This is a question I often see asked here, and it is a difficult question to answer, there are a number of key factors affecting the answer. First how much power do you plan to use, then also how much power can you store in your battery.
In the following I will try to provide a high level guide, I will do my best to avoid making it technical.
You want solar to provide power to charge your batteries, thus allowing you to use electricity for the things you need basically overnight. Thus you need to have enough battery capacity to be able to store enough energy to keep you going overnight. Indeed most people would like to have enough to keep them going for two or three nights (allows for bad weather).
So the first place to start is to understand how much energy you are going to use in a typical 24 hour period, you would measure this in amp hours (one amp hour is taking one amp of current from your battery for one hour). I will not go into how to calculate your usage here, but for the purpose of the example below lets assume an estimated usage of 40ah per day.
Now the amount of battery you need depends on when your power usage is, for this exercise lets assume it is all between sunset and sunrise, so we need to be able to provide all 40ah from the battery.
How big does the battery need to be?
That depends on the type of battery, lets assume it is a simple lead acid, where you do not want to discharge it below 50% charged, and you typically can not charge it past about 92 or 93% in the length of a solar day, thus a 100ah battery will typically yield about 42ah a day.
Thus with a 40ah per day usage, and a lead acid battery you would just make it through one night with 100ah, you would be better off with at least 200ah to get you close to two days. For the purpose of the example, lets assume we go with 200ah of lead acid battery, we aim to discharge the battery no lower than 50% and we want to build our system to get the battery back above 90% in a good sunny day. To refill the battery we will need to supply at least 80Ah.
How Much solar do I need?
I would assume five productive hours of sunlight per day, thus we need to divide the power we want to generate by 5, and that tells us how much we need to put in our battery each hour.
Now batteries do not charge in a linear fashion, thus we can put more in early in the charge dropping down to just a trickle when they get closer to full, so in reality we need to provide more to be able to get close to filling the battery. So I would recommend building a system capable of providing a quarter of our total power requirement each hour, remember it is always better to have too much power.
So in our example we need to supply at least 80ah, I would plan to put 20ah into the battery each hour, that means we need 20amps of current from going into the battery.
In a perfect world that would mean we needed 240 watts of solar (20 * 12 = 240, being Amps * Volts = Watts).
Unfortunately solar sitting flat on a caravan roof is not perfect, so I would assume the solar is only 70% efficient, and we would need to increased how much solar we need to compensate for this.
So now our 240W of solar panel needs to increase to about 340W (240 *100/70 = 342).
Confused?
It is not really that bad, just start out by working out what you need/want to run, how much power they take, and multiply it by the number of hours you want to run it for. Example Laptop charger 2.5 hours at 6amps = 15amp hours), then add them all together.
Then work through the story above, I am not able to tell you the percentage of useable power from the different common batteries being AGM and Gel, (I do not use them, I only use Lithium). But I am sure someone else will provide that information.
Good luck with working it out, and please remember it is always better to have more solar than you need rather than less.
rockylizard said
09:46 AM Jul 19, 2015
Gday...
Thanks for the info Plendo - that will be helpful.
May I offer the following "Solar Calculator" for interest.
I was fortunate to come across it from a helpful 'expert' when I was thinking about getting solar and found it handy.
It can be used as a guide to quantify what you will consume, what you will need to generate, and for length of time you can last.
I would add the following comments...
There are other aspects that influence the amount of solar you might need if you wish to be totally reliant on it.
1. The length of the day.
Short days give less charge time. The days are longer in Tasmania in the summer than they are in the north of Australia.
We are currently in northern Norway. At midnight last night we were still getting a tiny charge from the solar panels because the sun does not set.
2. The temperature.
Increased temperature makes the compressor fridge need to run longer, increasing its power consumption.
Higher temperatures also reduce the output of the solar panels.
3. Cloud cover.
Clearly, significant cloud cover will reduce the solar radiation and reduce the power the panels will produce.
The worst circumstance that you will come across is in the tropics when the sun is not shining. The solar output will be adversely affected by everything above.
The bottom line is that you can not have too much solar.
It has never been cheaper than it is now, so install a larger controller than you think you will need and as many panels as you can fit.
How much you need is determined by the bad days, not the good days.
