Solar + battery system design (help head spinning!)

[polomasta]

hey everyone - I've been trying to research details as I plan my camper van build and could use some help. I've read so much information at this point my head is spinning a bit from all the different units, conversions, etc, etc.

Here are the basics:

• will be living in the van full time
• two adults
• goal is to be fully off-grid for at least 5 days before needing any power supplements, but ideally indefinitely.
• I've got a rough calculation that I'll need 190 amp hours per day
• will have a 930 watts worth of solar panels
• plan to allow alternator to charge house batteries while driving (but assume 5 days stationary)
• plan on going w/ Lithium battery setup

my current challenge is I'm trying to figure out what my battery setup realistically needs to be to support this amount on consumption taking into consideration the incoming solar.

If I'm covering the 5 days.. that's essentially 1000 amp hours needed (or 10 x 100ah super pricey lithium batteries)

I realize calculating real-world input from the solar panels is tough... but I'm just trying to get a starting point in understanding if the solar will allow me to reduce the number of batteries I need and if so, by how many.

Thanks in advance for any insight/advice/thoughts!

 

[john61ct]

need 190 amp hours per day
930 watts worth of solar panels

If I'm covering the 5 days.. that's essentially 1000 amp hours needed (or 10 x 100ah super pricey lithium batteries)

You only need a couple days supply if you're in locations with reasonable weather conditions.

500+AH to start should be fine, see how you go.

Best to add more sooner than later if you find you need it so the batt's ages aren't too far apart.

You will cover your usage from solar only most of the time with reasonable weather conditions.

Tilting the panels and keeping them clean will help. Adding more is fine to do anytime you want.

A quiet little inverter gennie can help top up anytime.

 

[Rando]

190 Ah / day is a huge amount of power consumption. Is this a measured or calculated number?

I have budgeted to have about 3 days worth of energy stored - which for me is about 100Ah and if your usage numbers are right would be about 600 Ah for you. Even on the most overcast day you still get some solar power, so you don't need to have a full 5 days of battery capacity.

PVWatts will allow you to calculate your solar power input:
http://pvwatts.nrel.gov/

If your panels are flat mounted, 3-5 hours of 'full' power per day is a reasonable estimate, but that of course depends on time of year, latitude and weather.

 

[polomasta]

190 Ah / day is a huge amount of power consumption. Is this a measured or calculated number?

I have budgeted to have about 3 days worth of energy stored - which for me is about 100Ah and if your usage numbers are right would be about 600 Ah for you. Even on the most overcast day you still get some solar power, so you don't need to have a full 5 days of battery capacity.

PVWatts will allow you to calculate your solar power input:
http://pvwatts.nrel.gov/

If your panels are flat mounted, 3-5 hours of 'full' power per day is a reasonable estimate, but that of course depends on time of year, latitude and weather.

190 aH is calculated, and using the higher end of draw numbers and conservative (long) run times. But it includes electric fridge/freezer, MaxxFan, cell booster, and various other things. We'll be working from the van so 2 laptops that need to charged up all day and lots of camera batteries and other small electronics that need to stay charged.

Thanks for that link!

 

[Rando]

I am going to guess you are WAY over estimating your power usage. My Four Wheel Camper has similar appliances (65L Isotherm fridge freezer, two fans, furnace, lights, cell booster etc) and my daily average is around 30Ah , with the worst case ever around 60Ah.

 

[polomasta]

I am going to guess you are WAY over estimating your power usage. My Four Wheel Camper has similar appliances (65L Isotherm fridge freezer, two fans, furnace, lights, cell booster etc) and my daily average is around 30Ah , with the worst case ever around 60Ah.

hmmm that's sounding likely... the fridge is the biggest draw but I'm looking at something that also avgs 2.5amps (same as yours if I looked it up right)... so what do you estimate/measure your fridge drawing per day?

 

[john61ct]

hmmm that's sounding likely... the fridge is the biggest draw but I'm looking at something that also avgs 2.5amps (same as yours if I looked it up right)... so what do you estimate/measure your fridge drawing per day?

Average draw from a compressor fridge varies widely, how many minutes per hour is the compressor cycled on?

Start out low, test, add as needed. The LFP is not cheap!

 

[Rando]

Webasto specs the isotherm at 360W/24H average power (1.25A) which I would say is about right for an 80F+ day. On a hot (95F+) day in the sun it may go up to 2A during the day then drop down to 1.25A at night. The average temperature is a little lower here in the mountains - so around ~ 1A average.

 

[DiploStrat]

You may find this helpful: https://cookfb.files.wordpress.com/2016/09/plan-electrical-system.pdf

 

[polomasta]

Average draw from a compressor fridge varies widely, how many minutes per hour is the compressor cycled on?

Start out low, test, add as needed. The LFP is not cheap!

yeah that seems to be the hard part here... I can't seem to find any data out there about avg duty cycles on the fridge... so it's just a guess at this point. The fridge specs say 1.82 - 2.67 amp/hour average power draw.

