Wiring Solar Panels advice please?

[Raul]

We had a compressor coolbox in Benidorm a few years ago, 150w panel was well over the top the battery suffered over night though as I had the coolbox set on -5°c and it ran a lot in the high ambient temperature.

You are talking about over 20 deg latitude and solar irradiation difference is day and night comparison over that distance.
You plan for the weak time, not the good time.

 

[EML]

That’s not how you work out your energy requirements.

I don't want to get teachy here (I've got a Physics degree), but that's exactly how you work out your energy requirements. And it's simply not the case that you need 300W solar to run a compressor fridge. I run on 150W.

When averaged over a number of compressor cycles, fridges (mains or 12V) run at about 12W to 50W average. Sometimes they draw zero power, sometimes they whack up to several times this figure; it doesn't matter, as long as the battery can supply any instantaneous power requirement (easy, but with a minor complication if you're using an inverter), and the solar can put back enough energy to smooth everything out over a 24-hour cycle.

All the solar has to do is provide 12W to 50W average to make sure that the battery stays topped up, or (more realistically) that it drains over a period of a few days, when you have to start driving to charge everything up.

Your figure is 300-400 "Wh" "per day". The units don't really make sense, but I'll assume you mean "energy consumed in arbitrary units of Watt-hours measured over a period of one day", which is (400 * 3600) Joules. If so, then your average power consumption is actually 400/24 = 16.7W, which is pretty good. If you're sure of the 300/400 figure you should post your fridge model as a datapoint.

To find out if your panel can provide 16.7W average to keep your battery topped up you need to know how much the sun shines. At my latitude, we get 0.8 'Standard Test Condition' hours of sun in the winter, and 5-6 hours in the summer. So, if you've got a 150W panel on a bad summer's day (say 4 hours sun), and your panel is pointing at the sun, then you can expect to get maybe 150 x 4 / 24, or maybe 25W average out of your panels. On an average summer day you could expect 34W. You don't need to know anything else: "Amp-hour" figures are always meaningless; they're neither energy nor power.

Of course, real life is more complicated. When you're off-grid the fridge might be no more than 60% of your total power consumption. How long you can stay out depends on your battery rating and how far you can cycle it, how much solar you have, whether it's pointed at the sun, power for heating and pumps, inefficiencies, and so on. I've got a basic spreadsheet if anyone's interested.

 

[cmcardle75]

Very informative post, so all these PVCs sold with compressor fridges, if you haven’t got EHU, generator etc then they are not very good off grid unless you’ve got at least 300W of solar?

The compressor fridge in mine is very good indeed for wild camping. And I have 0W of solar. However, I do have a 30A B2B charger and a 300Ah Lithium setup. I can go almost a week without EHU or driving. However, if I wanted to sit around not moving for a week, I wouldn't have sold the caravan. I rarely stay in one place for more than 3 nights and in the intervening days, I'll do an hour or two's driving anyway.

My calculations showed that it would be better to put the money I had intended to spend on 400W of solar + MPPT into a much larger Lithium battery pack instead. I drive a lot, don't sit on beaches in the Med, but like to go away in the depths of winter, too. For me, a decent B2B gives me more than solar would, as it works year round and the larger Lithium that I bought with the freed up cash is more useful to me.

I did, however, need to sit for 2 weeks earlier this year at a scout camp. This isn't normal for me, I'd normally take a tent, but we needed another 3 tonne tow bar for the trip and the battery was useful for running an old 1970s freezer for a few days before we could get hold of a barn with a socket. I did buy a cheap and nasty 100W polycrystaline panel as well for £55 and connected that via a PWM controller I found in the garage. That was basically just enough to run the compressor fridge, lights and pump. I ended the 2 weeks on about 85% charge.

 

[Raul]

I don't want to get teachy here (I've got a Physics degree), but that's exactly how you work out your energy requirements. And it's simply not the case that you need 300W solar to run a compressor fridge. I run on 150W.

When averaged over a number of compressor cycles, fridges (mains or 12V) run at about 12W to 50W average. Sometimes they draw zero power, sometimes they whack up to several times this figure; it doesn't matter, as long as the battery can supply any instantaneous power requirement (easy, but with a minor complication if you're using an inverter), and the solar can put back enough energy to smooth everything out over a 24-hour cycle.

All the solar has to do is provide 12W to 50W average to make sure that the battery stays topped up, or (more realistically) that it drains over a period of a few days, when you have to start driving to charge everything up.

Your figure is 300-400 "Wh" "per day". The units don't really make sense, but I'll assume you mean "energy consumed in arbitrary units of Watt-hours measured over a period of one day", which is (400 * 3600) Joules. If so, then your average power consumption is actually 400/24 = 16.7W, which is pretty good. If you're sure of the 300/400 figure you should post your fridge model as a datapoint.

