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Old 11-11-2020, 12:28 PM   #1
Mini-Skoolie
 
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Am I estimating my solar needs correctly?

Hello! I'm in the early stages of planning my solar set up, and something just looks off, so I'd love extra sets of eyes on this spreadsheet to make sure I'm estimating my needs correctly.

My 6-window bus will be my full-time home. Although I hope to use my bus for travel in the future, I typically work the farm season and remain relatively stationary (it makes more sense when you know that rent where I live is minimum $1000/room in a house/mo). Shore power is... unlikely? So I'm counting on being entirely off-grid. This spreadsheet should have everything I hope to use, and yes, I did account for the Vitamix I hope to use for a few minutes every few days, which obviously needs to be run through an inverter.

From my calculations, it seems like I'll be needing about 600 watts of solar. This is either (a) a lot for one person, (b) a generous estimate, or (c) miscalculated. Or maybe a+b. Then there're cheat sheets like this one, which make me seriously question my calculations. Based on those descriptions, I think I'd safely put myself in Group B.

Thoughts/considerations/obvious miscalculations you can add are appreciated!

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Old 11-11-2020, 08:50 PM   #2
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Quote:
Originally Posted by veglorde View Post
Hello! I'm in the early stages of planning my solar set up, and something just looks off, so I'd love extra sets of eyes on this spreadsheet to make sure I'm estimating my needs correctly.

My 6-window bus will be my full-time home. Although I hope to use my bus for travel in the future, I typically work the farm season and remain relatively stationary (it makes more sense when you know that rent where I live is minimum $1000/room in a house/mo). Shore power is... unlikely? So I'm counting on being entirely off-grid. This spreadsheet should have everything I hope to use, and yes, I did account for the Vitamix I hope to use for a few minutes every few days, which obviously needs to be run through an inverter.

From my calculations, it seems like I'll be needing about 600 watts of solar. This is either (a) a lot for one person, (b) a generous estimate, or (c) miscalculated. Or maybe a+b.

Then there're cheat sheets like this one, which make me seriously question my calculations. Based on those descriptions, I think I'd safely put myself in Group B.

Thoughts/considerations/obvious miscalculations you can add are appreciated!
After a quick glance at your spreadsheet and the cheat sheet, I am more skeptical of the cheat sheet.

Their estimates seem unrealistically low for a system based on PV+Batteries. (400W solar and 200AH usable capacity for up to a family of 4, induction cooking, occasional A/C use, electric water heater, is so unrealistic its silly). They do specify modifying habits, and using all 12v systems where possible to maximize efficiency which is important to consider and can make a big difference. Their point about designing your system so that your inverter can be switched off is a good one as well, a decent sized inverter burns up a good bit of power just sitting there idle. Low power modes can help with this somewhat.

This brings me to my next point. Inverter inefficiency. In your calculations you should account for this for your AC devices. Inverters are in the ballpark of 70%-95% efficient. A good inverter will have a peak efficiency in the high 80s to mid 90s, a budget inverter will have a peak efficiency in the 80s. I use 85% as a generic ballpark for inverter efficiency.

So say you have a 100W AC load. To supply that 100W on the AC side of the inverter, the inverter will draw 100W / 0.85 = 118W on the DC side.

This is one form of inverter loss (conversion inefficiency), the other is idle power consumption / self consumption, which is the power an inverter consumes just sitting there idle, for a medium sized inverter 7-40W+ is a broad ballpark estimate, this depends on a few factors including inverter design and quality.

The only item on the spreadsheet that looks off to me at first glance is the phone. But that won't make a huge difference. You estimate 40Wh, using my own phone (pixel 2) as a guide that would be 4 full 0-100% charges per day. But in the realm of things how much your phone uses is just a rounding error in your overall system power budget. In your system it looks like the fridge accounts for ~50% of your consumption. How have you measured or calculated its power usage what style of fridge is it?
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Old 11-11-2020, 10:06 PM   #3
Mini-Skoolie
 
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Thanks dzl_! I thought the cheat sheet looked a little light...

