Load requirements for a new off-grid system

raindog

The new ~realistic~ system needs to power:

• small refrigerator    ~350 watts x 8hrs/day x 1.5 (efficiency loss due to hot environment) x 7 days/week

• laptop                         33 watts x 12hrs/day x 7 days/week

• smartphone/hotspot    43 watts x 12hrs/day x 7 days/week

• graphics display        ~120 watts x  4hrs/day x 5 days/week

• desktop                     ~500 watts x  4hrs/day x 5 days/week

• [alternatively, and probably the better solution for redundancy: new laptop with sufficient graphics capability to drive display/Photoshop/Adobe Apps (--online not an option--) ~120 watts x 4hrs/day x 5 days/week; then, reserve desktop for backup/failsafe, so ~500 watts 2hrs/day 1 day/week]

• printer                           23 watts x 0.5hr/day x 1day/week

• miscellaneous small tools used occasionally:

  • hand drill : 588 watts x 1hr/month (likely to increase);

  • rotary tool 36 watts x 1 charge/month (likely to increase);

  • dog clippers : (1) wired ~15 watts x 3hrs x 1 day/month, (2) cordless 8.85 watts x 2 (charges) x 1 day/month;

  • camera battery charger 5.04 watts (usage varies widely);

  • AA/AAA battery charger 10.5 watts (usage indeterminate);

  • weather radio .78 watts (emergency use only; can charge by solar or hand crank).

That's about it, I think.

I'm uncertain how to figure out the load values for the miscellaneous tool usage because there is so much variability in that category.

icarus

Don’t see any lighting, cooking, HVAC loads etc.

Talk any any given load multiply it out.  Fridge...350 watts X 8= 2800 watt/hours/day=2.8 kwh/day
Lap top...33X12=396wh/day=.39kwh/day.               
And so on.

You also have to factor in charging loses (~20%) inverter loses etc.to come up with a total load.

raindog

Lighting and cistern pump are running off a smaller, dc system. Stove and hot water are propane.

OK, thanks!  Still reworking these numbers ...

mcgivor

Conservation is always cheaper than building to suit large loads, so you are correct regards using a laptop over the desktop.

The refrigerator wattage seems high at 350W, I would expect it to be about half that or less, the ambient temperature dose have some effect but not as much as 50%, I've logged mine over 2 separate months one above 35°C one below 32°C, there may be a 10-20% difference. Energy ratings are conducted at 32°C I believe.

The small occasional loads will be fine with a properly designed system for the main loads as they are short duration or low consumption.
 
One point worth mentioning is time of useage, any large loads would be best used during the day as they can be powered directly from the PV array rather than by the battery at night, this will reduce battery capacity needed, PV panels are far less expensive than batteries, the refrigerator naturally needs power 24h, I was thinking along the lines of the computer equipment. 

softdown

You feel a need for that desktop - going to want to use that before ~3-5pm depending on the season. Others, perhaps, may fine tune the gist of using heavy power while it is being generated by panels vs batteries. Yesterday I used the microwave at ~4:30pm and the batteries failed to 100% recover due to the lateness of the day at 37 degrees latitude. At only ~25 degrees latitude you have fewer concerns about seasonal variations in solar availability. 

With a strong panel array you can freely use a lot of power while they are harvesting energy. 

Lighting? Fans? 

I'd like to see what may be accomplished by converting a chest freezer to a fridge via a temperature controller such as ..... and I just lost functional internet as workers log on at work. It is 8:10am here. 

raindog

@mcgivor : Yes, I just tried to find the average w for small fridges, but someone on these forums mentioned the Insignia brand; I could get one of these : 10.5 cu. ft./104.65w (very inexpensive too). I have revised the calcs along these lines.

Lol on the fate of my poor computers; the one described above was an older, recently rebuilt backup machine for the main computer, which was destroyed in Hurricane Irma when the ceiling caved in ... 

@softdown : Currently lighting is provided by the DC system, which is running 2, 6w bulbs, a small fan and the water pump (shurflo). This has performed mostly without issue, though I cannot run the fan on higher settings for very long without putting stress on the system (2x 6v 235ah MasterSolar batteries/ProStar 15 controller).  I do not usually need the fan unless the noseeums are bad. I may decide to add a fan in the new addition, but I don't plan to wire the structure (it will be build out of the native limestone, like the older structures on the island).

