need advice on upgrading off grid system

dlarson59 · December 2014

Hello all, first but I'm sure not my last post

I want to upgrade my current system that I have in a remote cabin in Green's Peak AZ. My current system consists of 4 Kyocera 140 watt panels, an older Trace 12V inverter, Morningstar 45 controller and 10 Trojan 105 6v batteries. This system has worked well for many years but had a bad lighting strike earlier this year and both the controller and the inverter took a hit. Overall we do not run a lot of appliances but as retirement approaches, I plan to spend more time at the cabin and since I need to replace some items, I was thinking now would be a good time to upgrade the system to hand future needs. We run mainly 12v lights, which I plan on changing over to LED and a few 120v lights which are LED, we run a 40" TV and Direct TV box, we have a microwave that I can't remember the last time we used but it could see use in the future. The main thing I want to add is a 17-18 cubic foot 120v top freezer refrigerator to replace an old 1950's propane refrigerator and a 120v stereo system. Any input on staying with 12v, converting to 24V?, going to L16 batteries?, more panels? brands to go with for new needed equipment, brands to stay away from would be greatly appreciated. Thanks

mike95490 · December 2014

Re: need advice on upgrading off grid system

First, you want to avoid parallel batteries, like your old system used...

2nd, do you want batteries to last you 2 years ($) or 8 years ($$$$$$) (golf cart 200A, 6V) (RE grade 600A, 2V)

Adding a fridge will for sure, put you into 24V territory, and maybe consider 48V. Higher voltage has less losses,
smaller cables..... And you will need more solar to keep the batteries charged. As you spend more time there,
and less idle days for the system to recover, you need to plan for a backup generator too.

If you are in a vulnerable lightning area, maybe adding a couple "flagpoles" to divert strikes away from your house/array, would be a consideration too.

BB. · December 2014

Re: need advice on upgrading off grid system

Starting with your loads--About 1,000 WH (1 kWH) per day is OK for an off grid/cabin type system (some LED lights, TV/Laptop Computer, water pump to pressurize the cabin water system, etc.).

A 12 volt system with a 300-1,200 watt or AC inverter usually works well.

Add a full size engerystar refrigerator (living full time off grid), you would be looking at a 3.3 kWH per day system and a 1,200 to 1,500+ Watt AC inverter. And you are in the realm of a 24 volt minimum AC battery bank. Above power+Refrigerator + well pump + washing machine + etc. (basically as near as you can get to having a normal electrical life in an off grid system--Obviously, need to be very conservation minded on power use).

So, assuming a 2 day "no sun" and 50% maximum discharge for the battery bank:

1,000 WH per day * 1/12 volts * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank
3,300 WH per day * 1/24 volts * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank

Recharging rates for an off grid battery bank run from ~5% to 13% for solar power. 5% can work OK for a weekend/seasonal system. 10%+ is recommended for a full time off grid home. So, using 10% rate of charge for solar and the above battery banks:

392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 738 Watt array minimum
647 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,437 Watt array minimum

How much power would a 10% solar array generate. Using PV Watts with a fixed array, tilted to latitude (33.5 degrees from horizontal), near Phoenix AZ:

Month
Solar Radiation
(kWh/m²/day)

1
5.09

2
6.06

3
6.61

4
7.54

5
7.53

6
7.28

7
7.13

8
7.17

9
7.15

10
6.75

11
5.59

12
4.88

Year
6.57

You have lots of sun there -- So assuming 4.88 hours minimum for December:

738 Watt array * 0.52 end to end system eff * 4.88 hours = 1,873 WH per day (average December day)
2,437 Watt array * 0.52 end to end system eff * 4.88 hours = 6,184 WH per day (ave December day)

So--Basically for stormy/heavy cloudy weather, you would use 1/3.3 kWH per day for two days, then start the genset.

For sunny weather, you can use additional power during the day (water pumping, shop tools, cooking, etc.) or 1.9/6.2 kWH per day minimum (and a lot more during summer).

Roughly you are looking at a 1:3.3 price difference between 1/3.3 kWH per day system. Energy usage is a highly personal set of choices. What would work best for your needs? Will this be eventually a full time home? Do you need A/C? Want solar hot water? etc...

You can build a larger system, but they do become even more costly to maintain and you may want to have it professional installed (always fun finding a good contractor). And you probably would want some sort of backup genset+AC charger too (don't want to kill your expensive battery bank with a lot of deep cycling).

More or less, look at your power costing around $1 to $2 per kWatt*Hour, or ~10x what grid power costs in the US. Some people have gotten it down to $0.50 or so per kWH--But start with conservation first.

-Bill

dlarson59 · December 2014

Re: need advice on upgrading off grid system

BB. wrote: »

Starting with your loads--About 1,000 WH (1 kWH) per day is OK for an off grid/cabin type system (some LED lights, TV/Laptop Computer, water pump to pressurize the cabin water system, etc.).

A 12 volt system with a 300-1,200 watt or AC inverter usually works well.