Just after posting, I made some significant changes to my spreadsheet (maybe not before you saw it though). I'll only be needing an inverter to run the blender and my laptop for a few hours a few days a week, and on a separate calculation, accounted for the ~85% efficiency loss when inverting. I'll definitely be planning on switching the inverter off when not in use (I'm sure I'll wind up charging/powering other gadgets besides the blender eventually, so I'd rather factor it into my set up).

You're likely right about the phone, but I'm accounting for nights I'll likely charge while asleep. (Apple seems to suggest 5W, but really I'm not sure if that's including the cube adapter or not.)

For the fridge, I'm looking at a few (haven't purchased but I'm estimating for the maximum just in case) that are dual compartment chest fridge/freezers. I've split the difference between a lot of different estimations, starting from the cooling cycle maximum at 50-60W. Then, I read several different reviews and comments that suggested the cooling cycle runs anywhere from 15-30% of the time, and another that suggested that one of these models averages out to about 35W/hr. This sounded like the best and most generous estimate, so I went off of that. If you have any suggestions for how I might rethink that number, I'm here to learn!

The last question I'm adding into this thread: do most people take the chance of minimal/no sun into consideration with their solar setups? As in, should I be doubling or tripling my daily needs? I'll be in California. It seems that even doubling brings my setup from 400W/~300Ah battery capacity --> 800W/~600Ah?!? That seems very unnecessary for me on my own...
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Old 11-11-2020, 10:51 PM   #4
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Originally Posted by veglorde View Post
The last question I'm adding into this thread: do most people take the chance of minimal/no sun into consideration with their solar setups? As in, should I be doubling or tripling my daily needs? I'll be in California. It seems that even doubling brings my setup from 400W/~300Ah battery capacity --> 800W/~600Ah?!? That seems very unnecessary for me on my own...
I think this depends very heavily on your use-case and your overall system design.

I'm sure you have noticed many people here do not build towards an off-grid mindset/model and plan for access to shorepower most of the time, or do plan to be somewhat off grid but rely heavily on a generator and/or alternator charging. Solar is often just a secondary supplemental power source. Others plan for mostly off-grid and some fall somewhere in between.

So, if you plan to be off-grid most of the time, and you plan for solar to be your primary or maybe only power source most of the time, then it definitely makes sense to account for a few 'days of autonomy' in your battery bank calculations, as well as an oversized PV array if you can find space for it, because the consequences of consecutive days without sun are a big deal in that context.

On the other hand, if you have a generator onboard, the extra cost of building for multiple days of autonomy may not be worth it, since the consequences of multiple days without sun is that you have to run the generator more often. Alternator charging is also a factor if you drive a lot consistently, or need power in a pinch, if you dont drive decent distances fairly often daily its not the most cost effective or efficient approach, but its nice to have to supplement other charge sources.

I don't plan to be in situations where shorepower is available often, and I don't plan use a generator, so maximizing solar and maximizing battery capacity are important. I also really like the idea of being dependent on a sustainable, silent, power source.

One more thing to factor into your calculation. If you design your battery bank for a few days of autonomy it costs more up front, but you will have a much shallower depth of discharge which will increase the life of your batteries saving you money over time.
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Old 11-12-2020, 01:05 AM   #5
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The above post has some parts that are a bit muddled, too late to edit, let me know if anything needs clarifying.
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Old 11-12-2020, 10:05 AM   #6
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DZL captures my thoughts on this very thoroughly.

The only thing I'd add is if you want confidence in your design, consider adding an overall derating factor of 80% on your solar system. Meaning, whatever precise specifications your system has on paper (including things like solar panel efficiency), multiply that by .80 to determine the system's capacity. Conversely you can divide your carefully calculated demand (which might include things like inverter efficiency mentioned above) by .80 to arrive at your estimated wattage requirements.

