Small Texas Off Grid System Sizing

Newton_Lass · February 2020

Hello all. Great forum! There's a ton of great information on here. I am looking for some suggestive input on the smallest set up I can get away with. Here are some details of the project-

Location: West Texas in the Big Bend/Terlingua region.
Use: Full time residence in a small, 600 sq/ft well insulated house.
Power items: Sanyo 9000 BTU Mini Split, LED porch and house lights, two ceiling fans, small water pump for pressure from rainwater tanks, laptop for 6 hours/day, cell phone charging, converted chest freezer for refrigeration, small chest deep freeze (5 cu/ft), radio and WiFi.

Grid hookup is out of the question. Panels will be ground mounted with zero shade obstruction. House is being built on 40 very remote acres. Cooking and hot water will be done with small propane tanks. That is all the items I can think of that we would need on a daily basis. We like to keep it as simple as possible. I have a backup generator for emergencies in case for some odd reason there is no sun for 3 days, so no need to oversize for worst case scenario. The temperatures at night are sufficient to not run the AC all night for 9 months out of the year. The 3 hottest months will probably require the AC running low most of the night.

I would like to find out how many watts worth of solar panels I need, smallest battery bank that would be sufficient, what size power inverter and charge controller I will need. I think that is all that I need. Any input is greatly appreciated! Thank y'all!

littleharbor2 · February 2020

Newton, Welcome to the forum. You should learn a lot here. The best way is to learn how to approach this and not just be given answers. There's plenty of members here who you can learn a lot from.

You have to start with an accurate load analyses, including total watt hours per day and maximum surge power from the large loads. This will allow you to size your battery bank, system voltage and inverter size, which will allow you to figure out how much charging current you'll need to properly charge those batteries. Doing it right requires it be done in this order.

Hint* a 12 volt system is very likely out of the question. You'll end up spending more on cabling, charge controllers and possibly early battery replacement than with a higher voltage system.

Newton_Lass · February 2020

Littleharbor, thank you very much for the response.

That helps a lot. I will get to work on the load analysis. To calculate the total watt hours per day do I simply try to estimate how much each appliance will take to run per hour and how many hours a day I expect it to run for? I assume that to estimate the maximum surge power would be to take a total of how ever many appliances would be running at a given time? Ex.- Air Conditioner + water pump + lights + laptop at the same time? I am not very educated in electrical so I am sorry if this is a silly question.

Again, thank you for your response. Very helpful.

littleharbor2 · February 2020

Yes, , try to include everything, including any anticipated additions you may have. Things with motors such as air conditioners and water pumps are where you need to look for surge loads.
Once you get a good load estimate you can figure your battery bank size. generally you would aim for a daily discharge of 20 % of your battery bank so the battery should have 5 times the watt hour capacity. This will give you two days + of usable capacity without discharging below 50 %. Watt hours are the battery's amp hour size times the voltage.
This is a rough explanation as I'm not so eloquent in my descriptions. BB, OTOH, can give a highly detailed breakdown once you come up with the needed load analysis. Oh, your location also needs to be factored into the solar array sizing as the amount of sun hours per day on the shortest days of the year is what you need to base solar charging on.

706jim · February 2020

Two fridges and an air conditioner?

I see at least 5kw in panels so this will not be a "small" system IMO.

Newton_Lass · February 2020

Ok. I think I have a total for watt hours per day:

Sanyo 9000btu mini split- 10 hours a day max at 400 watt run for a total of 4,000 watt/day
Water pump- 104 watts a day assuming 1 hour of run time a day
5.0 cuft deep freeze- 35 watts a day
converted chest freezer to fridge- 150 watts a day
ceiling fan- 155 watts a day assuming only running for 3 hours

Total watt hours per day: 4,304

Max surge power:

Sanyo mini split- 782 watt at max capacity, 6.8 A
Water pump- 104 watt, 0.9 A
Deep freezer- 800 starting watt
chest freeze conversion- 800 watt start
fan- 73 watts at high speed

I think that puts my max surge power at 1,847 watt

Does this look about right to move forward? Thank you very much for the help.

littleharbor2 · February 2020

You'll never have all items surging at any one time. That being said all A microwave, (ok because of short duration use) or toaster, (not off-grid friendly), are going to be higher draw than any one or two other items combined. a 3800 to 4000 watt inverter would easily cover your needs.

Go ahead and do the math . I have and see a 48 volt system in your future.

706jim · February 2020

Remember that the current use of your two fridges has to be multiplied by their running hours, typically about 40% of the time. So a 150 watt fridge x 24 hr x 40% works out to about 1.4 Kw-hr/day.

BB. · February 2020

Newton_Lass said:

Ok. I think I have a total for watt hours per day:

Sanyo 9000btu mini split- 10 hours a day max at 400 watt run for a total of 4,000 watt/day

That is possible... That is a 400 Watt load (mini-split set to low power/small difference between thermosat and room temp, etc.).:

400 Watts * 10 hours per day = 4,000 WH per day

Water pump- 104 watts a day assuming 1 hour of run time a day

Need to know more here... If you are talking about an RV style water pump (for example, 12 volts x 9 amps):

12 volts * 9 amps * 1 hour per day = 108 WH per day (at 12 volts).

Most other water pumps (shallow/deep well, 120/240 VAC) take quite a bit more power--But, if not used often (i.e., not 6 hours a day irrigation, high pressure sprinklers), water pumping is not usually a big energy hot. However, a well pump with an induction motor--Those starting loads can be very large (2,000+ Watts) pretty easily, and going for "solar friendly" well pump can make things a lot easier (i.e., ~900 Watts with no surge, AC or DC or Either).... These types of pumps (solar friendly) tend to be expensive.

