just want to be able to supply enough power during the day.

Cariboocoot · July 2014

Re: Array size and storage

Welcome to the forum.

You're sort of going about it backwards. You should start by figuring out how much power you need to supply. That will determine the size of the battery bank which in turn will determine how much solar is needed to recharge it and what charge controller should be used. You are starting with the controller.

Morningstar makes several different MPPT controllers: 15 Amp, 30 Amp, 45 Amp, and 60 Amp. The size is based on maximum output current. It will relate directly to the size of the battery bank as batteries have a minimum, 'ideal', and maximum charge current.

System Voltage depend on stored capacity and/or maximum Watts. A little bit about that: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

On 12 Volts a 60 Amp controller running at maximum ('ideal' 600 Amp hours of battery) would have about 935 Watts of array. The batteries would provide up to around 3kW hours AC.

A lot of RV applications use only two golf cart batteries (GC2's) at 220 Amp hours. This requires only a 30 Amp charge controller and about 350 Watts of array. It would provide up to 1 kW hour AC.

Those power figures, btw, are based on 50% DOD of the batteries and all power being stored in and removed from the batteries. This is not necessarily the case as daytime power usage 'direct from panels' can improve the system's efficiency considerably. It sounds as though you plan on doing at least some of that.

vtmaps · July 2014

Re: Array size and storage

Isolation wrote: »

at night the only thing running will be a chest freezer
<snip>
my LED tv (32" vizio with blu-ray uses 80w per hour)
<snip>
I read a few threads that recommended going to 24v systems if you have over 1300 watts but I wasn't sure if that meant stored power or just what you consume hourly. I'll never consume anywhere near that per hour.

Welcome to the forum. I recommend that you follow Cariboocoot's advice and design you system around your loads... after all, the purpose of your system is to satisfy your loads.

If you are going to do so, it would help for you to learn the language of power and energy.

Watts is a RATE of energy. Your TV does NOT consume 80 watts per hour. It consumes 80 watts. If your TV were consuming 40 watts at noon, and then 120 watts at 1 PM, and then 200 watts at 2 PM, and then 280 watts at 3 PM, its power consumption would be increasing at 80 watts per hour.

1300 watts is NOT an amount of stored energy. 1300 watts is NOT what you consume hourly. 1300 watts could be the amount of power that your solar panels produce, or it could be the rate at which you consume energy.

Energy is measured in wattHours. Your battery bank will store a certain amount of energy, measured in watthours. Your TV will, over time, consume a certain amount of energy. For example, if your TV draws energy at the rate of 80 watts, and you run it for 1.5 hours, it will consume 80 watts X 1.5 hours = 120 watthours.

You mentioned that you will never consume 1300 watts... don't be so sure. Your chest freezer may draw perhaps 120 watts while it is running, but when it starts up it could easily draw 1300 watts for a very brief amount of time (fraction of a second). Suppose it draws 1300 watts for 0.5 seconds. 1300 watts X .00014 hours = 0.182 wattHour of energy. That is an insubstantial amount of energy, but it is a substantial load while it lasts. This is important in choosing an inverter and battery... they must be able to handle that load, even though it is very brief.

Other than that freezer, it sounds like you could get by with the deservedly popular 300 watt morningstar inverter. Your freezer will push you up to at least a 1000 watt pure sine inverter. Unfortunately, larger inverters draw a fair amount of power by just being turned on (tare loss). Suppose your inverter has a 20 watt tare loss, that means 20 watts X 24 hours = 480 watthours per day. That's a lot... your freezer itself may draw only 1000 watthours per day (you should measure what your freezer draws with a kill-a-watt meter).

--vtMaps

Isolation · July 2014

Re: Array size and storage

That is the part that confuses me. To figure out my loads, do I even count the loads that I don't want to store power for? Some loads will only run during the day when the sun is up and after the batteries are charged so I don't want to figure those in because it will result in more storage than I actually need.

Loads I know will be drawing power 24/7 are a chest freezer, 2xlaptops which will stay plugged in, 2x12v lights (3-5 hours, if that), 12v water pump (1 hour or less) and maybe a clock radio@3w so 72watts total.

Other than that all loads will be temporary and I was hoping to run them off excess power during the day. If I can't do that because I'm not bringing in the power then they will not be run since I can only power something after the bank is charged. When the excess power drops off the loads are turned off. This is in Arizona so not many days without sunshine.

vtmaps · July 2014

Re: Array size and storage

Isolation wrote: »

Loads I know will be drawing power 24/7 are a chest freezer, 2xlaptops which will stay plugged in, 2x12v lights (3-5 hours, if that), 12v water pump (1 hour or less) and maybe a clock radio@3w so 72watts total.

So the clock radio is 3 watts X 24 hours = 72 watthours per day

Your 12 volt pump probably draws at least 70 watts... maybe more.

Your laptops don't draw much, but small loads that are on all the time can add up to large energy draws per day.

The freezer is the defining load in your system... buy a kill-a-watt meter (about $25) and find out for sure what it and the laptops draw.

--vtMaps

Isolation · July 2014

Re: Array size and storage

vtmaps wrote: »

So the clock radio is 3 watts X 24 hours = 72 watthours per day

Your 12 volt pump probably draws at least 70 watts... maybe more.

Your laptops don't draw much, but small loads that are on all the time can add up to large energy draws per day.

The freezer is the defining load in your system... buy a kill-a-watt meter (about $25) and find out for sure what it and the laptops draw.

--vtMaps

I have a killowatt meter and I did check the freezer a while back but I'm not sure how accurate it was. The freezer is a 5.0 cu.f freezer I got from my sister and it used just shy of 1000 watts in 24 hours which seemed excessive for a small chest freezer but it was badly in need of defrosting at the time so I don't know what effect that would have on its power consumption. Does it make it use more or less with all that ice built up? I will check the laptops.