Sounds like padding, eh? Thing is, derating is used in many industries and disciplines and it not considered 'padding the number'. It is a hypothesis that real-life conditions that cannot be known can reasonably be accommodated with the system design, therefore there is a chance it will not meet requirements in corner conditions. Just like using a pure sine wave inverter reduces the likelihood of burning out a motor, derating improves your confidence that the system will operate as intended without unintended issues.

That hypothesis can be tested over time, as you use your system. And by the way, if you don't use derating, you still have a hypothesis--that your system as designed accounts for all possible factors. You will also be testing that hypothesis, if you don't use derating.

I really enjoy reading posts from those who designed something, used it, then reported out on the results after a period of time. That learning is very valuable to all of us here.
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Old 11-12-2020, 12:00 PM   #7
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My setup is 600W with a 60A controller, two 200Ah AGM batteries and a 1000W inverter/charger. My calculations look similar to the spreadsheet, but I only calculated the fridge to run half the time. I also included 2 days of low-sun and I derated the panels 20% because they are mounted to the curvature of the roof.

I’m sorry I cannot really give you a real world performance review. My bus isn’t done yet. I have, however, hooked up the fridge and went to the desert for a few days. I ran the fridge, water pump, and an automotive drop light off the inverter.

The only time I saw the battery capacity drop was at night. By morning I’d be down to 85%, but in a few hours after sunrise they would be back up to 100%.

I think that inverter is a serious vampire!
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Old 11-12-2020, 02:59 PM   #8
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The spec on my 1200 watt pure sine inverter is 1.5A standby. That's 36 Amp Hours a day, if I left it on.

Haven't used it enough real-time to characterize actual consumption.
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Old 11-12-2020, 06:29 PM   #9
Mini-Skoolie
 
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Whew, y'all are so helpful! Using all of your input, 12 hours of research, and considering that I'll need about 140Ah/day, I think this is where I'm landing:
  • 400Ah LiFePO4 battery bank (2x200Ah Sok -- if anyone has experience with these, I'd love to hear your thoughts!)
  • 400W Renogy panels (4x100W) (can increase if necessary, but thought I'd start here as I'll have the DC-DC to run off the alternator, see below)
  • DCC50S 12V 50A DC-DC battery charger w/ MPPT
  • 2000W Renogy Pure Sine Wave inverter with on/off switch

With the deeper discharge on the lithium iron batteries, I should have more than 2 days worth of power onboard. I know a lot of supplies are on backorder/delayed delivery due to covid, so while I want to place orders sooner rather than later, using this forum to think it through with y'all helps a lot--thank you!
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Old 11-12-2020, 08:11 PM   #10
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Quote:
Originally Posted by Rucker View Post
The only thing I'd add is if you want confidence in your design, consider adding an overall derating factor of 80% on your solar system. Meaning, whatever precise specifications your system has on paper (including things like solar panel efficiency), multiply that by .80 to determine the system's capacity. Conversely you can divide your carefully calculated demand (which might include things like inverter efficiency mentioned above) by .80 to arrive at your estimated wattage requirements.

Sounds like padding, eh? Thing is, derating is used in many industries and disciplines and it not considered 'padding the number'. It is a hypothesis that real-life conditions that cannot be known can reasonably be accommodated with the system design, therefore there is a chance it will not meet requirements in corner conditions. Just like using a pure sine wave inverter reduces the likelihood of burning out a motor, derating improves your confidence that the system will operate as intended without unintended issues.

That hypothesis can be tested over time, as you use your system. And by the way, if you don't use derating, you still have a hypothesis--that your system as designed accounts for all possible factors. You will also be testing that hypothesis, if you don't use derating.

I really enjoy reading posts from those who designed something, used it, then reported out on the results after a period of time. That learning is very valuable to all of us here.
Yes!! I can't agree enough. And I really like how you explained it.

Proper de-rating / safety margins, are very important for (1) safety (2) prolonging the life of your equipment (3) managing your expectations (with PV panels for instance I ballpark 70% of rated Watts).