To find out if your panel can provide 16.7W average to keep your battery topped up you need to know how much the sun shines. At my latitude, we get 0.8 'Standard Test Condition' hours of sun in the winter, and 5-6 hours in the summer. So, if you've got a 150W panel on a bad summer's day (say 4 hours sun), and your panel is pointing at the sun, then you can expect to get maybe 150 x 4 / 24, or maybe 25W average out of your panels. On an average summer day you could expect 34W. You don't need to know anything else: "Amp-hour" figures are always meaningless; they're neither energy nor power.

Of course, real life is more complicated. When you're off-grid the fridge might be no more than 60% of your total power consumption. How long you can stay out depends on your battery rating and how far you can cycle it, how much solar you have, whether it's pointed at the sun, power for heating and pumps, inefficiencies, and so on. I've got a basic spreadsheet if anyone's interested.

Again , I stand by my comment. You don’t need 12w-50w or whatever for the solar to cover the compressor, you have a buffer, the battery. In real time to be powered directly by the production, it does happen when the battery is full on float. When the panel production is lower that’s the load, the difference comes out the storage, all the time.
I’m ok with your disagreement that you don’t believe a compressor need a 300w solar and you get by with the 150w. You have different conditions. And yes, the solar harvest is calculated in WH or KW. I have done consumption audits many times. In the van is no different same laws of physics.
The 300-400wh makes perfect sense for those that understands energy. It’s industry standard units. And you don’t divide to 24h as that is not true. The fridge cycles, there are times with 0w and times with 40-40w load. Over a day I total 300 winter and 400wh Sumer. Sometimes less, sometimes more. Depending on fridge usage, ambient,etc. The fridge is a dometic crx50, and the freezer is a rebranded mobicool 48l. Both run same compressors and are more or less equal in consumption.
Please don’t take it as offence but, you need to understand solar production. There are simulators on line for solar harvest, by location and time. The data is based on historic solar irradiation and works out prediction of harvest for various tilts, various panels and installation. A panel on the ground mount will produce more than a panel on the roof. I did residential and industrial solar installations few years. I learned few thinks and try to share as much as I can. Some are repeating same mistakes and learn the hard way. But I can not help there. By the way, we live of grid since 2012. Even the house build has been done on solar power. I have data from few years and I know what to expect from a given panel.

 

[autorouter]

Your figure is 300-400 "Wh" "per day". The units don't really make sense,

Watt's the problem? As you say, 1 watt-hour is 3600 joules, so the daily energy consumption (average) can be calculated. Motorhomers, like most-non-physicists, prefer using watt-hours not Joules.

An amp-hour is a perfectly respectable unit of electric charge. Technically since 1 amp is 1 Coulomb per second, 1 amp-hour is 3600 Coulombs. Again like most non-physicists, the amp-hour is the preferred unit of charge. Most of us, and even the electric companies use kWh. All the battery manufacturers quote Amp-hours. Nobody talks about Joules or Coulombs.

As you will realise, the energy carried by one amp-hour of charge is measured in 'watt-hours per amp-hour'. That's too much even for motorhomers, so we all agree to refer to 'watt-hours per amp-hour' as 'Volts'.

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[EML]

Watt's the problem? As you say, 1 watt-hour is 3600 joules, so the daily energy consumption (average) can be calculated. Motorhomers, like most-non-physicists, prefer using watt-hours not Joules.

Yeah, I agree with you, but unfortunately there's a problem. I said:

"Your figure is 300-400 "Wh" "per day". The units don't really make sense, but I'll assume you mean "energy consumed in arbitrary units of Watt-hours measured over a period of one day", which is (400 * 3600) Joules. If so, then your average power consumption is actually 400/24 = 16.7W, which is pretty good."

To which Raul replied:

"The 300-400wh makes perfect sense for those that understands energy. It’s industry standard units. And you don’t divide to 24h as that is not true."

Which I didn't bother replying to. Fair enough, he's the only person who knows what he actually measured as "300-400". However, if he did measure it over one day, then you must divide by 24 to get a figure, in Watts, of 16.7W, which is plausible. OTOH, if you don't divide by 24, which is what he's saying, you get a figure of 400W, which is implausible and nowhere near Ok, which is where I lost interest. Dometic did exactly the same when I asked them for power consumptions on the CRX fridges a couple of years ago, and then their guy bizzarely complained that I didn't understand power, and that he did.

