Advice on solar panel size please

G'day Russell

Seems to me your problem goes way beyond panel size. Without doing the sums I think you'll find the battery won't go the distance running both the fridge and CPAP at night. Added together we're looking at somewhere around 80W after making allowance for losses etc. You'll likely be in strife after a couple of hours I think.

That CPAP is quite power hungry.

Hi Russel,

As Rosco suggests, I think you have a battery capacity problem.

I agree with your 30Ah per day for the fridge.

CPAP: If you can do without the humidifier (especially without a heated tube system) the demand will be much less, but running a nominal 75W CPAP through an inverter will probably call for up to 50Ah per night, more than half being for the humidifier.

Add in some lighting and charging cameras, ipods, torches etc, say a total of 5 to 10 Ah, and your daily energy requirement is 80 - 90 Ah. It isn't good for your 120Ah battery to have to deliver that amount - generally, batteries should not be repeatedly discharged down below their 1/3 of capacity point ( i.e. try to avoid taking more than 80 Ah from your 120 Ah battery.) I'd suggest, unless you can get by without the heated humidifier, that you consider doubling up on your battery.

How much solar do you need? Depends on your travel and usage patterns. To keep up with the above demand you need to be able to push close to 100 Ah per day into your battery/s. When travelling some of this could come from your alternator. It may be worth considering a dc-dc charger to assist in this. (Mentioning a dc-dc charger here may start the usual vitriolic argument. They do have their place though and your's may be one of them!) Regardless, it is very desirable that you use heavy cables running from front to back to minimise voltage loss.

Each 100W of solar capacity can deliver to the battery/s a maximum of about 6 amps if using a simple PWM controller, or 7 amps with a MPPT controller. If we assume good full strength sunlight for say 6 hours per day, and keep the panels reasonably close to normal incidence (its a sine function so no need to be too accurate) we could harvest a maximum of about 35-40 Ah per day from each 100W of solar capacity.

On this basis, on the perfect sunny day 200W of solar would go close to meeting your daily demand. Add some charge from the alternator, double your battery capacity to provide some flexibility and 200W looks a good size for your purposes. If you were to run the CPAP without the humidifier this would give you a handy buffer.

HTH

Cheers

John

Hi Russell,

Thankfully, unlike yourself and John, I have no need for CPAP....... yet!
It is bad enough to have to support a CPAP in the 'field' without all the issues of battery supply and charging, what with overcast weather and wishing to 'stay-put' and lose the benefit of the car alternator charge.

So, have you considered the alternative of a small petrol driven 230v generator? I have encountered several caravaners who run their CPAP machines this way. It could eliminate a lot of hassles but I can understand if you have an aversion to motor gennies.

If i see you out there Russell and you need a cold beer i will give you one.

Yeh I think all considered, you need to get that battery capacity up....there realy is no way arround it......120 Ah just does not go far..especially if you expect decent service life from your batteries......ya gota be talking 200Ah pluss to be comfortable

Sticking my thumb in the air...your worst case demand could be arround 100Ah per night

as far as the charging capacity...well that depends on how far you plan on traveling in a day and how long you plan to be in one place off grid.

A generous solar array would be best for sure.....but even a solar array that wont keep up with the demand will considerably lengthen your stay time between drives or shorten the necessary drive time to get a full charge back.

If you want to keep up wth demand well you'd have to be talking 160watts of pannels to be even in the race....200 to 300 watts would be more comfortable...particularly traveling in winter at low latitudes.

OH that 12 volt fluro...ditch it in favour of LED.

cheers

I am on to my second machine now. The first was a 240v only unit and when run via the inverter it was hungry on battery, necessitating a second battery install. However the latest (DeVilbliss brand) has 12v and 240v power input. Without the humidifier it is very frugal on 12v power and when tent camping I just use a 22ah sealed battery overnight in the tent with the unit. Alternatively I run an anderson plug extension lead from towbar 12v output plug to tent and draw from the vehicle setup. I have used the 22ah battery for two nights without charge on a couple of occasions as well and not discharged below 50% soc. Daily charge is usually via the vehicle solar setup or if travelling, just connected within the vehicle charging system.

Sorry, but in CPAP world, 12v is the go. Otherwise you need to compensate with extra battery / solar as others have covered.

Greg N

Russell, I estimate that to supply 75 W your inverter might consume 120 W so draw 10 A for 8 hrs. With a 110 Ah battery thats a big percentage discharge every night. Maybe you need another battery in parallel to deal with such a big load. I'd look at a 250W panel as they are cheap because of volume - just bought a 40 panel system with good inverter and installation for $10k so panels are less than $200.

Hi Russell,

I've got a resmed S9 and am actually on a trip at present. I am using the 12v adaptor. According to the Arkpak I used less than 5% of the 95a/h agm battery last night.

I recommend you consider sourcing a cpap with 12v capabiliies from the US. You will find the net price better than Aussie prices less private health rebates. I used secondwindcpap.com for mine.