Especially so with cheap off-brand stuff sourced from Amazon. In these cases it pays to be even more conservative especially with things like wire and fuses.
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Old 11-18-2020, 08:29 PM   #11
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Quote:
Originally Posted by veglorde View Post
Hello! I'm in the early stages of planning my solar set up, and something just looks off, so I'd love extra sets of eyes on this spreadsheet to make sure I'm estimating my needs correctly.

My 6-window bus will be my full-time home. Although I hope to use my bus for travel in the future, I typically work the farm season and remain relatively stationary (it makes more sense when you know that rent where I live is minimum $1000/room in a house/mo). Shore power is... unlikely? So I'm counting on being entirely off-grid. This spreadsheet should have everything I hope to use, and yes, I did account for the Vitamix I hope to use for a few minutes every few days, which obviously needs to be run through an inverter.

From my calculations, it seems like I'll be needing about 600 watts of solar. This is either (a) a lot for one person, (b) a generous estimate, or (c) miscalculated. Or maybe a+b. Then there're cheat sheets like this one, which make me seriously question my calculations. Based on those descriptions, I think I'd safely put myself in Group B.

Thoughts/considerations/obvious miscalculations you can add are appreciated!
I can't see your spreadsheet for some reason. Ruth and I live 100% off grid in our bus. Being comfortable in the heat and cold is important to us. So, part of our power consideration is the ability to run our mini split 24/7. If you are not planning to read heating or Cooling on your solar, this will considerably reduce the needs for the size of your battery storage, and number of solar panels. However, I see in your text that you're planning on running a Vitamix from time to time, so you will need at least 1000 watt inverter. Things with motors in them tend to need considerably more than they're running wattage at startup.

Ruth and I built our system by adding panels and batteries as our electrical needs demanded. We currently have a seven and a half cubic foot deep freeze, 2 4 1/2 cubic foot refrigerators, an 800 watt microwave, 1000 watt toaster oven, a 6-quart instant pot, a 50-inch television set, various LED lights, laptops, cell phones, Sound Bars, and of course the mini split. We are able to run any combination of these devices at any time of the day or night and never need to plug into Shore power or generator. Our AC house current is provided buy a power jack 8000w Power Jack pure sine wave inverter, about $400. We have 12 325 Watt SunPower solar panels on the roof, about $1,800. 3 60 amp solar controllers about $175 each. Our big investment was our lithium iron phosphate batteries. We bought two matched pairs byd 200 amp hour battery packs, used on eBay, $880 per pair. The current price from the same seller is $1,012 per pair. You could probably run comfortably with half the solar panels and a quarter of the batteries if you didn't want to run a mini split or air conditioner. It's a really good idea in terms of efficiency to go 24 volt instead of 12. A 24-volt pure sine wave inverter is more efficient than a 12 volt inverter. Going 24 volts also allows you to utilize more of the output from your solar panels / solar controller.
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Old 02-28-2021, 04:12 PM   #12
Mini-Skoolie
 
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Hello again, friends...

Back with a question I know has been covered through and through on this forum. But I'm still struggling, so here we go:

- no shore power
- Renogy DCC50S 50A DC-DC charger (charge controller)
- 12V 400ah LiFePO4 batteries
- 2000W inverter

I'm ~mostly~ using this guide to get myself situated:
https://www.mobile-solarpower.com/si...-charging.html

but... it says "ground to chassis/starter battery negative" and this confuses me so much.

I've read all of the warnings about "hot skin" and everything in my DC load goes back to negative bus on fuse panel, so no accessories are being grounded locally. The only things I will have to ground are all of the normal things -- batteries, inverter, panels, DC load.

Because this is a DC-DC unit, I'm needing some clarification: do I NEED to run a negative cable from my car battery to my charge controller, or can I skip this (i.e. the car battery is already grounded to chassis)? If I do need to run from charge controller to car battery, does everything else get grounded that way, too, or do I ground everything else to chassis?

Getting lost in the "use chassis as common ground" mentality when it comes to the alternator charging, and wondering if EVERYTHING should be grounded to one OR the other, or if I should be running to both.

Thank you so much (I'm losing my mind)!!!!!
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