Luci lights, candles, ... & the computer screen is lit!

So, here's where I am re: the new system :

It looks to me that I will need about 3700w/h/day (max = 4499/min = 3044). 

If I assume an inefficiency of 1.15, need becomes 212.4wh/day. I think this number may be lower for the LiFePO4 batteries, but am not sure, and do not know what the correct factor is.

Using the 212.4 number, and assuming 5.5 average sun hours (I think this figure is low: 7.5 average/5.5 peak), i'll need 38.6ah/hour from the solar array.

• [side note: it's 11:45AM and I just started the fridge (maybe could have started earlier); the Schneider shows full charge]

Required solar panel watts: 926.4 
Battery bank w/3 safety days storage: 12.2kwh ; 462.5a

Thank you all!

softdown

I feel free to start placing loads on the system as soon as the voltage starts to climb. That gives me several hours to conduct biggest loads. But this valley is unique in being very sunny. 

Wish I could use the microwave for dinner but that would seem to be out sometime in early October. I just started constantly watching system voltage recently. Being able to easily monitor system voltage is one of my best moves ever. The three best moves?
*Starting with a quality Outback charge controller and inverter - I don't worry too much about pre-mature failure.
*Starting with a quality solar panel array and pretty good wiring.
*Getting quality batteries the second time around. Seems like everyone kills their first bank while learning things the old fashioned way. 

Estragon

At this stage, I'd stick to watts and watt-hours.  With no system voltage chosen yet, mixing amps and watts can get confusing.

Using 4kwh/day for loads, and having most/all supplied through an inverter with efficiency of ~85%, that's 4000 ÷ .85 = 4700wh/day.  Three days no sun is 4700 x 3 = 14,100 storage.

Assuming lead acid batteries max 50% design discharge, that would suggest 14,100 ÷ .5 = 28,200wh total bank.

Assuming lithium with max 80% discharge, 14,100 ÷ .80 = 17,625wh total.

Overall (charge/discharge) cycle efficiency is higher with lithium, but the difference is mainly near the end of charging.  Both are quite efficient in bulk charging, so we can leave this aside for now.

Solar panels are (STC) rated at room temp, but normally operate at higher temps with lower output, so we estimate 75% for normal output.  We want to be able to replace 14,100wh in 5hrs, so we need 14,100 ÷.75 ÷ 5hrs = ~3,760w array.  In theory, we need to also handle loads while charging, but this may be offset by getting at least some solar production on the cloudy days.  

There are other variables, choices, and inefficiencies, but hopefully this helps get the thought process going.

softdown

Three days no sun? Pretty harsh requirement in my book. Being prepared for two "sunless days" is usually considered a pretty strong system - in generally sunny locales.  

A largish solar array sitting on top of a cliff *might* draw unwanted attention. 

Estragon

IIRC, there's no generator in the plan for now, and OP is trying to cover longish periods of gloomy skies as was recently experienced.  More days of autonomy does = bigger and more costly system though.

At this time of year in my climate it can be weeks of gloom at a time, so a generator is pretty much unavoidable.  In other climates a generator may be avoidable, albeit at a cost.

softdown

I seem to remember that as well but suspect there may be a change in view. Generators are often a bargain these days. I may recommend a dual fuel genny since propane last forever and ethanol gas goes bad in a fast year. Plus I would not be surprised if Bahama'x gasoline could be "hit and miss" quality wise. 

There are many things I love about living in less developed countries (privacy is far more possible) but several prices to pay as well. 

raindog

Using Estragon's numbers and assuming good, 325w panels; 3 days safe, I get 14 panels. I'll need to do some measuring.

