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Solar is all really maths ... and no, I am not the expert on maths.   :D 

I am using a guy named Bill, from around San Francisco, brain power here.

 

 

Having been on this site for a while, I see very few calculations here on i.e. size of panels required for the time devices must run taking into account inefficiencies.

 

Calculating Load in WH

Power rating (watts) * Number of hours the appliance is used (hours) = WH

Battery Bank calculation
Wh x 1/Battery voltage x Number of days of storage x 1/maximum discharge = Ah @ Battery Voltage

Array required for Charging a certain battery bank
Ah x Charging Voltage x 1/0.77 panel+controller derating x % rate of charge (in fraction) = Watts, array size

 

For example:

What panels watts do I require to run a 40w fan for 10 hours per night, assuming 52% inefficiency from DC to AC for the whole system:

Calculation is: 10hours x 40w x 1/0.52 system eff x 1/4 hours of sun = 192 watts of solar panels required, minimum

 

Now say, we are in winter, and you want to add 1 or 3 days of no-sun backup with max 50% discharge on a 12v volt battery bank, with a inefficiency of 85% for system:

For 1 day   : 40w x 10hours x 1/0.85 inverter eff x 1/12 volt battery x 1 days no sun x 1/0.50 max discharge = 78AH @ 12 volt battery bank.

For 3 days : 40w x 10hours x 1/0.85 inverter eff x 1/12 volt battery x 3 days no sun x 1/0.50 max discharge = 235AH @ 12 volt battery bank.

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Watts

The poor watt is often misunderstood. Watts are basically just a measure of how much power a device uses when turned on, or can supply. A watt is a watt - there is no such thing as "watts per hour", or "watts per day". If a something uses 100 watts, that is simply the voltage times the amps. If it draws 10 amps at 12 volts, or 1 amp at 120 volts, it is still 120 watts. A watt is defined as one Joule per second, so saying watts per hour is like saying "miles per hour per day".

 

Watt-hours

A watt-hour (or kilowatt hour, kWh) is simply how many watts times how many hours that is used for. This is what most people mean when they say "watts per day". If a light uses 100 watts, and it is on for 9 hours, that is 900 watt-hours. If a microwave uses 1500 watts, and runs for 10 minutes, that is 1/6th of an hour x 1500, or 250 WH. When you buy power from your friendly utility (look at your last bill), they sell it to you at so much per kWh. A kWh is a "kilowatt hour", or 1000 watts for one hour (or 1 watt for 1000 hours).

 

Amps

An amp is a measure of electrical current at the moment. (Amps do not come in "amps per hour" or "amps per day" either). Amps are important because it determines what wire size you need, especially on the DC (low voltage) side of an inverter. All wire has resistance, and amps flowing through a wire makes heat. If your wire is too small for the amps, you get hot wires. You can also get voltage drops in the wire if it is too small. This is not usually a good thing. An amp is defined as 1 Coulomb per second.

 

A Coulomb is the charge of 6.24 x 1018 electrons. Therefore, 1 Amp is equal to the charge of 6.24 x 1018 electrons passing a point in a circuit in 1 second.

 

Amp-Hours

Amp-hours (usually abbreviated as AH) are what most people mean when they say "amps per hour" etc. Amps x time = AH. AH are very important, as it is the main measure of battery capacity. Since most inverters run from batteries, the AH capacity determines how long you can run. See our battery page for much more detailed information.

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Volts / Ohms = Amps (Ohm's Law).

Volts * Amps = Watts (Power Formula without power factor correction).

MPPT output current = panel Wattage * 0.77 efficiency / nominal battery Voltage.

PWM output current = input current.

Amp hours * battery Voltage = nominal total capacity in Watt hours.

Watt rating * hours of use = Watt hours consumed.

Watt hours per day / system Voltage = Amp hours per day (nominal).

Usable AC Watt hours = Rated array Watts x hours of good sun * 0.52 over-all efficiency.

Expected grid-tie system production = Rated array Watts x 0.77 efficiency * hours of good sun.

 

To determine breaker or fuse size for panels: Multiply Isc by 1.56 and round up to the next available size.

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To determine breaker or fuse size for panels: Multiply Isc by 1.56 and round up to the next available size.

 

 

HI TTT 

 

That is an exhaustive list and I think should be pinned somewhere for future reference. One recommendation when I was reading up on solar fuses was to de-rate them by 1.25 and round to nearest size whereas you calculation is higher (1.56).

 

I will use my own system as an example. 