Cheers,
Peter
Reed said
02:44 AM Jul 20, 2015
Plendo - excellent post
There is some very good information in the above posts. We have been solar autonomous for two years
Much happier when the terminology used is in watts and W-hrs since the US seems to use 12 V battery banks and others use 24 V banks. We utilize a 48 V nominal LFP suite. We are currently mootchdocking at older son's place in mountains of northern New Mexico at 7800' (2500 m) since Elaine just had total knee replacement (1 July) and so are not traveling as usual. The temperatures are moderate with a maximum of 32 C during day and 15 C at night. We are having sunny mornings and cloudy afternoon (it is the Monsoon season in New Mexico). TriStar MPPT-45 shows we are getting up to 1400 W insolation around noon and a maximum harvest of about 8500 W-hrs on cloudless days. We are getting about 6500 W-hrs currently but we do enjoy the cooler afternoons.
Our usage is different from most caravaners since we run the Dometic fridge on solar/battery suite /inverter during the day and at night on battery/inverter at night when we know it will be a sunny day following. This does gives us a deficit of around 3 kW-hrs at dawn. We utilize the AC for water heating when it is a sunny day. We change our energy management SOP when it is overcast or we are "camped" in the shade. Then everything is run on propane/butane and the inverter is turned off when not required. The Magnum 4 kW PSWI uses about 60 W just sitting there.
It is a most slippery slope when you decide to go to solar, and soon you will go over to the Dark Side, the more you have the more you want. So it is a good idea to wire your system for more solar panels (may wind up with entire roof of caravan filled with panels) and to get an MPPT controller quite a bit in excess of original plans.
Our power comes off the roof at 90 V so that 1400 W is only 15 amps. The battery bank is 48 V nominal and requires a converter (a 508 W Mean Well 48V to 12 V converter for us). Higher voltage means smaller cabling and controller/longer run from panels to controller
We have a friend who will wind up with 2 kW of panels on his camper shell/motorcycle trailer combo. The trailer has its own battery bank and inverter. Even he is not quite sure why he will have so much power, guess it just seemed like a good idea since panels are so cheap.
Reed and Elaine
03_Troopy said
10:14 AM Jul 20, 2015
And thanks go to Richard Graylin for that spreadsheet. I have used it myself for a while now, and it's useful for simulating the effects for different numbers of useful hours of sun per day. You can play around with battery sizes and total watts of panels and see the effect it has on your battery state of charge and you can see the rough number of days you can spend off grid in different sunshine situations. Now, if only we could predict the weather accurately we'd be set.
Good article too Plendo
dogbox said
07:36 AM Aug 16, 2015
Cheers,
Peter
the days are longer in Tasmania in the summer than they are in the north of Australia.??????
Ondabeach said
09:08 AM Aug 16, 2015
Yep, Peter's right dogbox. For example, this year on the summer solstice which is December 22nd, in Hobart the day will be 15 hours, 21 minutes and 06 seconds long. In Darwin on the same day, the day will only be 12 hours, 15 minutes and 42 seconds long.
Solar power however is more about the angle of the sun in relation to the earth's surface where you are than about the time of exposure. So, even though the day is shorter in Darwin, you'll still get a lot more power into your batteries there than in Hobart. Angling your solar panels will help a bit, but the main problem is the distance the light has to pass through the atmosphere which greatly diminishes the sun's power. So in Darwin, during the 4 hours between 10am and 2pm you'll get more power than the whole day in Hobart.
Bottom line is that the closer you are to the equator, the more power you'll get in a day.
dogbox said
10:10 AM Aug 26, 2015
good thing it wasn't a trivia question
SnowT said
01:16 AM Sep 25, 2015
Peter this is Not a dig..
Peter_n_Margaret wrote:
I would add the following comments... There are other aspects that influence the amount of solar you might need if you wish to be totally reliant on it.
1. The length of the day. Short days give less charge time. The days are longer in Tasmania in the summer than they are in the north of Australia. We are currently in northern Norway. At midnight last night we were still getting a tiny charge from the solar panels because the sun does not set. -- The Further South you go,as the solar panels are sitting Flat on the Roof they are more inefficient as in the factor of energy.. The further North the more power you get.. In a perfect world the Solar should be hitting the Panel at 90 degree's. So unless you can angle your panels to the sun you Don't get much extra power..