House orientation:

Current house is the left rectangle (~ 16' wide, sloped to the sw); addition is the right rectangle (~ 24 x 18' shed roof sloped to the se). The 2 250w panels are  currently on the sw corner, which is a porch roof (porch is about 16 x 7'. Don't know the slope yet). I will have to see how much the tree (which I plan to keep) off that se corner shades that roof. The sw facing portion of gable roof over the main house (~16 x16.5') looks like this (with the 75w panel atop):

May get some shade from those palm trees up there, eventually, but I'll be gone by then. Trees, in general, do not get very tall here, and grow slowly.

raindog

I could, perhaps, build a trellis right along the outer edge of the new, south facing deck, to hold panels. It would help shade the house too.

softdown

You must avoid any shade at all on the panels. 

raindog

@Estragon & softdown : re: generators :

• I would have to cart gasoline in small containers (~2.5 gal. is all I can lift) over several miles of steep, rocky, dirt roads that are not maintained, ever;
• machines of this kind are are very difficult to keep running in this environment;
• gasoline is likely to be scarce to none when I need it (hurricanes);
• I think it would have to be of a kind that could be used indoors, or, it would have to be able to be started from indoors and firmly mounted somewhere outside in order to be useful in an hurricane (the most likely time I would need it);
• I would just hate hearing the noise it would make.

I think I would rather go into full meltdown.
 
But, I think the converted chest freezer may be a workable idea (if it helps), and terracore's clever idea, here, might actually work for me, if it works. I'll do a bit of research around that. Problem with Sun Frost and the like, is the cost and the near impossibility of getting it fixed if something goes wrong. The former owner had an EcoSolarCool refrigerator here, before my purchase. I think the compressor died. He tried to get parts/repairs, but I think has, so far, failed.

Would it be reasonable to configure the system to run only a portion of the load during a stretch of cloudy/stormy weather? 

Or, maybe better, just use the current system to run the refrigerator (+/- other things: battery chargers, etc.)?  It appears to me that the old system, with 2 new 200ah batteries/500w panels, etc. should be ample to run a fridge + a few ~~~.

mcgivor

 
The Bahamas has on average 340 sunny days per year without much seasonal difference in length of daylight, as long as there is enough charging capacity to recoup a few days of reduced input it may be possible to get away with slightly less capacity depending on chemistry.

With a similar sunny day count where I'm located in Thailand, I've not needed to use a generator since my change to LiFePo4 a little over a year ago, not the case when using LA batteries of equal nominal capacity with the same array.. Even an overcast day will provide ~ 20-30% PV output of a normal day, in my case the array can produce ~6 KwH, per day, maximum daily useage 3.5 - 4 Kwh and storage capacity is 10 Kwh.

Designing a system for use in the tropics would be vastly different to that of one in higher latitudes for obvious reasons, the type of battery used is also a factor but looking at what you described it would appear our load demands are very similar, as are climates, although perhaps much hotter here at times being away from the ocean and a little closer to the equator at 17°N. 

Given the existing system is 24V nominal, it would be wise to make the new one the same so there is interchangeably should an inverter fail for example, my advice is build to suit needs if this is for the long term, tring to live on bare minimum is fine for short term but soon gets old. You may want to try 400Ah to start with, if more capacity is needed it's not a problem to add capacity later with LiFePo4, but based on what I've read so far of your description, I think it would be adequate even with a regular refrigerator.

You may notice my focus is on battery capacity, the first step, I would recommend using drop in replacement type for your application despite the fact that is not what I used, mine is a DIY bank, cheaper but more complicated. Have you researched a battery source yet?

softdown

kWH usage per day become less meaningful when you have direction over when, and if, the power is used. Your desktop can be used when the panels are harvesting power I would think. You could get by with a laptop when the conditions are cloudy enough to warrant that.

I didn't do the math but a glance at your rooftop has me thinking that space could be an issue with an array the size that Estragon mentioned.

I also like to keep things simple. So I might look at things this way:

"I got by with xxx amount of power for years. So if I triple it that should be fairly comfortable." Well, it is a simple way to look at things. A way that will make engineers throw their calculators at me of course. You did say the new addition would remain unpowered.

