question about wiring panels in series

[Dave S]

I am new to solar. If I had 2-300 watt/12 volt solar panels, and I wired them in series...Would I would obtain 600 watts/ 24 volts? ...would the resulting amperage be 25 amps? a=600w/24V...and if I then wanted to run a Straight DC system, no inverter, would I be able to expect the 2 panels would produce 600 watts and 25 amps of power for DC devices? Say I had lights and fans, all DC, all 24v rated power, and those devices all together added up to 500 watts, could those 2 panels produce the power to drive those devices? If not help me to understand my incorrect thinking and explain the math. Thank you.

 

[Horsefly]

Watts are watts, independent of the voltage (mostly). That is, two 300W panels put together either in parallel or series gives you 600W. Of course, that is the nameplate rating (STC - Standard Test Conditions), which is not what you will see in real life.

 

[solarsimon]

Each 300W panel can produce 25A at 12V
It'll be 25A at 24V when in series. 25*24=600W
That's assuming your loads use all available power. If you have a 24W load, it'll just draw 1A. etc.
Sub optimal sunlight & orientation gives you less.

 

[chrisski]

Say I had lights and fans, all DC, all 24v rated power, and those devices all together added up to 500 watts, could those 2 panels produce the power to drive those devices? If not help me to understand my incorrect thinking and explain the math. Thank you.

You’d want a battery as a buffer to keep them running when the clouds go overhead.

Also, you’d want a charge controller to keep the voltage constant, which the charge controller will require a battery.

The other reason you’d want a battery is those panels will require assistance to meet that wattage except for a cloudless day around noon. Other than those couple of hours, you’ll fall short of your 400 watts.

 

[400bird]

Are these theoretical panels? Or do you have a spec sheet? I've never heard of a panel outputting 25 amps.

Most "12 volt" panels will output some higher voltage.

To answer your question:
Wired in series, add up the voltage (current remains constant)
Wired in parallel, add up the current (voltage remains constant)
Watts = volts x amps

So, wires in series, or parallel, or any combination, you can always add up the watts.

 

[Supervstech]

Yeah, let's get the panel specs before we go down the rabbit hole with theory

 

[Bud Martin]

I am new to solar. If I had 2-300 watt/12 volt solar panels, and I wired them in series...Would I would obtain 600 watts/ 24 volts? ...would the resulting amperage be 25 amps? a=600w/24V...and if I then wanted to run a Straight DC system, no inverter, would I be able to expect the 2 panels would produce 600 watts and 25 amps of power for DC devices? Say I had lights and fans, all DC, all 24v rated power, and those devices all together added up to 500 watts, could those 2 panels produce the power to drive those devices? If not help me to understand my incorrect thinking and explain the math. Thank you.

Can we see the spec of this 300W 12V panels? How did you figure out that they are 12V panels?

 

[time2roll]

Yes the math is correct and will run the items as described.

 

[Dave S]

Thank you one and all for your help. I appreciate your time and help more than you know. Some background ... I am recently retired. There is an organization called Schools for Haiti that has about 12 schools and about 2500 kids...The person who started the ministry when he retired did everything out of his own pocket, and the ministry grew, now every penny of donations go to Haiti, so he's a guy I want to help. He asked me if I could help with putting solar into his schools. So I am learning...I'm a former master electrician, so I am not totally clueless, only mostly clueless. The only needs are lighting, ceiling fans, and to charge laptop computers. So I figure why not stay at 24 V DC, use DC fans and lights, and use buck transformers to step down 24 V to 19.5 to charge computers. The first building he would like power in is 100' x 50', two story, 20 rooms. I have designed 10 circuits, 400 watts each, need power for 6 hours a day, so I figure 7 hours for margin, which gives me 28,000 Wh a day I need to build a system for. Not a lot of use of the building at night, so I do not have to have a whole lot of battery backup. Area is always sunny. Rains at night. Gets 7 sun hours a day in winter, 12 in summer. Here is where I really covet your help. Am I thinking this out correctly? I'm just using ballpark figures. 2-350 watt solar panels, would produce 700 watts, and at 7 hours, would produce 4900 watthours of power at day at 24 volts? So if I need 28,000 Wh/day, and 2 panels in series produce 4900 Wh/day, am I correct in thinking 12 panels would potentially give me the 28,000 Wh? I know I would need more panels for margin...I am just trying to learn, and need to know if I am thinking this through correctly. Thank you all once again.

 

[400bird]

A noble cause, hopefully I can help.

Solar panels only produce rated wattage in lab conditions. Plan on 60-80% of the name plate rating.

There are online calculators that will help estimate actual output. 7 hours of sun does not equal 7 hours of perfect production as the sun raises and set the output makes a bell curve sort of shape.

