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Old 01-14-2019, 09:33 PM   #1
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Solar vs. generator run time

Iím tossing around the thought of going with some solar on the top of my motorhome. Iíve read a lot about what you need or want to produce. My main question is, how long would I have to run my 7.5k generator to produce the same amount of power that solar would produce all day. With the high cost of solar power, I can buy a lot of diesel for my generator. When we dry camp we have generator time in the mornings and then we are good for all day unless we need to run the A/C.
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Old 01-15-2019, 08:11 AM   #2
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So many variables to your question. What batteries? Size of battery bank? Daily power consumption? Size of solar system? Amount of dry camping days?
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Old 01-15-2019, 03:22 PM   #3
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As stated above, lots of variables.

Long post, but hope it will help you out.

There is more to the cost of the generator than just diesel fuel. Maintenance costs (oil/filter, air filter, fuel filter, etc.) go along with it, as well as the wear and tear. It is a moving part. Then the noise and smell.

"how long would I have to run my 7.5k generator to produce the same amount of power that solar would produce all day?"

First, leave the A/C time out of it. Generator for A/C if needed. Solar size grows exponentially with A/C use.

On a "balanced" system (enough solar to get you charged after a night of use) 2-3 hours of generator would typically charge the batteries. It would take about half of the day (5-6 hrs?) for the solar to charge the batteries on a sunny day. But then, this is all dependent on power usage and solar system size/setup. The more use, the longer to charge. The bigger the solar setup, the less time to charge.

If you figure 3 hours of generator time, at half load, at 0.5 gallon of diesel an hour you get 1.5 gallons of diesel a day. 1.5 gallons of diesel at $3.00/gallon equals $4.50 a day for talking purposes, agreed?

I gather you have a battery bank, as you dry camp. If so, then the "the high cost of solar power" may be lower than you anticipate. You only need enough solar to recharge your batteries, unless you plan to use "power" all day long.

I self-installed 300watts (2-150w) of tilt-able solar panels, wiring from panels to controller, Tristar 45amp controller, Trimetric RV-2020 battery monitor, relocated inverter/charger near batteries, all new matched/balanced cables, fuses, and fused panel disconnect.
Total installed cost was just over $1250.00, including screws and bolts!

So: My $1250.00 (solar cost) divided by the $4.50/day for diesel comes out to 288 days / 42 weeks / 10 months of generator use for talking purposes. That is a lot of generator time in this scenario.

Comes down to weather or not you want to spend the money for solar. Once the money is spent for solar, the diesel cost disappears.
Another factor to consider is if you want to hear the generator running or if you want the silence of no generator. Solar doesn't make noise.

My 42' Class A DP has (4) 6-volt house batteries serial/parallel with a total of 430AH at 12-volts. We full-timers and are trying boon docking. I work on a desktop computer with (2) 20" monitors, 8 hours a day, 5 days a week. There is typically a 40" LED TV running, also. Frig and water heater on Propane. Propane heat, also. All our interior lights are LED.
On a good, sunny day, I can work AND get the batteries recharged if I am careful about my power usage. (lights, use 1 monitor, limit TV use). If I am not working, I can catch up by abut 12:00 or 1:00. That includes having the furnace come on several times at night. (amp hog)

I am planning to add another 200watt panel on the roof to help keep up with my work power usage and quicken up the recovery time. (pushing up to 500watt total) The new panel will cost me under $200. I already have the wires and connectors to add it in, since my initial installation costs also included excess material, as I purchased more than I needed, just in case.

Good luck!
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Old 01-15-2019, 06:22 PM   #4
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No offense but that old a rig is NOT a energy hog and doubt you have a residential refrigerator so run your generator. Problem with solar is you need to park in the sun...AC often needed.
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Old 01-15-2019, 06:49 PM   #5
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Easy answer, as much battery and solar as you can afford.
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Old 01-21-2019, 01:45 PM   #6
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Solar isn't for everyone and it's hard to justify the cost and trouble to add it if you're generator is working well for you. It's a personal choice.

I started on solar because every summer I spent 4 days in a place where I couldn't run my generator. I started with a single panel which didn't keep up with the drain, but did extend the time I could run off batteries just long enough. Over time, I added another panel. I liked the fact I didn't need to run the generator so I decided to keep going and added a third panel. Sold that motorhome and moved the system to my current one. When we decided we want to full time in the future I added another more panel and a better controller. I call it doing solar on the payment plan because it was done in stages.

