what charge controller do I need for a 250watt panel

[russ]

Where are you getting 5.6 Sun Hours from.

If you need 4 Kwh per day and truly have 5.6 Sun Hours worse case then yeah you got it wrong. [4 Kwh x 1.5] / 5.6 SH = 1071 watts.

Ghana

[fopoku2k2]

Where are you getting 5.6 Sun Hours from.

I am getting 5.6 sun hours from http://goo.gl/5O4zGQ . the actual reading is for GHANA as @russ stated.

If you need 4 Kwh per day and truly have 5.6 Sun Hours worse case then yeah you got it wrong. [4 Kwh x 1.5] / 5.6 SH = 1071 watts.

the 4.0 Kwh I gave already includes the fudge factor.

[Sunking]

I am getting 5.6 sun hours from http://goo.gl/5O4zGQ . the actual reading is for GHANA as @russ stated.



the 4.0 Kwh I gave already includes the fudge factor.

Is 5.6 worse case month? For off grid you design on shortest Sun Hours for a month. In my part of the world is December/January. You cannot use yearly average for battery systems. If you do you go dark in any month less than average.

[Sunking]

the 4.0 Kwh I gave already includes the fudge factor.

Then how do explain 714 watts?

[4 Kwh x 1.5] / 5.6 = 1071 Watts, not 714. Round that up to 1100 or even better 1200 and use 200 watt panels. Do not use a Prime number of panels like 5, 7, 11.....

Here is the deal. If you are fortunate enough to live in a area that actually gets 5.6 Sun Hours in the shortest month, the design process changes for panel wattage calculations. Once you go above 4.4 Sun Hours you fail to meet the C/12 minimum charge rate of FLA batteries.

That leaves you two options.

1. Use AGM batteries which do not need a minimum charge rate.
2. Provide a panel wattage that meets the minimum C/12 charge requirement. Put another way you over size the panel wattage to get to C/12 minimum requirement.

Don't be fooled into think Going AGM will be less expensive because you would be dead wrong. AGM batteries cost twice as much as FLA and only last half as long. Much less expensive to up the panel wattage and charge controller size. Doing that also means a lot less generator run time during the year.

So for example if you require 4 Kwh per day or a 24 volt 800 AH battery minimum panel wattage is 1700 watts and a 63 amp CC. Odd numbers so go to 1800 watts and 80 amp CC. That gives you room to grow.

Better yet and less expensive is 48 volt battery @ 400 AH, 1800 watt panel with 40 amp MPPT controller. Still the same battery capacity and panel wattage, but a much smaller less expensive controller requiring smaller wiring.

You got to cover all angles to find the least expensive route.

[Bucho]

Wrong, not if the batteries are discharged enough to demand full power, they are not above nominal voltage. The voltage rise is determined by Ri plus OCV. 16 amps into .001 ohms is only .016 volts. Take that Bucho. Your math needs a lot of work.

I don't think the problems my math, issue is that this imaginary system of yours is fubar.

I mean 24v at noon on a bright sunny day isn't something he should be seeing unless he's done something wrong or the neighbors played a trick on him by covering his panels with a tarp all morning.

[Bucho]

Do you think I might have made a mistake using sunkings calculations?

Sunking explained it, hopefully that makes sense now.

P.S. Do you really need to use that desktop so much when the grid's down, could you use your laptop instead?

[Sunking]

I don't think the problems my math, issue is that this imaginary system of yours is fubar.

I mean 24v at noon on a bright sunny day isn't something he should be seeing unless he's done something wrong or the neighbors played a trick on him by covering his panels with a tarp all morning.

When the battery reaches 80% 12.4 volts, charge controllers go into Constant Voltage mode. CV is well below full power or Constant Current mode. That is no FUBAR or SNAFU, it is simple math.

[Bucho]

When the battery reaches 80% 12.4 volts, charge controllers go into Constant Voltage mode. CV is well below full power or Constant Current mode. That is no FUBAR or SNAFU, it is simple math.

Yes that's nice we've all heard of absorb, but it doesn't explain how you've managed to f this imaginary system of yours up
so badly that it's just managing to get up to 50% DOD at high noon on a beautiful sunny day.

[paulcheung]

Yes that's nice we've all heard of absorb, but it doesn't explain how you've managed to f this imaginary system of yours up
so badly that it's just managing to get up to 50% DOD at high noon on a beautiful sunny day.

Easy, with 3 days on rainy season without sun, the SOC of the battery gone straight down to 30%. one morning can't charge it up to 80% SOC.

[Bucho]

Easy, with 3 days on rainy season without sun, the SOC of the battery gone straight down to 30%.

Yep, that would be fubar all right. Don't let that happen, run a generator first.

Which is the same thing Sunking would recommend when he isn't giving a sales pitch for MPPT.

[paulcheung]

Yep, that would be fubar all right. Don't let that happen, run a generator first.

.

If you fire up the generator any day is raining then what is the sense to size the battery for five days reserve?

Actually most off grid people in Jamaica do the two days capacity for the battery, so the system cost will be much less as the battery bank s smaller, we don't afraid to fire up the generator when is raining.

[fopoku2k2]

Then how do explain 714 watts?

[4 Kwh x 1.5] / 5.6 = 1071 Watts, not 714. Round that up to 1100 or even better 1200 and use 200 watt panels. Do not use a Prime number of panels like 5, 7, 11.....

Here is the deal. If you are fortunate enough to live in a area that actually gets 5.6 Sun Hours in the shortest month, the design process changes for panel wattage calculations. Once you go above 4.4 Sun Hours you fail to meet the C/12 minimum charge rate of FLA batteries.

That leaves you two options.

1. Use AGM batteries which do not need a minimum charge rate.
2. Provide a panel wattage that meets the minimum C/12 charge requirement. Put another way you over size the panel wattage to get to C/12 minimum requirement.

Don't be fooled into think Going AGM will be less expensive because you would be dead wrong. AGM batteries cost twice as much as FLA and only last half as long. Much less expensive to up the panel wattage and charge controller size. Doing that also means a lot less generator run time during the year.

So for example if you require 4 Kwh per day or a 24 volt 800 AH battery minimum panel wattage is 1700 watts and a 63 amp CC. Odd numbers so go to 1800 watts and 80 amp CC. That gives you room to grow.

Better yet and less expensive is 48 volt battery @ 400 AH, 1800 watt panel with 40 amp MPPT controller. Still the same battery capacity and panel wattage, but a much smaller less expensive controller requiring smaller wiring.

You got to cover all angles to find the least expensive route.

Sorry for disappearing on you for a while. Got some family emergency. My actually daily requirement is about 2,500 wh.

2,500 wh x 1.5 = 3,750 wh rounded up to 4,000 wh = 4.0 kwh

then 4,000 wh / 5.6 h = 714 watts

[fopoku2k2]

Sunking explained it, hopefully that makes sense now.

P.S. Do you really need to use that desktop so much when the grid's down, could you use your laptop instead?

I usually run my plex media server from my desktop. but do you think the desktop pulls all that 250 watts from its power supply esp when it's idle or normal operations? if not then i guess i would have to go with your suggestion.