 

My panels have a Isc of 8.79 A. I have 4 strings so should never see more than 8.79 x 4 = 35.15 A for the most part 80% of this would be normal with it peaking close to 35A. If we multiply but 1.25 we get 43A.  8A more than our expected maximum. I would want my system to react if those conditions occurred. With 1.56 my DC trip switch would be 35 x 1.56 = 54 nearly 55 A (20 A higher than the maximum) . If this current flow was due to a dead short both systems would trip. If for some bizarre reason the Amps increases over a longer period of time your wiring is going to be subjected to heat.

 

I suppose in a rambling sort of way I am saying too high a breaker/fuse is as bad as the nuisance trip/blowing for a trip/fuse which is too low.

 

Edit: perhaps we can have this discussion and then delete the discussion or pin TTT's post as a new post since I think it is a valuable resource and I was hesitant to comment on it as now I have opened a can of worms and a elegant thread like this needs to be presented "clean" to newbies.

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Hi Chris,

 

Yes, it is a can of worms.

We HAVE to discuss, share expand our thinking BUT we must keep the thread clean for future references. I started this because I ALWAYS forget the something.

So I want to put the core down here, if you all like it.

Maybe Admin can assist with some ideas?

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I suppose in a rambling sort of way I am saying too high a breaker/fuse is as bad as the nuisance trip/blowing for a trip/fuse which is too low.

Quoting from my source:

Not always true ... depends on the fuse or breaker.  

There may be two multiplications by 1.25, in other words, that 1.56 multiplier is 1.25 X 1.25 = 1.56.  
You always use one 125% multiplier, but the second multiplier depends on the type of fuse or circuit breaker.

Fuses and breakers have different types of ratings. Some are designed to blow/trip when they carry their rated current, others are designed to carry their rated current without blowing/tripping.
If your fuse/breaker is designed to carry its rated current without blowing/tripping, then you only use 1 multiplier. The second multiplier is for fuses/breakers that blow/trip at their rated load.  

Now, having said all that, why do you want a fuse between your panels and your charge controller? 

There is no need for one. Whatever fuse or breaker you put in there will be sized so that there will never be enough current from your panels to blow/trip it. I recommend that you put a DC circuit breaker between your panels and your charge controller. The breaker serves as a switch and it is very convenient to have a switch. Most charge controllers must be powered up by connecting the battery before connecting the PV, and they must be powered down by disconnecting the PV before the battery. Therefore it is very convenient to have a breaker/switch.

VERY IMPORTANT: Whereas the breaker between your panels and your charge controller is more for convenience than safety, the breaker between your controller and your battery is absolutely necessary for safety.

 

Edit: I did not know that either.

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Some of the most important things to keep in mind when selecting fuses to be used for PV lines and in line with the batteries - taken from http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf

 

NORMAL OPERATING CURRENT: The current rating of a fuse is typically derated 25% for operation at 25
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Now, having said all that, why do you want a fuse between your panels and your charge controller? 

 

 

They are there to protect the cable and limit the possibility of a fire. I have a solar fuse on each string (12A). If you have a short on one of your strings it will "draw current from the remaining strings. The 4mm2 solar cable was not designed to carry that amount of current therefore each string is fused to protect it. Once the current is "upped" in the combiner box I have 25mm2 armoured cable running into the house which should take a max of 35A. It is rated at much more but if we see 40 A something is wrong. The solar fuses should blow at 46A but a 40A DC circuit breaker is added layer of protection and yes I use it as a convenient switch as it is right next to the inverter but it is part of the safety requirement. 

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A one stop shop for solar information brilliant.

 

I think we must centre it around TTT's post.

 

I certainly would want the "Three Wise Men" (Mike, Wetkit and SuperDIY) to contribute. There are many others plonky, jhay Gnome to name a few who have oodles of technical stuff at their finger tips. I have learnt so much in the 6 months I have been on this forum. It is time to pay it forward. Contributions from folk who are installers/ in the industry is important since they come up against these problems/issues daily.

 

Edit: I probably going to get into trouble for having left peoples' names off this list but I am a a big boy now and can look after myself. "Mom where my shoes?" 

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I agree.

 

One request: It MUST be for the really new guy. 

One can either whack a lot of technical terms together that 99.9% of the people do not and will not want to understand.

 

Along the lines of the Dummies Guide to Solar ... 

With Excel formulas that gives you the basics.

 

Our own, but along these lines.

Battery size Calculator (22.8.12).xlsx

Off-Grid - Master - Solar-Panel-Design1.1.15.xlsx

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Using a AC inverter, the generic calculations powering a 1.2kW AC inverter running at full rated load:

12v system: 1200 Watt x 1/0.85 inverter eff x 1/10.5 inverter cutoff x 1.25 derating for fuse/wire = 168 Amp min rating fuse/breaker/cable.

24v system: 1200 Watt x 1/0.85 inverter eff x 1/21 inverter cutoff x 1.25 derating for fuse/wire = 84 Amp min rating fuse/breaker/cable.