In my case I don't expect solar to do it all when boondocking. I have a propane generator which is less efficient but I only expect to have to run it for a while before going to bed.

Will the solar pay for itself? As you said, $1000 will buy a lot of propane so only time will tell. For me, it's being able to camp with a minimum of noise and it's fun looking at the power panel and seeing the batteries being charged silently.
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Old 01-21-2019, 02:20 PM   #7
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Youíll never come out ahead vs a generator thatís already purchased on a strictly dollar basis.

It might be worth it to you to stretch time between generator runs though...to the point where youíre eliminating them entirely unless you need AC. Of course, with a larger converter/charger and more batteries you could reduce runtimes and increase intervals if youíre not adverse to running the gen and donít camp in no-gen places.
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Old 01-21-2019, 02:47 PM   #8
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Quote:
Originally Posted by Ken Thompson View Post
Iím tossing around the thought of going with some solar on the top of my motorhome. Iíve read a lot about what you need or want to produce. My main question is, how long would I have to run my 7.5k generator to produce the same amount of power that solar would produce all day. With the high cost of solar power, I can buy a lot of diesel for my generator. When we dry camp we have generator time in the mornings and then we are good for all day unless we need to run the A/C.

For example, if you have 4 - 100 watt solar panels , they will produce about 5 amps each for a total of 20 amps. With 5 hrs of sunlight , that would be about 100 amp hrs of charging. Now as for your generator , it all depends on your converter/ inverter for charging your battery bank as well as the maximum current you can charge your battery bank at. If your charger is capable of charging 100 amps , and your battery bank will take the full 100 amps , you would have to run your generator for 1 hr. Once the batteries are more than 80% charged , they will not accept same amount of charging , so you generator may only be charging at 20 amps , so a longer run time is needed.

Where the solar works real good , is if you run your generator for a short time in the morning to get the bulk charging completed , the solar panels will slowly charge your batteries to 100% during the day. This will ensure the bank is fully charged and will extend the life of your batteries.
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Old 01-22-2019, 08:57 PM   #9
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There really is no payback or cash return, solar vs gen, but the convenience is great, gets you camping where the solitude and lack of crowding is was your looking for. Wouldn't be without it no matter the cost.
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Old 02-01-2019, 06:49 PM   #10
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Update
So, my system now is:
500 watts of solar panels, (4) 6volt batteries at 430 total AH, Tristar 45amp charge controller, trimetric battery monitor, 2000watt Xantrex inverter/charger, and 300watt pure sine wave inverter.
Total cost is at $1450. Self installed.
We can live more or less normally. I work on a computer all day. We watch tv, day and night.
Batteries will be at 100% by noon on sunny days. Overcast days will take until late afternoon.
We are going on 2 weeks off grid with no generator time. Quartzite, AZ at the moment.
Yes, we have a factory installed Onan QD 8500 generator. Just trying not to use it.
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Old 02-01-2019, 07:50 PM   #11
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Yes a lot of variables, but consider lithium ion batteries as well, charge time by solar or generator is generally 1/3 to 1/4 charge time, compared to Lead Acid batteries. So instead of 8 hours by solar, or 3 hours by generator, now 2 hours on solar and 1 hour on generator. No more bulk, absorption, float. Only bulk at .5 charge rate instead of .2 for LA. so charges much quicker.
Costs a lot more, but worth it if you are going to keep your coach and do a lot of dry camping, and don't want to listen to generators.
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Old 02-02-2019, 10:18 PM   #12
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Lithium battery are not all positive. Please consider your your physical location. Lithium is not good in very cold weather.
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Old 02-02-2019, 10:30 PM   #13
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Lithium battery are not all positive. Please consider your your physical location. Lithium is not good in very cold weather.
Lot of threads on keeping lithiums above their cut off temps. Not really a big problem. Plenty of forum members have lithiums and use them in cold climates with no problems.
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Old 02-03-2019, 08:15 AM   #14
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Lithium battery are not all positive. Please consider your your physical location. Lithium is not good in very cold weather.
It's all in the planning of the system, what folks didn't know or realize just a couple/ few years ago about heat / cold for lfp batteries can easily be worked around now knowing that these conditions can have adverse effects of the batteries life. If the end user can't build for these possible conditions than lfp might not be the right chemistry.
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