How important is it that the school has power every day? If you design at the bare minimum, a cloudy day would mean no lights and no laptops.

Also, if you want to stick with DC power distribution, wouldn't 12 volts be a better choice for product availability? I've never heard of a 24 volt LED light. And you can get 12v boost converters as laptop chargers. They're pretty common up here as they plug into a cigarette lighter.


I've got one last consideration. You're going to need a charge controller, you can't hook the panels directly to the battery. As such, you aren't limited to having 24 volts (or 2* 12 volt) panels.
You can get higher voltage charge controllers to save tons of wiring on the roof. Also, larger panels (with high voltage) will require significantly less racking and wiring from the roof to the charge controller.

 

[MisterSandals]

I'm just using ballpark figures. 2-350 watt solar panels, would produce

I'd check to see what the biggest and cheapest (cost per watt) panels are in your area. Before buying a controller, let us do a sanity check to make sure the electrical specs match up with SCCs that you are considering.

Buying plenty of cheap solar panels is good place to maximize production at the lowest cost.

 

[zanydroid]

24V LED is a standard voltage for

I've never heard of a 24 volt LED light

24V is a standard LED strip voltage, and superior to 12V for higher power / longer wiring run situations.

 

[Dave S]

Thank you all. I have bought a couple 100 w solar panels, 24 volt charge controller, small 12 v 14 Ah batteries so I can wire them in series/parallel, 24 volt fan, 24 volt LED, buck transformer and other stuff I need so I can do some testing and try to teach myself. I'm in education mode right now. I am trying to duplicate on a small scale what I might want to build in Haiti. I am in contact with a solar distributor in Haiti where I will buy my equipment. The solar panels I will have to buy are from a company called LAC Solar. Shipping to Haiti is prohibitive, and the country is in such turmoil I'm not sure what I ship will make it there anyway. So I am at their mercy. I contacted them about what a system would cost based on what I designed for power requirements and I got a price of $61,000 US. So, that is why I think I need to learn all I can and try to do it myself. No way $61K is available to the ministry. Of course they were going to use inverters...They told me I needed 48- 350 watt solar panels...That is why I have so many questions...what am I missing where I think I can build a system with around 20 panels and they say i need almost 50... I just think--and I am open to you telling me I am wrong--that by staying at DC I can do it cheaper...a lot cheaper. DC ceiling fans so much more efficient...the 24 Volt LED throws a lot of light. I am thinking 24 volt over 12 volt because of wire sizing.

 

[MisterSandals]

I am thinking 24 volt over 12 volt because of wire sizing.

There are a lot of unanswered questions but a system with 20x 350W panels would need to run at 24V or probably 48V.

Which brings up the question about batteries. How much power do your require outside of solar charging hours (the other 19h of the day)?

Just messing with numbers...
20 panels x 350W = 7500W
7500W / 28V charging = 268A (yow, that's a lot)

 

[zanydroid]

That’s a pretty high price for materials.

$61000/(48*350) = $3.6 Wdc, which is a full service solar only install with premium materials in the US. I would have expected $1/Wdc with lower regulations and construction standards. It’s even $1/Wdc in Australia

That price needs to include batteries, or factor in risks/extra tariffs of doing business down there.

With DC you need more wires to compensate for the voltage and some appliances are likely more expensive. You anyway need some kind of power electronics to charge batteries. Generic Chinese string off grid inverters really aren’t that expensive. I would imagine the relative cost between different kinds of power electronics from China would be the same there as up here.

 

[Dave S]

Thank you, Gents. I love all you are teaching me. Looking for one of you to show me where I am wrong. The 24 V LED I have tested use 18 watts. The DC fans use 36 watts. So I designed the system so that I allowed 20 watts per light, and 40 watts per fan, plan on piping them all together in a circuit (emt) to add up to 400 watts per circuit. Then for battery capacity, since 400w/24v=16.6 A per circuit, then give each circuit a 200Ah of battery capacity, which 200Ah/16.6=12+ hrs, which would cover for the occasional cloudy day or occasional night-time usage. I would wire batteries in series parallel to allow 24v and 200Wh per circuit. For laptop charging, I would allow 40 watts per computer, so my charging station would charge 10 at a a time per circuit. Same battery capacity. So, in the following, am I wrong? 10 circuits, each circuit using 400 watts...400 watts x 7 hours = 2800 Wh per day--2 solar panels, 350 watts each, producing 700 watts x 7 hours = 4900 Wh per day. So if I had 2 - 350 watt solar panels per circuit would they produce 4900 Wh, and since I need 2800Wh, that is enough power to provide my needs for power and charge my batteries? I am told the sun is always out, in winter they get 7 sun hours in winter, in summer 12 sun hours per day. And then, if I am correct, since I don't really know what I am doing, any suggestions on how you personally would build the system, or resources that would give me instruction? Thanks for your time and thought into my silly little problem. I truly appreciate it.