Add a full size engerystar refrigerator (living full time off grid), you would be looking at a 3.3 kWH per day system and a 1,200 to 1,500+ Watt AC inverter. And you are in the realm of a 24 volt minimum AC battery bank. Above power+Refrigerator + well pump + washing machine + etc. (basically as near as you can get to having a normal electrical life in an off grid system--Obviously, need to be very conservation minded on power use).

So, assuming a 2 day "no sun" and 50% maximum discharge for the battery bank:
1,000 WH per day * 1/12 volts * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank

3,300 WH per day * 1/24 volts * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank

Recharging rates for an off grid battery bank run from ~5% to 13% for solar power. 5% can work OK for a weekend/seasonal system. 10%+ is recommended for a full time off grid home. So, using 10% rate of charge for solar and the above battery banks:
392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 738 Watt array minimum

647 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,437 Watt array minimum

How much power would a 10% solar array generate. Using PV Watts with a fixed array, tilted to latitude (33.5 degrees from horizontal), near Phoenix AZ:

Month
Solar Radiation
(kWh/m²/day)

1
5.09

2
6.06

3
6.61

4
7.54

5
7.53

6
7.28

7
7.13

8
7.17

9
7.15

10
6.75

11
5.59

12
4.88

Year
6.57

You have lots of sun there -- So assuming 4.88 hours minimum for December:
738 Watt array * 0.52 end to end system eff * 4.88 hours = 1,873 WH per day (average December day)

2,437 Watt array * 0.52 end to end system eff * 4.88 hours = 6,184 WH per day (ave December day)

So--Basically for stormy/heavy cloudy weather, you would use 1/3.3 kWH per day for two days, then start the genset.

For sunny weather, you can use additional power during the day (water pumping, shop tools, cooking, etc.) or 1.9/6.2 kWH per day minimum (and a lot more during summer).

Roughly you are looking at a 1:3.3 price difference between 1/3.3 kWH per day system. Energy usage is a highly personal set of choices. What would work best for your needs? Will this be eventually a full time home? Do you need A/C? Want solar hot water? etc...

You can build a larger system, but they do become even more costly to maintain and you may want to have it professional installed (always fun finding a good contractor). And you probably would want some sort of backup genset+AC charger too (don't want to kill your expensive battery bank with a lot of deep cycling).

More or less, look at your power costing around $1 to $2 per kWatt*Hour, or ~10x what grid power costs in the US. Some people have gotten it down to $0.50 or so per kWH--But start with conservation first.

-Bill

Thank you Bob for your reply and the helpful information. I will start looking at components that will get me to the 3.3 KWH/day level. Although I will not be running a well pump (solely rainwater harvest, but we do run a DC on-demand pump) or washing machine (have an old ringer machine that we run directly of the generator when needed) but will as time goes on add a few additional items like a ceiling fan and such as time goes on. If you don't mind, a couple of other questions - We do not plan to live in the cabin fulltime in the winter months (maybe a few trips when snow level permits) but my question would be since there could be inactivity say four months long, would it be best to disconnect the system or just make sure the batteries get some attention during a long absence? the other question is right now is there is a 2008 Xantrex XW4024 inverter with its system control panel currently listed on my local Craig's List that just got removed from a off-grid system that got power, would this be the level of inverter I should be looking at? Thanks

dlarson59 · December 2014

Re: need advice on upgrading off grid system

Thanks Mike for the response. I agree a couple of flagpoles would be a good idea, we get a record number of lighting strikes in our area. This cabin I have has been in the family a long time and an "old timer" told my dad that grounding the equipment was a bad idea??? I can't see this logic since every piece of equipment grounding lugs? As far as $$ for batteries, are the RE grade batteries that much better in both performance and long life or just long life. Thanks

dlarson59 · December 2014

Re: need advice on upgrading off grid system

Hi Bob, I have been searching the web and have found a local outfit that is selling a complete 2040 watt array and 3,500 watt 24v Outback controller and inverter, based and what my needs are, would this system work? Thanks Dave

BB. · December 2014

Re: need advice on upgrading off grid system

What size battery bank (ah/voltage) are you looking at?

And how big of ac inverter? 3.5 kWatt is pretty large and would like to see a ~700 ah @ 48 volt battery bank (out 350 ah @ 48 volt battery bank).

Do you really need such a large inverter?

A 2kw would work pretty well unless you really want to run the microwave oven a lot, plus a bunch of other loads at the same time.

Conservation of electricity is your friend here.

-Bill

dlarson59 · December 2014

Re: need advice on upgrading off grid system

Hi Bill, This system is from Northern Ariz Wind and Sun and comes with four Crown 430ah batteries. I'm way ok with conserving just a little naïve in understanding all the ins and outs of this. If I went with a 2Kw inverter on a 24V system, knowing I want to add a engerystar refrigerator in the mix, what would be the recommended total Ah battery bank be and array size in watts should I look at. Thanks for indulging all my questions.

BB. · December 2014

Re: need advice on upgrading off grid system

I always suggest doing a paper design first (based on you loads/needs/amount of sun/etc.). if you find a kit that meets your needs and price point--Great.