I was hoping you could clarify something about the T.V. consumption. I said it used 80 watts per hour because that is what the meter showed. It actually shows the tv and blu-ray are drawing 60-80 watts with a movie playing so of course I went by the worst case which was 80. At the end of an hour it showed the tv had consumed .08 kwh so unless I am reading it wrong the tv and blu-ray uses 80 watts each hour. I have ran it longer but it never seems to change much, always between 60-80watts being drawn over the course of an hour.

The part that confused me is you said it doesn't use 80 watts per hour, just 80 watts. I'm not understanding and just want to make sure I'm not way off base when calculating the loads.

I do realize the freezer has a surge and if I remember correctly it was around 600 watts because when I first plugged the freezer in the compressor kicked on.

Cariboocoot · July 2014

Re: Array size and storage

Terminology: Watts is a rate, Watt hours is a quantity.

So your TV consumes power at a rate of 80 Watts maximum. If you run it for one hour it uses a quantity of 80 Watt hours (.080 KW h). If you use it for two hours it will uses 160 Watt hours.

Notice the variation in your rate: 60 to 80 Watts. This kind of fluctuation occurs in many things that have variable consumption according to what they are doing. Computer, for example: might use 190 Watts with everything running (laser burning a CD for example) but sitting idle drops down to perhaps 60 Watts. Refrigerator start-up surge >1000 Watts, running power at 130 Watts, defrosts at 500 Watts: depending on how much of which it has to do in a given day this will vary the total daily Watt hours it consumes.

Small freezers are not necessarily good on energy. We've done a lot of testing of various refrigeration devices and found that the little ones can use as much as the big ones because they're not built very well; little effort is put in to efficiency. Your 1kW hour consumption figure is not unexpected, but is nearly as much as a full-size refrigerator-freezer that is not "Energy Star rated".

This is the value of the Kill-A-Watt: measuring things under normal usage to get a realistic consumption figure. Plug it in and let it go for a week if you can. That will give you a better sampling time for average consumption.

Isolation · July 2014

Re: Array size and storage

I have another question. I've read that the array size should be matched to the battery bank for proper charging. Clearly if you don't have enough power coming in your bank will be undercharged. But I've read that too large of an array and you could overcharge your bank. Why is it a bank would overcharge if the MPPT controller cuts back the charging voltage when the bank reaches full charge? I thought the MPPT would cut charging to the battery bank and then you could either lose the excess power being made or divert it to something else. Am I misunderstanding something regarding overcharging the battery bank?

Cariboocoot · July 2014

Re: Array size and storage

Isolation wrote: »

I have another question. I've read that the array size should be matched to the battery bank for proper charging. Clearly if you don't have enough power coming in your bank will be undercharged. But I've read that too large of an array and you could overcharge your bank. Why is it a bank would overcharge if the MPPT controller cuts back the charging voltage when the bank reaches full charge? I thought the MPPT would cut charging to the battery bank and then you could either lose the excess power being made or divert it to something else. Am I misunderstanding something regarding overcharging the battery bank?

It isn't the Voltage that's the issue. Battery banks aren't smart. If the charge controller is capable of putting out more current than is good for the battery this can happen. So we try to keep the array in the realm of < maximum current the battery can take. Some controllers can have a current limit programmed so that you can run a much larger than normal array. This means they may still be able to put out sufficient charging on less-than-perfect days.

BB. · July 2014

Re: Array size and storage

Isolation wrote: »

I have a killowatt meter and I did check the freezer a while back but I'm not sure how accurate it was. The freezer is a 5.0 cu.f freezer I got from my sister and it used just shy of 1000 watts in 24 hours which seemed excessive for a small chest freezer but it was badly in need of defrosting at the time so I don't know what effect that would have on its power consumption. Does it make it use more or less with all that ice built up? I will check the laptops.

To be clear... That was (most likely) 1,000 Watt*Hours of AC energy usage for the Freezer... About normal for typical small freezer.

I was hoping you could clarify something about the T.V. consumption. I said it used 80 watts per hour because that is what the meter showed. It actually shows the tv and blu-ray are drawing 60-80 watts with a movie playing so of course I went by the worst case which was 80. At the end of an hour it showed the tv had consumed .08 kwh so unless I am reading it wrong the tv and blu-ray uses 80 watts each hour. I have ran it longer but it never seems to change much, always between 60-80watts being drawn over the course of an hour.

80 Watt TV * 1 Hour = 80 Watt*Hours = 0.080 kWH

New LED TVs are using much less power--Perhaps as low as 15 Watts or so... Or you can use a laptop that would use around 20-30 Watts.

Conservation will be your friend here... It is almost always cheaper to conserve that to build out a solar power system to run inefficient loads.

The part that confused me is you said it doesn't use 80 watts per hour, just 80 watts. I'm not understanding and just want to make sure I'm not way off base when calculating the loads.

Watts is a "compound" unit and is really Joules/Second (we use hours just to keep the number smaller--per second would be 3,600x larger numbers).

It is the same as if I named Miles per Hour the "Bill"... The speed limit would be 75 Bills (75 MPH) and you will have driving 750 Bill*Hours (750 miles) in 10 hours.

I do realize the freezer has a surge and if I remember correctly it was around 600 watts because when I first plugged the freezer in the compressor kicked on.

Many times, you will need around a 1,200 to 1,500 watt minimum AC inverter to run a standard refrigerator/freezer plus a few small AC loads. Starting surge current is really bad on these refrigeration compressors.

-Bill

Array size and storage

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