So there's the problem. "300-400 Wh per day" makes lots of sense when you know exactly what it means and how it was measured, and no sense otherwise. OTOH, most people do actually know what a Watt is. Go to a gym, cycle hard on a bike, and it tells you you're doing 100W. Slow down for a couple of seconds and it drops to 50W. Energy is much the same; cycle hard for 30 minutes and you've got through 20% of a Mars bar, in ergs, MeV, Joules, Wh, cals, whatever.

And the only reason battery manufacturers quote Ah is because of a peculiarity of wet battery chemistry - once the battery is charged, you have a fixed amount of charge that you can get out over some period of time, so amps x hours is actually natural. 'Ah' is only of value to a chemist, though - the rest of us need to know how much stored energy there is.

And we'll agree to disagree on whether charge carries energy...

 

[Raul]

How about 0,3-0,4kwh per day, does that make sense? Makes you feel better if you divide it by 24h, that’s ok. But then you need to put the h after your calc as 16,7wh. Because you divided the energy by 24 and the result is still energy. It is very important to very specifically say wh or W. Otherwise we will never understand, only guess. The Bly reason I quoted total energy per day, is, nobody is interested in energy per hr when you run that appliance all day. All you need to know is daily consumption and peak draw. These are the only crucial pieces of info if you need to design a system.
And the reason Pb batteries are in AH rather WH is to do with peukert law. One amp at 10,5v is not the same with one amp at 14 or 13v. Since not one battery resistance is the same, the peukert will differ,so the total energy will differ, despite the AH is the same. You can draw from one battery 80ah and you get 960wh or there abouts, and you can draw from another the same 80ah but a different amount of energy. Could be less, could be more, depending on the internal resistance. If high resistance will drive the voltage down resulting less total energy, and viceversa. Li are most quoted in WH. Most 100ah Li are 1200wh batteries.

 

[stevewagner]

 

[autorouter]

If I were to use 20l of water for a shower, and 5l for washing up (twice), I have used 30l of water. I could divide it by 24 and say I use on average 1.25 litres per hour. Does that help anyone? All that matters is the total water consumption, not the average rate.

The fridge uses a basic background quantity of energy, plus extra because it's warm, you open the door more often, you just bought a slightly warm 2l bottle of lemonade from the shop, etc, etc. What matters is the total energy consumption over a 24 hour period, not the average hourly rate.

And we'll agree to disagree on whether charge carries energy...

One interpretation of the units of Volts is Joules per Coulomb. Energy per unit Charge.

 

[autorouter]

However, if he did measure it over one day, then you must divide by 24 to get a figure, in Watts, of 16.7W, which is plausible. OTOH, if you don't divide by 24, which is what he's saying, you get a figure of 400W, which is implausible and nowhere near Ok,

400 watt-hours / 24 hours is 16.7 watt-hours per hour, ie 16.7 watts, as you say. If you don't divide by 24, you have 400 watt-hours of energy, consumed over a 24-hour period. That's a definite quantity of energy. 400 x 3600 = 1440000 Joules or 1.44MJ. It's not 400W, which is a rate of energy consumption, not a quantity of energy.

It's important to distinguish between rate of energy consumption and quantity of energy consumption. Rate of energy consumption is watts. Quantity of energy consumed is the rate x the time. Measured in either Joules or watt-hours.

If the rate is varying, as in your gym example, then (since you mentioned a Physics degree) you integrate the rate (watts) with respect to time, to give a quantity of energy. Watts = dE/dt, so E = Integral of W dt.

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[jumar]

What a fantastic thread, fully enjoyed if not fully understood it all.....

 

[Richard n Ann]

Well I thought I was OK with electrics... I know nothing
autorouter what's your background?

 

[EML]

400 watt-hours / 24 hours is 16.7 watt-hours per hour, ie 16.7 watts, as you say. If you don't divide by 24, you have 400 watt-hours of energy, consumed over a 24-hour period. That's a definite quantity of energy. 400 x 3600 = 1440000 Joules or 1.44MJ. It's not 400W, which is a rate of energy consumption, not a quantity of energy.

So... confused... was that intended for me? The problem here (I think, unless it has changed) is to work out what the power consumption of Raul's fridge is. I think we're all agreed that it appears to average out to 16.7 W - yes?

That's all fine and dandy, except that Raul insists that we shouldn't divide by 24. At least twice. And he also says that, if you do divide by 24, the answer is in 'Wh', not Watts, so it's not a power anyway. So, I'm just trying to be nice, without actually using the words "Raul, you're utterly wrong", because this isn't a GCSE class, and my life is too short for that. So, my point was to show the calculation without the division by 24, to show that the resulting power consumption would have been 400W, which is obviously completely incorrect.