My home is on the grid and we only get an hour or so power failure every few years (usually when a car or truck takes out a power pole)--So, I do not have/need a battery based off grid power system.

For a first time install--And especially if the install is in the middle of nowhere--A kit (and prewired E-Panel with breakers/bus bars/etc.) is pretty nice. While it is possible to go and buy piece parts for everything--Having a kit with all the pieces, can save you weeks of ordering/shipping the missing small parts.

A few folks have actually taken a 4x4 sheet of heavy plywood and installed/wire the entire system at home, debugged, then throw in the back of the truck and haul it to the site. Can save a lot of time and frustration.

Regarding your system... Power usage is a highly personal set of choices. What works for me or Cariboocoot/Marc, is going to be different for you.

However, your choices seem like a good start for an energy efficient/conservation minded life style. Starting with your battery bank:

430 AH * 24 volt battery bank * 0.85 AC inverter Eff * 1/2 days storage * 0.50 max discharge = 2,193 Watt*Hours per day

That may be a bit light for a refrigerator system--But if you are not there full time in the winter, probably OK (with a small backup genset). If you need more power later (less generator run time or want to use a larger AC inverter for more loads/shop tools/etc.), adding a second string of 430 AH batteries should be a very capable system.

More or less to give you some ideas of how much power you can draw from your battery bank:

430 AH * 24 volts * 0.85 inverter eff * 1/20 hour rate = 439 Watt average AC load (5 hours per night, two nights)
430 AH * 24 volts * 0.85 inverter eff * 1/8 hour rate = 1,087 Watts max recommended continuous load)
430 AH * 24 volts * 0.85 inverter eff * 1/5 hour rate = 1,754 Watt short term load (minutes to hour or so)
430 AH * 24 volts * 0.85 inverter eff * 1/2.5 hour rate = 3,509 Watt maximum surge (seconds for starting well pump, etc.)

Not saying the bank is bad or good--Just an idea of how much power you can pull from it. You decide if this meets your needs.

Charging wise, 5 to 13% rate of charge--10% or more recommended:

430 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,619 Watt array Nominal
430 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,105 Watt array typical "cost effective" maximum array

And because you live in a very sunny area, 4.88 hours of sun (December, you may need to use a genset for bad weather days):

1,619 Watt array * 0.52 end to end system eff * 4.88 hours of sun = 4,108 WH per day

Certainly enough to run your cabin system + Energy Star refrigerator during sunny weather.

For a genset, depending on the exact AC charger (or inverter-charger) you choose, roughly a 10 to 20% rate of charge:

430 AH * 29 volts charging * 1/0.80 charger eff * 1/0.80 genset derating * 0.10 rate of charge =1,948 VA rated genset
430 AH * 29 volts charging * 1/0.80 charger eff * 1/0.80 genset derating * 0.20 rate of charge = 3,897 VA rated genset

So, for your proposed battery bank, a Honda eu2000i or eu3000i would be good choices (above numbers are very rough)... The eu2000i is only rated for 1,600 VA / Watts continuous (note I have an 80% derating here)--A bit on the small side, but probably "good enough" for your needs unless you need electric start (getting old, spouse would prefer electric start which the eu3000i has).

For another genset, the Honda em4000sx might be a good choice (can be configured for auto start--Note you want to find the "sx" version with "auto choke"--really computer controlled fuel mixture).

Note the above numbers are not that accurate. I carried the digits out so you can reproduce my math and see where the numbers are used. If you are withing ~10% -- That is virtually identical for solar power (variations in weather and even measurements are not gong to be much more accurate/consistent).

-Bill

dlarson59 · December 2014

Re: need advice on upgrading off grid system

As always, thank you Bill for the information. I will use this information and start a worksheet and make some calculations. I have some time before the next season so taking advantage of gathering all the information I can and upgrade to the right system and level. I agree on the kit view, they can be less expensive in the long run and probably have 95% of everything needed.

AuricTech · December 2014

Re: need advice on upgrading off grid system

Here's a bit of good news: assuming that this link about Greens Peak, AZ is accurate, your elevation of around 9000 feet above mean sea level will increase the efficiency of your PV array, as there's less air to absorb our local fusion reactor's energy. The cooler temperatures at such an elevation will also work to increase the efficiency of your PV array, since PV panel output Voltage drops as the panel's temperature rises.

I suggest basing your PV system calculations around the derating constants you find used on this forum, with any additional power produced by your system being a welcome bonus.... :cool:

One last note: always remember that your PV array is only one part of your overall PV system.

BB. · December 2014

Re: need advice on upgrading off grid system

I believe that NAWS will also custom design/build/test/supply kits and sub-assemblies to your needs too. Call and talk with them when you have a chance.

Note that none of us Moderators work for/or have any connections with NAWS other than as volunteer moderators for this forum. We do not speak for or represent NAWS, and have no special access to NAWS.

-Bill

need advice on upgrading off grid system

Comments

Categories