5.0 cuft deep freeze- 35 watts a day

Back to looking for details here... A 120 VAC deep freeze can easily take 1,000 WH per day. For example:

120 Watt motor * 0.50 duty cycle (runs 1/2 the time in warm weather) * 24 hours per day = 1,440 WH per day

Picking the "right appliance" can help... A chest type deep freeze can be more energy efficient than an upright freezer... However, the spouses usually like upright/frost free types instead.

converted chest freezer to fridge- 150 watts a day

Typically, converted chest freezer to chest refrigerator have been reported more like the 250 WH to 500 WH per day. 150 WH per day is a bit on the low side.

There are AC and 12/24 VDC versions of fridge/freezers now... Picking the right device (AC units are generally cheaper and easier to find service persons) can save costs (for appliance) but cause issues for the solar power side (running a fairly large AC inverter 24 hours per day at 20 Watts Tare--no load power draw):

20 Watt AC inverter On (no load) * 24 hours per day = 480 WH of "wasted energy"

Larger solar power systems, 480 WH is not a huge factor... For small solar power systems, that is more power than your converted chest refrigerator may draw.

ceiling fan- 155 watts a day assuming only running for 3 hours

Need to confirm that this is (155WH / 3 hours = ) 51.7 Watt average load

Ceiling fans are all over the place regarding energy usage... There are DC fans available, and (should be) energy conserving AC fans too.

Total watt hours per day: 4,304

The AC usage of the mini-split... Really need to measure actual power usage... Some sort of Kill-a-Watt meter and running in your application. AC system energy usage are not small (and there are heat pump systems too--These are looking very interesting even down towards 0F).

Might even be worth building the home first, and powering for a few weeks on Genset power--Monitor actual energy usage. (of course, paper designing and costing out your solar power system before you cut earth, very important too--Avoid surprises).

Max surge power:

Sanyo mini split- 782 watt at max capacity, 6.8 A
Water pump- 104 watt, 0.9 A
Deep freezer- 800 starting watt
chest freeze conversion- 800 watt start
fan- 73 watts at high speed

I think that puts my max surge power at 1,847 watt

Does this look about right to move forward? Thank you very much for the help.

For larger power systems, surge is not usually a big issue (and the mini-split) is a very surge friendly pick.

Most inverter power systems will surge ~2x rated capacity (2.5 kW inverter will surge ~5.0 kWH/kVA with a good sized battery bank and heavy DC Bus wiring).

Here are some links to measuring devices... Just to help you in your education/search. Some I have used, many I have not.

https://www.amazon.com/s?k=kill+a+watt&ref=nb_sb_noss (Kill-a-Watt type power/energy meters... Cheap and good for measuring one appliance at a time)
https://www.amazon.com/s?k=dc+wh+ah+meter&ref=nb_sb_noss (DC versions of power/energy meters--Lots of low cost units out there)
https://www.amazon.com/gp/product/B019CY4FB4 https://www.amazon.com/gp/product/B019CY4FB4 (mid priced AC/DC current clamp DMM)
http://theenergydetective.com/ (a "whole house" 120/240 VAC energy monitor--With options for multiple branch circuits)

Units like Inverter based Mini-Split systems can have highly variable energy consumption based on how you use the system, crank case heater, spread between room setpoint and actual room temperature (typically start AC earlier in the day, and keep on low power, vs running in the afternoon/evening for fewer hours at high power). Sanyo was purchased by Panasonic, and these 120 VAC units are no longer available (have to pick 240 VAC units--As far as I know). Just some light reading:

Sanyo mini split AC (inverter/variable speed)
Sanyo Single Split systems (3,000-9,000 BTU)
smallest, most efficient A/C ?

Also a general purpose FAQ... Lots of different subjects cataloged here:

https://forum.solar-electric.com/discussion/4426/working-thread-for-solar-beginner-post-faq/p1

-Bill

Photowhit · February 2020

Here's link to energy star freezers, The best listed 5 cuft uses l72 kWhs per year or (l72/365=) 47l watt hours day. Converted to fridge temps maybe half that...

https://www.energystar.gov/productfinder/product/certified-residential-freezers/results

If you have them now, you could plug them into a Kill-a-watt meter bout $20 home depot;

Kill-A-Watt Electricity Monitor

How you define 'well insulated' likely will effect how close your air conditioner numbers are... 6" r24 or better walls? 35+ roof and floors. Out of direct sunlight?

I think you are getting the idea, remember laptops, lights, modems, TVs...

It's a sunny area, so long days of summer will help with air conditioning.

mike95490 · February 2020

Attached (I hope) is a simple spreadsheet I use to play with load sizing.

I've left examples in, retype with your gear and values.

AC watts are not the same as DC watts, a nasty bugger called Power Factor (PF) causes the inverter or generator to work harder than expected. Good appliances have PF listed on their name tag, other things, you can measure PF with a Kill-a-Watt meter

Also, it's unusual for all loads to be OFF when your air con kicks on, so you don't have the full surge capacity at all times, I never like to load a generator or inverter beyond 70% of max continuous rating.

Mike

BB. · February 2020

Just to clarify Photowhit's freezer specifications.

Freezer is rated for 172 kWH per year
Or 172,000 WH per day
172,000 WH per day year / 365 Days per year = 471 Watt*Hours per day

-Bill

mcgivor · February 2020

BB. said:

Just to clarify Photowhit's freezer specifications.
Freezer is rated for 172 kWH per year
Or 172,000 WH per day
172,000 WH per day / 365 Days per year = 471 Watt*Hours per day
-Bill

Should read 172,000 WH per year / 365...............

BB. · February 2020

Thank you McGivor... I have fixed the post.

-Bill

Photowhit · February 2020

Thank you @BB. and @mcgivor ...

Small Texas Off Grid System Sizing

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