Solar Installation Questions

[Bob Denman]

Originally Posted by DanChateau33SW

I like to think I have a pretty great mind...but there are probably several others that would disagree. A matter of my personal opinion I guess!

I've been told that I have a mind like a comic book...


...i tend to agree with that assessment!

[Chance]

Originally Posted by DanChateau33SW

.....cut....

We live in south Florida and most of our travel is within the state, but we do regularly travel up to GA and looking to take a few other, longer trips. It would be nice to stop over night at a rest area to sleep and not have to be woken up by the generator starting or the inverter alarming on a low voltage.

Any experience, recommendations, or general direction to go are definitely appreciated.

My 2 cents...

There is a real possibility that adding solar won’t help with the “stop overnight in rest area” scenario because your batteries may be 100% charged by the motorhome’s alternator when you stop for the night, at which time it may be too late in day/night for solar to contribute much, or anything. There are a lot of variables to consider though, like what other power you’re using, battery bank capacity, alternator charging rate (delivered to batteries), etc...

Because solar only produces power during a few hours of the day, energy balance can’t be looked at on a steady-state basis. The same goes for alternator since engine must be running, or converter since generator must be running. Just saying that if goal is to cut back on generator time (particularly in middle of night), there is quite a difference between boondocking at the beach for a week and driving everyday from rest area to rest area during a long trip.

[bigben]

As I said - I don't understand (and yep Ted thanks for pointing that out). Our coach on 4 Trojan 105s makes it easily through the night (11 to 7) running our residential frig, CPAP and some SAT TV. Batteries set at about 11.9-12 volts each morning. We are not wanting to eliminate the gennie in total but rather to help top up the batteries during the daytime. Other choices are to run a 2000 watt generator for about 8 hours a day. As I said, we are no hook ups for about 2.5 months each winter.

So here is what I don't understand - how many batteries do you have? Is your alternator not charging your household batteries while you driving? What else are you running beyond your residential refrig - based on our experiences over the last 4 winters - our 4 Trojan T105s are more that equal to the task of 8-10 hours of residential refrig. Oh yes - I turn the ice maker off each night to avoid the draw of the automatic ice maker. So I'd recommend you look at your battery bank as well as the charging source.

[DanChateau33SW]

Originally Posted by bigben

As I said - I don't understand (and yep Ted thanks for pointing that out). Our coach on 4 Trojan 105s makes it easily through the night (11 to 7) running our residential frig, CPAP and some SAT TV. Batteries set at about 11.9-12 volts each morning. We are not wanting to eliminate the gennie in total but rather to help top up the batteries during the daytime. Other choices are to run a 2000 watt generator for about 8 hours a day. As I said, we are no hook ups for about 2.5 months each winter.

So here is what I don't understand - how many batteries do you have? Is your alternator not charging your household batteries while you driving? What else are you running beyond your residential refrig - based on our experiences over the last 4 winters - our 4 Trojan T105s are more that equal to the task of 8-10 hours of residential refrig. Oh yes - I turn the ice maker off each night to avoid the draw of the automatic ice maker. So I'd recommend you look at your battery bank as well as the charging source.

I w\started checking the water levels yesterday and found most of the cable loose on the batteries. I tightened all of them and had a lot better luck with the batteries after that. I ran a test for an hour with the fridge on the inverter and the voltage only dropped from 12.5 to 12.1 under a full compressor run. the fridge had been off for the past few days, and it has been in the upper 80s here in south FL the last few days, so the fridge was pretty warm. I am going to give this a run for the next trip, but will add a 400W Renogy array very soon to help out with day to day maintenance charge and helping to offset battery power when we are stopped.

[bigben]

Originally Posted by DanChateau33SW

I w\started checking the water levels yesterday and found most of the cable loose on the batteries. I tightened all of them and had a lot better luck with the batteries after that. I ran a test for an hour with the fridge on the inverter and the voltage only dropped from 12.5 to 12.1 under a full compressor run. the fridge had been off for the past few days, and it has been in the upper 80s here in south FL the last few days, so the fridge was pretty warm. I am going to give this a run for the next trip, but will add a 400W Renogy array very soon to help out with day to day maintenance charge and helping to offset battery power when we are stopped.

Dan - right way to proceed. All the number crunching you can do will only give you a start point.

[Chance]

Originally Posted by petefoss

Isn't the fridge about 8 amps of 120V? Almost 1000 watts when the compressor is running?

You can guesstimate an average number by looking up the Energy Star rating of a refrigerator. Many newer refrigerators (obviously depends on size, efficiency, and conditions in which it operates — so I picked one below only as an example) use roughly (in order of) 2 kWh of energy daily. Averaged over a 24-hour day, even when adding a little for inverter inefficiency, power demand should be around 100 watts, which is close to 8 Amps at 12 Volts.

Refrigerators “may” draw close to 1,000 watts for very short periods, but it can’t be that high for very long or else they wouldn’t operate very long on 4 golf-cart batteries.

[Chance]

Originally Posted by bigben

All righty - I guess I don't understand. A 100 watt panel produces about 6 amps per hour. A residential refrig uses about 8 amps per hour with the compressor running. ....cut.....

Ben, for all practical purposes, there isn’t such a thing as “Amps per hour”. Trying to use such units of measure is just going to lead to wrong estimates/answers in my opinion.

If your panel puts out 6 Amps at rated solar conditions, and you get about 5 hours of sun per day, then you’ll get 6 Amps X 5 hours, or 30 Amp-hours.

Amp-hours isn’t a real unit of energy, but since it’s assumed these systems are all at 12-Volts, then Amp-hour is used as if it were a measure of energy to compare systems.

As example, 30 Amp-hour X 12 Volts = 360 watt-hours, or more commonly expressed in kilowatt-hour as 0.360 kWh.


For reference, a Trojan T-105 golf-cart battery stores up to 1.5 kWh of energy (at 100%), so 0.36 kWh isn’t much when comparing to 4 batteries.

[bigben]

Chance - couldn't agree more. In my experience, I've tried to use it as a guide to size our use and then an appropriate size for our battery bank. We've struck a sweet spot with our 4 six volts. In dry camp and given our usage (battery bank and on board gennie usage), we are going to start with a 200 watt solar install to charge our battery bank during daylight vs running a small generator. If that isn't enough we'll make sure to size the system to allow for upgrading to 400 watt solar. Same approach we've taken to our battery bank.