 

[Rocketman]

I think you should make a smaller “unit” install it and then use real life measurements to decide on the final.

According to what you said, I understand you will not need a very big battery- mostly it will be harvest and use immediately.

Because you will need a lot of solar charge controllers, it may be better to use 48v for the solar and battery- then drop the voltage down to 24v for lights and 19v for laptops.

I am familiar with Victron gear (it’s what I use in my MotorHome) so this is the way I would set it up.

This is one “unit”.

48v batteries. (This item needs a lot of thought - Lithium’s would probably be best - but flooded lead might be what is available locally and would not get taken).

Victron mppt 250/100 Solar Charge Controller. It will handle 5800w of solar (round up and overpanel to about 6000w). $950

I would look at used solar panels. Suntan solar has used 255w panels for $71. 24 panels @ 255w = 6,120w. In Haiti you could install those as 6s4p. Voc= 224v $1704

Combiner box and wires
Racking (could this be locally sourced)?

Battery Monitor- use a shunt based like Victron BMV712 or Smartshunt

Also use Victron Battery Protect. This will shutdown the voltage at a level you pick, so you set it above where damage happens on the batteries. BP-100 48v $150. Use one on the fan circuit - so if batteries get low fans stop. Then another on computer/light circuit.

48v to 24v converter
48v to 19v converter
Fuses - don’t forget those!

For the first unit, install a Victron Cerbo - you can then track (from home on the internet) how this first unit is working.

You could find… you need another row of panels going to 6s5p, because the used panels are not preforming as well as you wished. (That may not be a bad idea to start with). Clouds, dirt on panels,etc.

You could find that you can extend this unit to more classes - they don’t use as much power as you originally thought or they use more power than planned.

You could find you need a bigger battery.

It will take four or five “units” for the school.

Good Luck on your project!

 

[400bird]

I would look at used solar panels. Suntan solar has used 255w panels for $71. 24 panels @ 255w = 6,120w. In Haiti you could install those as 6s4p. Voc= 224v $1704

I doubt there is much availability of used panels on an island in the path of hurricanes. I don't know what international shipping of used panels looks like, but I'd expect to spend at least 2-3 times the cost of the panels to get them there.

 

[Dave S]

I think you should make a smaller “unit” install it and then use real life measurements to decide on the final.

According to what you said, I understand you will not need a very big battery- mostly it will be harvest and use immediately.

Because you will need a lot of solar charge controllers, it may be better to use 48v for the solar and battery- then drop the voltage down to 24v for lights and 19v for laptops.

I am familiar with Victron gear (it’s what I use in my MotorHome) so this is the way I would set it up.

This is one “unit”.

48v batteries. (This item needs a lot of thought - Lithium’s would probably be best - but flooded lead might be what is available locally and would not get taken).

Victron mppt 250/100 Solar Charge Controller. It will handle 5800w of solar (round up and overpanel to about 6000w). $950

I would look at used solar panels. Suntan solar has used 255w panels for $71. 24 panels @ 255w = 6,120w. In Haiti you could install those as 6s4p. Voc= 224v $1704

Combiner box and wires
Racking (could this be locally sourced)?

Battery Monitor- use a shunt based like Victron BMV712 or Smartshunt

Also use Victron Battery Protect. This will shutdown the voltage at a level you pick, so you set it above where damage happens on the batteries. BP-100 48v $150. Use one on the fan circuit - so if batteries get low fans stop. Then another on computer/light circuit.

48v to 24v converter
48v to 19v converter
Fuses - don’t forget those!

For the first unit, install a Victron Cerbo - you can then track (from home on the internet) how this first unit is working.

You could find… you need another row of panels going to 6s5p, because the used panels are not preforming as well as you wished. (That may not be a bad idea to start with). Clouds, dirt on panels,etc.

You could find that you can extend this unit to more classes - they don’t use as much power as you originally thought or they use more power than planned.

You could find you need a bigger battery.

It will take four or five “units” for the school.

Good Luck on your project!

Thank you!!

 

[Dave S]

I doubt there is much availability of used panels on an island in the path of hurricanes. I don't know what international shipping of used panels looks like, but I'd expect to spend at least 2-3 times the cost of the panels to get them there.

Thank you...Haiti is a place no one wants to send their ship to right now. I think I am stuck buying panels and batteries in Haiti, but the other stuff I think I can carry on as extra luggage