You may find this helpful: https://cookfb.files.wordpress.com/2016/09/plan-electrical-system.pdf

Thanks! I remember coming across that a while back.... very helpful!


So, this was the original estimate worksheet I threw together:

The fridge estimate is probably really high considering the condenser won't actually be running 24hrs a day.

Cell booster, laptops and charging of other peripherals (camera batts, ipads, phones, etc, etc) will be running a lot because of what I do for work.

 

[Rando]

The fridge definitely seems high - 50% duty cycle is probably a better guess. I also have a WeBoost - it is under 1A at 12V (the power supply on it is rated for a max of 2A at 6V). I have a 15" Macbook Pro which has an 8 Ah 12V battery inside and will run for 6-8 hours off that - so it needs about 10 - 12Ah to run it for an 8 hour day (assuming some losses). Similarly an iphone has a 7 - 10Wh battery - so ~1Ah at 12V to fully recharge it, I am guessing the Android and camera are about the same. It is up to you to decide an acceptable risk tolerance, but I think you can safely half your power budget.

There is a tendency for folks to design for the 1 - 3% case without considering behavioral factors - keep an eye on your battery and for those one or two times a year you have a string of hot days with no sun and important project to complete and are really hitting your battery hard, go for a drive as opposed to hauling around an extra 300lb (or $3000) worth of batteries.

 

[polomasta]

The fridge definitely seems high - 50% duty cycle is probably a better guess. I also have a WeBoost - it is under 1A at 12V (the power supply on it is rated for a max of 2A at 6V). I have a 15" Macbook Pro which has an 8 Ah 12V battery inside and will run for 6-8 hours off that - so it needs about 10 - 12Ah to run it for an 8 hour day (assuming some losses). Similarly an iphone has a 7 - 10Wh battery - so ~1Ah at 12V to fully recharge it, I am guessing the Android and camera are about the same. It is up to you to decide an acceptable risk tolerance, but I think you can safely half your power budget.

There is a tendency for folks to design for the 1 - 3% case without considering behavioral factors - keep an eye on your battery and for those one or two times a year you have a string of hot days with no sun and important project to complete and are really hitting your battery hard, go for a drive as opposed to hauling around an extra 300lb (or $3000) worth of batteries.

Thanks, really appreciate your insights here!

 

[DiploStrat]

Some Ranging Shots

I think your numbers for laptops and phones are way over the top. I use a MacBookPro and I can basically ignore it. The big consumers are:

-- Refrigerators
-- Microwaves
-- Cooktops
-- Furnaces

(We'll ignore air conditioners as those are simply off scale.)

I calculate one amp per hour for everything else and ignore it.

In two years of multi-month trips, running the heat (Webasto Dual Top) 24 hours a day as needed and cooking electrically we found that our overnight consumption was typically 125Ah. A less if we grilled outside and sometimes a bit more if we had a cooked breakfast. The lowest numbers were down around 75Ah overnight when it was warmer.

YMMV, of course. As others have noted, cook with propane, don't use a microwave, and don't run heat all night and you will be below 50Ah.

None of this is hard, but it can be a bit complex; simply proceed one day at a time.

During the design stage we assumed three days of solid rain without starting the engine. We have never had conditions that bad and you can always run the engine in the morning when fixing breakfast.

 

[polomasta]

I think your numbers for laptops and phones are way over the top. I use a MacBookPro and I can basically ignore it. The big consumers are:

-- Refrigerators
-- Microwaves
-- Cooktops
-- Furnaces

(We'll ignore air conditioners as those are simply off scale.)

I calculate one amp per hour for everything else and ignore it.

In two years of multi-month trips, running the heat (Webasto Dual Top) 24 hours a day as needed and cooking electrically we found that our overnight consumption was typically 125Ah. A less if we grilled outside and sometimes a bit more if we had a cooked breakfast. The lowest numbers were down around 75Ah overnight when it was warmer.

YMMV, of course. As others have noted, cook with propane, don't use a microwave, and don't run heat all night and you will be below 50Ah.

None of this is hard, but it can be a bit complex; simply proceed one day at a time.

During the design stage we assumed three days of solid rain without starting the engine. We have never had conditions that bad and you can always run the engine in the morning when fixing breakfast.

appreciate these insights based on your experience, thanks for sharing! Since I'm in the planning phase here I'm simply going off of hardware specs and calculations, so it's nice to see some real world experience and numbers!

I had to laugh when I saw including 1.5amp coffeegrinder !
But 1.5A would do it if you are not in a hurry.. 10-15A (assuming 12V) is more realistic. But for what ? 60 seconds per day ??
Anyway, Not trying to pick a fight, Just thought it amusing...

heh, good point! that's actually a 120V appliance not 12v so should have left it out of this particular calc w/ all the other 12v stuff.

 

[DiploStrat]

To one of your original questions, you can expect 5-6A per hour of good sunlight, per 100w of solar panel. How many hours of "good" sun? Depends on where you are, angles, etc.

Again, these are all guesstimates/planning numbers, but they do have the advantage of being taken from real world experience.

Again, YMMV!