With a smaller array than laid out by Estragon you could invest the savings in a propane generator. Always pros and cons.

icarus

^ I don’t quite agree.  Solar harvest is a “fixed” number under ideal conditions.  In other words, a certain PV system would put out say 2.5 kwh total over the course of a day.  It matters little if that power is being used to charge a battery or used to power a desk top, (not including charging loses etc).  So calculating total use (regardless of time of day use) give you a base line of loads to use as a design criterion.  

Yes, there are some efficiencies to be gained by load shifting to when the sun is good, as well as when charge controllers go from bulk to absorb to float charging, BUT...that requires micro managing your system daily, taking into account the vagaries of weather.  If you know you know loads, and design accordingly, then you know how much PV capacity you ideally need in lieu of a generator, factoring in however many days of autonomy you wish to provide for.

If you figure it so close that you have to micro manage you system, I think that is a recipe for failure in most case.  all that said, I do add loads on good solar days.  I charge pall the portable tool batteries when the house batteries are near full, I charge other small batteries, and I pump water in the afternoon to avoid having to pump after dark if I can.  I don’t “need” to do any of this, but it serves to add tot he autonomy time.

Tony

raindog

@mcgivor : Yes, the last calculation I ran, though, without the fridge, was for 2.5kwh system. The recommended ah for lithium batteries, storing 3 days safe, was 345.8; close to your estimate.

I am still looking at the various LiFePo4 batteries, no particular supplier.

@softdown : Well, no, I said it would be unwired, meaning that there would not be wiring running through the walls or fixed in place for lights, etc.  I may have one outlet installed where the addition joins the old structure. I am using an extension cord running from the controller to power the laptop, smartphone, etc., at present. It seems I always need extension cords anyway, even when there are outlets. Need for flexibility, simplicity.

Problems with this way of functioning? If I want to add lights, e.g., I'm thinking I'll just add solar-powered lights (like these, e.g., [guy next door uses something like these]) as needed (leaving access at the roof soffits to run cables; e.g., soffits on northeast side of the current cottage are covered only with screen). Realize that the addition will not even have glass windows; just large screen windows with wooden, handmade, Bahama-style shutters. That's all that's really needed to fit my lifestyle here and there are many homes like this around.

Better than a generator, in my case, lol.

@icarus : I think you are as wise as the learning suggested in your avatar. I think I would be a great failure at micro-managing a system. But, I too have learned a bit about managing loads and still have a lot to learn.


Is there a reason that I should not use the current system primarily, if not exclusively, to run a fridge in the future?

I think about 400ah is a good and reasonable number.

It's expensive, but, I'd rather use a portion of what I have to enhance my life rather than giving it to the billionaires when the market crashes.

spacebass

raindog said:

@mcgivor : Yes, the last calculation I ran, though, without the fridge, was for 2.5kwh system. The recommended ah for lithium batteries, storing 3 days safe, was 345.8; close to your estimate.

I am still looking at the various LiFePo4 batteries, no particular supplier.

@softdown : Well, no, I said it would be unwired, meaning that there would not be wiring running through the walls or fixed in place for lights, etc.  I may have one outlet installed where the addition joins the old structure. I am using an extension cord running from the controller to power the laptop, smartphone, etc., at present. It seems I always need extension cords anyway, even when there are outlets. Need for flexibility, simplicity.

Problems with this way of functioning? If I want to add lights, e.g., I'm thinking I'll just add solar-powered lights (like these, e.g., [guy next door uses something like these]) as needed (leaving access at the roof soffits to run cables; e.g., soffits on northeast side of the current cottage are covered only with screen). Realize that the addition will not even have glass windows; just large screen windows with wooden, handmade, Bahama-style shutters. That's all that's really needed to fit my lifestyle here and there are many homes like this around.

Better than a generator, in my case, lol.

@icarus : I think you are as wise as the learning suggested in your avatar. I think I would be a great failure at micro-managing a system. But, I too have learned a bit about managing loads and still have a lot to learn.


Is there a reason that I should not use the current system primarily, if not exclusively, to run a fridge in the future?

I think about 400ah is a good and reasonable number.

It's expensive, but, I'd rather use a portion of what I have to enhance my life rather than giving it to the billionaires when the market crashes.

Yes 400 ah is twice what I have today , like I say work out your loads (at first in the amateurish way) then double it.