brian wrote:

Wirroff wrote:

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OK. So, thanks you to you kind knowledgeable folk on here, we are starting to understand motorhome electrics. I have a couple more questions if you would be so kind.....

1. Lets assume we have 2 x 80w solar panels on the roof. Total = 160w.

2. I have read on this forum (and others) that a good 'rule of thumb' to ascertain the daily amps that could be expected out of your solar panels is to use the equation 0.33 x Total Wattage (0.33 x 160w) = 52.8amps. This is based on average daylight hours etc. This is actually borne out by my solar controller (PL20) which stated that 55 amp hours were put in, so pretty accurate.

3. OK, I know that when I looked at the Solar Controller and everything else was switched off, the fridge freezer (Waeco 141 Litre) had a load of 6.3a. The only reference to power etc in the fridge manual I can find, is that is it a 75 watt fridge. Using my high school electrics knowledge (Power (Watts) is equal to Amps x Volts), I get 75 watts is equal to 6.25a x 12 volts, so again, pretty accurate. This also shows that there is no voltage drop between the fridge and the battery as this is what I expected the voltage to be at the battery.

4. The question I have is this; If, at the time I measured it, the fridge was drawing 6.3a, does it do this for 24 hours a day? If so, the draw from the fridge alone would be 151 amp hours. My solar panels only put in 55 amp hours, so my shortfall would be 96 amps a day, or in other words, one full battery. This can't be right surely? I understand that we do have a 'deficit' each day from the fridge, but how much is it really? Does the fridge 'cycle' between power needs? What is the REAL amp use of a fridge in 24 hours?

5. The only other question left remaining is; Given some cash restraints, what would you guys choose to have fitted to assist us? (We will be having a new large charger fitted, that's a given)
A) An extra battery fitted (i.e 120 a/h AGM) to go with our two current 100 a/h lead acid batteries) or.....
B) An extra solar panel (say 120 watts) to compliment the existing two panels and the existing two 100 a/h batteries

As someone else pointed out, I don't want to throw away two decent batteries if they can be fully charged up with a decent charger. Your thoughts are most welcome. Which would you go with, A or B??

Cheers,


Paul & Val

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 I'll run with Brian's numbering...

2. Actual performance will depend a lot on whether your panels are positioned in the sun or the shade. Were all of those 55 amp hours supplied by the panels, or were some supplied by the generator?

4. Yes, the fridge consumes 75 watts when operating. Your fridge's compressor only operates at 100% or off, it doesn't vary its output according to the conditions. The fridge manages the temperature by switching the compressor on when the internal temperature gets too high, and switching it off when the internal temperature is back down to target. You will probably hear it switching on and off, and running, if it's quiet indoors. The percentage of time that the compressor spends running is the "duty cycle" that Brian's referring to. The higher the ambient temperature (and sunshine hitting the fridge surrounds), the higher the duty cycle and hence the higher the fridge's average consumption over time. Waeco's product page for your fridge gives a couple of average consumption figures at different ambient temperatures, so you can see that the amount of refrigeration energy you require will depend on your location and the season. One energy management tip is to position your vehicle to avoid or minimise the sunlight striking the wall behind the fridge.

5. You haven't told us anything about your system's other loads (e.g. pumps, TV, computers, etc), or your travel habits or intentions. E.g. do you normally seek to park in the open or in shade? Do you stay put for long periods or move often? Does your power system need to sustain your load indefinitely or just for a few days at a time?

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For comparative purposes, here's what I have, which is relatively minimal as I'm a former bicycle tourist and haven't kicked the habit of travelling lightly:

- Waeco CF35 fridge, which has published consumption figures almost exactly one third of yours. I don't have a freezer of any kind, the fridge is set to 4C.

- 80Ah deep cycle AGM battery (my battery travels inside a car so floodeds were never an option).

- The cheapest portable 80W panels and controller I could find on eBay, which I position to maximise capture (not that full sun is attainable at all camp spots).

- No alternative means of charging the battery. My fallback strategy would be to switch off the fridge or stay somewhere with power available, but I've never come close to needing to do this.

I tend to stay put for around a week at a time. The fridge stays in the car, which I park such that the fridge is not excessively exposed to the sun. I don't much like hot weather so I mostly stay in cooler places, and often at elevation, which will naturally improve system performance because the temperatures are lower and the sunlight stronger.

With this setup my battery voltage never drops below 12.6 unless I don't put the panels out for a day or place other significant loads on the battery (e.g. recharging laptop) when the weather is wet and the panels are mostly shaded. Even then, in ten months' use only once has the voltage dropped as low as 11.9 (under load). In summer the battery will be on float charge with less than two hours of morning sunshine, while in winter it'll take half that time or less. I don't need to move the panels during the day to keep up with my load, but I do anyway, as I like the feeling that the battery is fully charged when the sun goes down.

moblet wrote:

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You could probably do with a couple of cold ones after all that. Enjoy!

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He might just get one now that the fridge is working.