1. So, solution A, which I'm sure we agree is (trivially) correct, is 1.44MJ/(3600 * 24) = 16.7 W
2. And solution R, which I think we agree is (obviously) incorrect, is 1.44MJ/3600 = 400 W. It's not even worth discussing this, except that Raul insists that we shouldn't divide by 24. Which is, interestingly, exactly what Dometic said when I asked them about CRX power consumptions.

Which goes back to my other point, which is that giving a figure of "300-400 Wh per day" really is not particularly helpful, because it leads to long pointless threads like this one. It's pretty difficult to get this much confusion over Joules and Watts.

For reference, I never managed to find any useful figures on Danfoss compressor power consumptions. The graph Dometic gave me made me think it was probably in the 40-45W range in a Dometic box. I see that Jackson's is now showing 40W. Raul's figure implies 17W. The 240V fridges I looked at range from 7.3 - 19.6W. Unfortunately, the 7.3W fridge (which is also incredibly quiet at 35W) is way too expensive, at £429 (which is still a couple of hundred less than the Dometics/etc).

 

[autorouter]

The total quantity of energy consumed in a 24-hour period is 1.44MJ. Since watts = joules per second, and there are 3600 x 24 seconds in a 24-hour period, the average energy consumption rate ie power in watts, is 1.44MJ/(3600 x 24) = 16.7 watts, as you say.

The total quantity of energy consumed in a 24-hour period, 1.44MJ, can be converted into an alternative energy unit, called the watt-hour. This unit of energy is used throughout the energy industry, for example on all electricity bills. One kilowatt-hour is referred to as a 'unit'.

The conversion factor from Joules to Watt-hours is 1/3600. So 1.44MJ is 1440000/3600 = 400 watt-hours.

As you say, to call this 400 watts instead of 400 watt-hours is a mistake.

 

[Raul]

…

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[Raul]

Like I said, I have no problem with your divide by 24h. As you discovered, nobody uses it. What you did you divided 400wh by 24h and resulted 16,7wh. This is the average energy used per hr, not power consuption. If your calc came with 16,7w, this is what it throws you off. It’s WH you divided, and the result it’s not power, it’s energy. But you use it as power. So don’t look for 16,7w in specs as you will not find it. The problem you stumble across is as you keep repeating it: energy and power. The dometic is right spot on , it takes 40w of power. And over a period of 24hrs it mounts to 400wh. Which will average a 16,7wh per hr. Some hrs in the night less, and some hrs in the day more. So the compressor will run intermittent, few mins every hr drawing 40w of power. Another way to look at it, the compressor will run in average 10 hrs eating 40w in a hole 24h period.

 

[cmcardle75]

So... confused... was that intended for me? The problem here (I think, unless it has changed) is to work out what the power consumption of Raul's fridge is. I think we're all agreed that it appears to average out to 16.7 W - yes?

That's all fine and dandy, except that Raul insists that we shouldn't divide by 24. At least twice. And he also says that, if you do divide by 24, the answer is in 'Wh', not Watts, so it's not a power anyway. So, I'm just trying to be nice, without actually using the words "Raul, you're utterly wrong", because this isn't a GCSE class, and my life is too short for that. So, my point was to show the calculation without the division by 24, to show that the resulting power consumption would have been 400W, which is obviously completely incorrect.

1. So, solution A, which I'm sure we agree is (trivially) correct, is 1.44MJ/(3600 * 24) = 16.7 W
2. And solution R, which I think we agree is (obviously) incorrect, is 1.44MJ/3600 = 400 W. It's not even worth discussing this, except that Raul insists that we shouldn't divide by 24. Which is, interestingly, exactly what Dometic said when I asked them about CRX power consumptions.

Which goes back to my other point, which is that giving a figure of "300-400 Wh per day" really is not particularly helpful, because it leads to long pointless threads like this one. It's pretty difficult to get this much confusion over Joules and Watts.

For reference, I never managed to find any useful figures on Danfoss compressor power consumptions. The graph Dometic gave me made me think it was probably in the 40-45W range in a Dometic box. I see that Jackson's is now showing 40W. Raul's figure implies 17W. The 240V fridges I looked at range from 7.3 - 19.6W. Unfortunately, the 7.3W fridge (which is also incredibly quiet at 35W) is way too expensive, at £429 (which is still a couple of hundred less than the Dometics/etc).

Dometic claimed an average of 24Ah/day (i.e. 1A or 12W) for my CRX80. That is pretty much what I'm seeing. The compressor draws about 4A when operating.

 

[Raul]

Dometic claimed an average of 24Ah/day (i.e. 1A or 12W) for my CRX80. That is pretty much what I'm seeing. The compressor draws about 4A when operating.

In winter in UK I’m getting 25ah ish to. Your figures are about right. The 400wh is about 30ah and in summer with 35-38C ambient is what I get. It drops of towards autumn.