Tuesday, 1 August 2017

Calculating Solar Needs - me thinks.

Okay as promised I am writing today about how to calculate your solar needs. Now lets be clear, I am not an expert!!! In fact, I am a blonde, so please do verify everything that I am about to tell you.

I have been studying solar and its required pre-calculations for almost a year. Brad uses a CPAP when he is sleeping for his Sleep Apnea and so having power is a requirement for us - Every Night! We are leaving in 60 days to go RVing for the winter in our new-to-us RV and so it is imperative that I get this Solar issue finalized. I have written on this topic before and someone (I forget who) asked me for the details of the calculations... again, I am not 'the man' and the following may require tweeking but this is what I learned...

Every electrical item be it CPAP or coffee maker has the electrical requirements stamped on it somewhere (except the desktop computers for which I have learned 300w should be typical). Now these numbers are not always in the same configuration. What I mean by this is that some are written in Watts (w) some in AC Amps (A) and some in DC Amps (A). The only way to know if the amps are AC or DC is to check the cord. If it plugs into your home outlet it is AC Amps. If it plugs into your cigarette lighter it is DC Amps.
North American AC outlet (yes I know they are upside down- thats another story) European sockets look different but they are still AC so the same rules should apply.

What you need to do is convert all these numbers to DC Amps. Sounds complicated and I got really frustrated at first until Pippi Peterson saved the day... (Incidentally and for the record, Pippi is not a professional either and the same 'don't hold her to it' rule applies).

Here are the conversion rules:
AC Amps to DC Amps = multiply by 10.
Watts to DC Amps = divide by 12.

For example:
Brad's CPAP plugs into the house outlet and states 1.43 Amps therefore the calculation must be: 1.43 x 10 = 14.3 DC Amps
My CrockPot plugs into the house outlet and states 275 Watts therefore the calculation must be: 275/ 12 = 22.91 DC Amps

The appropriate conversion must be done for all appliances you require to power, once complete you will have to ask yourself how much you will use each item while 'boondocking' ie. not plugged into shore power.

While Brad almost never sleeps 8 solid hours I am calculating the power requirement for his CPAP as 8 hours. The CrockPot will only be on approximately 1.5 hours a day if at all and that is what I am calculating its power requirements on.

For example:
We now know that the CPAP uses 14.3 DC Amps per hour and assuming 8 hours of sleep the power requirement for the CPAP alone is 14.3 x 8 = 114.4 Amp Hours
The CrockPot uses 22.91 DC Amps per hour but will be used approximately 1.5 hours/day 22.91 x 1.5 = 34.36 Amp Hours

Let me remind you to consider your rig itself. Now for us the whole rig runs on Gas/Elec/Propane options except for the lights and plugs, which is not necessarily the same for you. Newer rigs are coming off the line fully dependent on electric ie. Fridge, Hot water heater, water pump, etc. So your calculations may not only be about the items you wish to bring but also the function of the rig itself.

Once you have completed this calculation for all of your required items add the Amp Hours (A.H.) together. This number is your required Amp Hours. Now the trick is that you can't just say I need 400 AH and ensure your batteries cover that. Oh no, not that simple Chuck!

In our case, we have 2 - 160AH batteries for the coach itself which of course is 320 AH and our total requirements are 450AH but its not about just getting one more battery and your good. It is 'prohibited' to allow your batteries to run below 50% so for a 450AH consumption we will require 900AH of batteries. Again, this is just my understanding from a lot of research but I will update you as we progress as to exactly what happens when 'the big boys' install what we actually need. Suffice it to say for a power requirement of 100AH/day a minimum of 200AH battery storage will be required.

Oh yes, we are getting this done professionally. Neither of us is electrically minded and this stuff is complicated and expensive. I will be having everything verified especially since I just realized I did not include the lights or the Fantastic Fan in the rig- told you I was blonde. Now we will be switching all of our lights to LED which use very little power (1Amp) and the fan I believe is 1.5Amps. Minimal but it all adds up and we don't want to drop our battery levels to a point where they will need to be replaced as Deep Cycle batteries (which we have) are expensive.

The panels are not the only item you need in order to get Solar working on your rig, there is also a Charge Controller, an Inverter, specific gauge wiring and a Monitor. I have also learned that you should buy your Charge Controller and then get the Panels dependent on the Controller. What this is about is beyond me but we will figure it out as we go.

Nothing money can't fix, hey Brad?!? LOL (private joke from our recent house build)

So I hope this helps. The biggest take away from this is the conversion calculations themselves which i had a great deal of trouble finding, I'm sure its out there and I just wasn't looking properly but regardless if this helps anyone it is worth it. Good luck with your Solar install and/or follow along on this Blog to learn with us as we travel through this maze.


  1. I think you will find you use a lot less amp hours. My husband uses a CPAP, we watch TV for 4 hours each night and 2 fantastic fans all night and we average 45 - 55 amp hours/ night. We boondock full time in Quartzsite for the winter we have 660 amp hours of battery and 730 watts of solar on the roof. WAY more than we need.

    1. Thanks for that Brenda, I had been concerned about a calculation problem and yes, sure enough the wattage was incorrect (mentioned in yesterdays post). We are now looking around the 400w range which seems to be the recommended 'full-time' allotment. Hopefully we'll meet up with you in Quartzite.

    2. Hey Brenda, I just came across this comment. Of course, I recognized that it was you. I remember when you stopped by you had said you had commented... hope your having a great time, say hello to Len also.


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