2. The temperature. Increased temperature makes the compressor fridge need to run longer, increasing its power consumption. Higher temperatures also reduce the output of the solar panels. --Yes Temperature is a factor in any system.. One of the big one's and forget about the extra fridge usage.. think about the Heating effect of the sun on the panel's taking them about the Lab tested 25 degree's.. As Panel heat up they lose efficiency.. it need to have the ability to have airflow under them.. and Most systems are fixed to the roof with Minimal Air gap..
3. Cloud cover. Clearly, significant cloud cover will reduce the solar radiation and reduce the power the panels will produce.
-- Now Cloud cover is a tricky one.. It all come down to the lvl of cloud cover... For example.. our 5.4kw system on our house roof was still giving us 2.4kw of power on a grey afternoon.. NO SUN..
- Now if the day has those lovely fluff white clouds, the solar gain can actually be Excellent.. even better than the day of say NO cloud cover at all.. If the day is cool the system can be nice.. But Ill personally take a day of fluffy clouds and cool wind..
The worst circumstance that you will come across is in the tropics when the sun is not shining. The solar output will be adversely affected by everything above. The bottom line is that you can not have too much solar. Peter this is where I personally think Most people fall over, NOT enough Solar.. It has never been cheaper than it is now, so install a larger controller than you think you will need and as many panels as you can fit. How much you need is determined by the bad days, not the good days. and your Expected Load's..
Cheers, Peter
the worst thing overall is Not sizing the system Correctly..
How much solar should you have ?
This is a question I often see asked here, and it is a difficult question to answer, there are a number of key factors affecting the answer. First how much power do you plan to use, then also how much power can you store in your battery.
In the following I will try to provide a high level guide, I will do my best to avoid making it technical.
You want solar to provide power to charge your batteries, thus allowing you to use electricity for the things you need basically overnight. Thus you need to have enough battery capacity to be able to store enough energy to keep you going overnight. Indeed most people would like to have enough to keep them going for two or three nights (allows for bad weather).
So the first place to start is to understand how much energy you are going to use in a typical 24 hour period, you would measure this in amp hours (one amp hour is taking one amp of current from your battery for one hour). I will not go into how to calculate your usage here, but for the purpose of the example below lets assume an estimated usage of 40ah per day.
Now the amount of battery you need depends on when your power usage is, for this exercise lets assume it is all between sunset and sunrise, so we need to be able to provide all 40ah from the battery.
How big does the battery need to be?
That depends on the type of battery, lets assume it is a simple lead acid, where you do not want to discharge it below 50% charged, and you typically can not charge it past about 92 or 93% in the length of a solar day, thus a 100ah battery will typically yield about 42ah a day.
Thus with a 40ah per day usage, and a lead acid battery you would just make it through one night with 100ah, you would be better off with at least 200ah to get you close to two days. For the purpose of the example, lets assume we go with 200ah of lead acid battery, we aim to discharge the battery no lower than 50% and we want to build our system to get the battery back above 90% in a good sunny day. To refill the battery we will need to supply at least 80Ah.
How Much solar do I need?
I would assume five productive hours of sunlight per day, thus we need to divide the power we want to generate by 5, and that tells us how much we need to put in our battery each hour.
Now batteries do not charge in a linear fashion, thus we can put more in early in the charge dropping down to just a trickle when they get closer to full, so in reality we need to provide more to be able to get close to filling the battery. So I would recommend building a system capable of providing a quarter of our total power requirement each hour, remember it is always better to have too much power.
So in our example we need to supply at least 80ah, I would plan to put 20ah into the battery each hour, that means we need 20amps of current from going into the battery.
In a perfect world that would mean we needed 240 watts of solar (20 * 12 = 240, being Amps * Volts = Watts).
Unfortunately solar sitting flat on a caravan roof is not perfect, so I would assume the solar is only 70% efficient, and we would need to increased how much solar we need to compensate for this.
So now our 240W of solar panel needs to increase to about 340W (240 *100/70 = 342).
Confused?
It is not really that bad, just start out by working out what you need/want to run, how much power they take, and multiply it by the number of hours you want to run it for. Example Laptop charger 2.5 hours at 6amps = 15amp hours), then add them all together.
Then work through the story above, I am not able to tell you the percentage of useable power from the different common batteries being AGM and Gel, (I do not use them, I only use Lithium). But I am sure someone else will provide that information.