48v system: 1200 Watt x 1/0.85 inverter eff x 1/42 inverter cutoff x 1.25 derating for fuse/wire = 42 Amp min rating fuse/breaker/cable.

Reason the inverter cutt off is used is because the lower the volts, the higher the amps. It is something that I never knew.

 

The fuse/breaker/wiring needs to be rated as above for each circuit that leaves the + battery pole. 

 

Because DC fuses/breakers are a bit more difficult to find, are quite expensive and there can be issues between 12/24/48 volt rated battery systems, fuses may be cheaper to install, but many high current DC fuses are not cheap so it may be simpler to use a DC Circuit Breaker instead. You then get the protection of a fuse, plus a "on/off" switch in one unit.

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I see this post was missed: http://powerforum.co.za/topic/446-all-about-batteries/

 

Also thinking along the lines of having the parts needed, for sale here. There are SUCH awesome devices overseas.

Idea is to try a few, the ones that are easy and quick to connect, try them out between us, and then once tried and tested, only propose those parts. 

Too many choices out there, confusing the whole thing.

 

This is a battery fuse I like a lot.

post-122-0-29578200-1453022410_thumb.jpg

Or even better, like this: https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A

 

The above fuse connected to a busbars like this, red (+) and black (-), to get all the wires neat and tidy:

post-122-0-31675900-1453022764_thumb.jpg

 

And a DC circuit breaker between solar panels and the charge controller that has a built-in switch, allowing the solar panels to be disconnected from the system.

post-122-0-43088100-1453022645_thumb.jpg

 

Stuff like above not only looks very professional but also make it very convenient to connect / disconnect wires.

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HI TTT 

 

That is an exhaustive list and I think should be pinned somewhere for future reference. One recommendation when I was reading up on solar fuses was to de-rate them by 1.25 and round to nearest size whereas you calculation is higher (1.56).

 

I will use my own system as an example. 

 

My panels have a Isc of 8.79 A. I have 4 strings so should never see more than 8.79 x 4 = 35.15 A for the most part 80% of this would be normal with it peaking close to 35A. If we multiply but 1.25 we get 43A.  8A more than our expected maximum. I would want my system to react if those conditions occurred. With 1.56 my DC trip switch would be 35 x 1.56 = 54 nearly 55 A (20 A higher than the maximum) . If this current flow was due to a dead short both systems would trip. If for some bizarre reason the Amps increases over a longer period of time your wiring is going to be subjected to heat.

 

I suppose in a rambling sort of way I am saying too high a breaker/fuse is as bad as the nuisance trip/blowing for a trip/fuse which is too low.

 

Edit: perhaps we can have this discussion and then delete the discussion or pin TTT's post as a new post since I think it is a valuable resource and I was hesitant to comment on it as now I have opened a can of worms and a elegant thread like this needs to be presented "clean" to newbies.

 

One of the benefits (apart from re-usability) of circuit breakers is that they have two mechanisms for dealing with over currents.

Circuit breakers have a slower thermal sensor for normal over currents, this would be the typically the breakers rated current , i.e. 40A.

This slower trip follows a curve so the higher the over amp, the shorter time is will trip, also dependent on outside temperature.

They also have a fast magnetic trip for short circuits, typically rated 2.5kA or 6kA for domestic breakers. Though this short circuit refers to an short between conductors.

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I think I need to start updating here, consolidating it all, for new solar people.

 

Want to go as far as putting down parts locally available, that one can just go and buy based on calcs of your installation.

One step further is maybe to get the parts sourced for forum members?

 

We keep this thread open for discussion on what is the best, we then open a new thread that no-one can add to, bar one editor, where we then list the salient info clean and tidy, as we agreed on here.

 

What say you all?

Who can bring what to the table.

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  • 3 weeks later...
  • 3 weeks later...

Hi All.,

Should  we all put our heads together and start hammering out a work sheet for the beginner? Lets Upload a Document here and work on it until it is official to publish.

I will add the completed version to the new "Downloads Section"

We can break them up in "Books"

Such as .

Book 1: How to size MPPT

Book2: How to install Solar Panels

And so on.

I will upload the books to the downloads section. Lets start off on hows to size MPPTS so long. Might be a good one?

What you guys think?

Jason

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My first reaction is: There are thousand of books and PDF's on the web.

Initial idea was to get the core down, as I started at the top, for in the end, following another site's info, it always comes down to a few questions, then calcs.

At THAT point you are either smiling, really get into it ... or give up. :D

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The one follows the other:
Terms
What is your load and expected run-times
Battery Calculations for the above
Panel Calculations to charge the batteries and power the load
Wire Calculations for connecting it all up
Fuse Calculations to protect your system
 

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