Good luck with working it out, and please remember it is always better to have more solar than you need rather than less.
Gday...
Thanks for the info Plendo - that will be helpful.
May I offer the following "Solar Calculator" for interest.
I was fortunate to come across it from a helpful 'expert' when I was thinking about getting solar and found it handy.
It can be used as a guide to quantify what you will consume, what you will need to generate, and for length of time you can last.
Cheers - John
There are other aspects that influence the amount of solar you might need if you wish to be totally reliant on it.
1. The length of the day.
Short days give less charge time. The days are longer in Tasmania in the summer than they are in the north of Australia.
We are currently in northern Norway. At midnight last night we were still getting a tiny charge from the solar panels because the sun does not set.
2. The temperature.
Increased temperature makes the compressor fridge need to run longer, increasing its power consumption.
Higher temperatures also reduce the output of the solar panels.
3. Cloud cover.
Clearly, significant cloud cover will reduce the solar radiation and reduce the power the panels will produce.
The worst circumstance that you will come across is in the tropics when the sun is not shining. The solar output will be adversely affected by everything above.
The bottom line is that you can not have too much solar.
It has never been cheaper than it is now, so install a larger controller than you think you will need and as many panels as you can fit.
How much you need is determined by the bad days, not the good days.
Cheers,
Peter
There is some very good information in the above posts. We have been solar autonomous for two years
Much happier when the terminology used is in watts and W-hrs since the US seems to use 12 V battery banks and others use 24 V banks. We utilize a 48 V nominal LFP suite. We are currently mootchdocking at older son's place in mountains of northern New Mexico at 7800' (2500 m) since Elaine just had total knee replacement (1 July) and so are not traveling as usual. The temperatures are moderate with a maximum of 32 C during day and 15 C at night. We are having sunny mornings and cloudy afternoon (it is the Monsoon season in New Mexico). TriStar MPPT-45 shows we are getting up to 1400 W insolation around noon and a maximum harvest of about 8500 W-hrs on cloudless days. We are getting about 6500 W-hrs currently but we do enjoy the cooler afternoons.
Our usage is different from most caravaners since we run the Dometic fridge on solar/battery suite /inverter during the day and at night on battery/inverter at night when we know it will be a sunny day following. This does gives us a deficit of around 3 kW-hrs at dawn. We utilize the AC for water heating when it is a sunny day. We change our energy management SOP when it is overcast or we are "camped" in the shade. Then everything is run on propane/butane and the inverter is turned off when not required. The Magnum 4 kW PSWI uses about 60 W just sitting there.
It is a most slippery slope when you decide to go to solar, and soon you will go over to the Dark Side, the more you have the more you want. So it is a good idea to wire your system for more solar panels (may wind up with entire roof of caravan filled with panels) and to get an MPPT controller quite a bit in excess of original plans.
Our power comes off the roof at 90 V so that 1400 W is only 15 amps. The battery bank is 48 V nominal and requires a converter (a 508 W Mean Well 48V to 12 V converter for us). Higher voltage means smaller cabling and controller/longer run from panels to controller
We have a friend who will wind up with 2 kW of panels on his camper shell/motorcycle trailer combo. The trailer has its own battery bank and inverter. Even he is not quite sure why he will have so much power, guess it just seemed like a good idea since panels are so cheap.
Reed and Elaine
Good article too Plendo
Cheers,
Peter
the days are longer in Tasmania in the summer than they are in the north of Australia.??????
Yep, Peter's right dogbox. For example, this year on the summer solstice which is December 22nd, in Hobart the day will be 15 hours, 21 minutes and 06 seconds long. In Darwin on the same day, the day will only be 12 hours, 15 minutes and 42 seconds long.
Solar power however is more about the angle of the sun in relation to the earth's surface where you are than about the time of exposure. So, even though the day is shorter in Darwin, you'll still get a lot more power into your batteries there than in Hobart. Angling your solar panels will help a bit, but the main problem is the distance the light has to pass through the atmosphere which greatly diminishes the sun's power. So in Darwin, during the 4 hours between 10am and 2pm you'll get more power than the whole day in Hobart.
Bottom line is that the closer you are to the equator, the more power you'll get in a day.
the worst thing overall is Not sizing the system Correctly..
Juergen