new to the field, setting up an off-grid sys, possibility for upgrade

DanSel Registered Users Posts: 5


So I'm setting up a 3000W 24V system for budget reasons, and would like to upgrade it later on to 5000W. Would it be possible to get instead a bigger inverter/charger for 5KW (5000W 48V charger inverter with 80A MPPT regulator - NOT the one in the set: Inverter + Charger + Controller MPPT 3000W 24V) to fit with panels (6x Solar panel 275W of 60 cells Bauer) and batteries (4x Ultracell UZS 600Ah 6V Stationary Battery) for 3KW, and on a later date get more panels & Batteries? Being so new to the topic and with little technical experience, what else am I not thinking off..?



PS, I'm in Portugal, the set is from Spain (as local prices are ridiculous) and probably have different brands to offer.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin

    Welcome to the forum DanSel,

    Adding/upsizing a solar power system is pretty difficult to pull off cost effectively and reliably. It is sort of like buying a compact car and trying to upsize it to a 1/2 ton pickup truck.

    My suggestion is to do several paper system designs and see what you can reuse and what needs to be replaced.

    In general, solar panels can be moved / added to -- HOWEVER, a particular model of solar panels (Watts, Vmp, Imp) may only be avialable for ~18 months, then a few years later, you want to add to your existing array, and you find out that you cannot find any of the "old style" panels, or the old style panels you find are 2-3x more expensive than the new panels you can find ($$$/Watt).

    Higher end solar charge controllers generally are rated for 12/24/48 volts at XX amps... So if you got an 80 Amp charge controller for your 24 volt battery bank, you can move it to a 48 volt battery bus and double the Watt rating of your array on the new battery bank.

    AC inverters--Typically, you are "stuck" at whatever battery bus voltage you purchase. Your 24 volt AC inverter will not work on a 48 volt battery bank. And, you should not "tap" 24 volts from your 48 volt battery bank--It will ruin your batteries (discharging 1/2 the batteries while trying to recharge the whole battery bank--1/2 the batteries are overcharged, and 1/2 are under charged--It is possible and is done with 12/24 volt vehicles--But won't go further here unless you ask).

    Mixing 2-3+ year old batteries with new batteries to take the battery bank from 24 to 48 volt battery bank... I suggest trying to avoid this. Even if your batteries last 6 years, you will be debugging/replacing part of your bank every 3 years as 1/2 the bank ages out.

    So, what to do...

    Build a smaller system that meets your initial needs and run a genset when you need more power (such as running shop tools) until you can justify the new/larger system. (I suggest a 3,300 Watt*Hour per day system would give you a near normal electrical existence--Refrigerator, LED lighting, Washing machine, small well pump, LED TV/Laptop Computer, cell phone charging and lots of conservation). Use something like 6 volt @ 200 AH "golf cart" flooded cell lead acid batteries (configured for 24 volts @ ~650 AH, last 3-5 years, and in the USA, pretty cheap).

    Then design and build your larger system and bring the charge controller and (hopefully) the solar panels (mix and match--may or may not be practical 3+ years out--Don't know).

    Sell the old system (or system components), or keep the old system (i.e., old house becomes a guest house and the new family home has the new system).

    Or, go with a 48 volt system now. Use smaller 48 volt inverter for now, and smaller battery bank, and later replace batteries+old small AC inverter with new large inverter (or even keep the smaller inverter as spare). And add solar panels. You can try paralleling another string of 48 volt batteries (mix old and new)--But still something I would avoid, if I could (all new bank, much fewer problems, and issues with old and new batteries not sharing current well).

    And, are you doing this yourself or are you hiring a local (experienced) solar installer? Is this a vacation home (part time), or full time off grid? Whatever you do, I highly suggest that you do paper designs before purchasing any equipment. I/we are into "balanced" system design. Until you know your loads and energy needs (backup generator, Watt*Hours per day by season, etc.)--It is very difficult to design a cost effective system that will meet your needs.

    Solar electric power is expensive (something like 2-5+x the cost of utility power). So, lots of conservation (insulation, LED lighting, efficient appliances, etc.) is critical to avoid building more solar power than you need. It is almost always cheaper to conserve energy than to generate it.

    Your thoughts?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanSel
    DanSel Registered Users Posts: 5

    Hi Bill,

    Thank you so much for all that:)

    The system is intended for our small off-grid not-built-yet home on a very nice river with no other option to connect to the grid. we only just bought the place and it's bare nature. After fixing the road, I'll put up a small shed for starters and I'm expecting a 3kw system will be enough for us to begin with. The set I'm looking to buy is this

    and add a generator (which i'm not sure about yet).

    And yes, I will connect it by myself and hopefully with a neighbour who has some experience. It's not easy finding good professionals around, most of them will do more harm then good (I'll go into it if you want:), but i'm a fairly reasonable man that can read a manual and ask around. With so much to buy, I can't afford the best, knowing fully well this is something I will later have to amend. But having no choice is often the best way to go ahead without the headache of tough decisions. I do, however, want to check if there is anyway I could change this set, for a few hundreds more, in such a way that will allow to upgrade it in the future. yes, something like 2 years.

    Now, suppose I take this inverter instead - if I understood your remark on the 48v thing

    so it's a 5000w but set for 24v. would this be any better?

    Thanks again!

  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,032 ✭✭✭✭✭

    Keep in mind that a 5000 watt array @ 24 volts will need 3, 80 amp MPPT charge controller$ being that it's capable of producing over 200 amps. Not to mention twice the fusing and combiner capacity than a 48 volt system would need. Don't forget all that expensive larger wire it will require.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭

    From what I've seen, kits are rarely the best way to go. They often appear to be assembled to a price point, and the trade-offs may not be best for a given application.

    The choice of inverter size, for example, should be driven mainly by large (and surge) load needs. A regular fridge will generally start and run fine on a ~1500w PSW inverter. Running a big PSW inverter all the time for small loads can use a lot of power just running the inverter. If tare load is 40w, that's ~1kwh/day just to run the inverter. MSW inverters tend to be cheaper but can be a problem with some loads.

    Best to itemize major loads, and the figure out how best to supply them.

    When I started out, I used a generator for tools, etc during the day while building/renovating stuff, and also charged a couple of cheap golf cart batteries for lights, etc at night. This allowed time to plan the solar system better, and also helped figure out what we could and couldn't live without.

    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin


    Sounds like it will be an idyllic site and home.

    Without knowing your loads, it is difficult to really figure out what system would work well for you.

    If you are building your home--Leaving a (relatively) expensive and desirable solar power system out in "the middle of no-where", some of your things (solar, tools, building supplies) may develop "feet" if you are not there during much of the time (building on weekends/summer vacation/etc.).

    A pair of gensets--Perhaps a small ~2,000 Watt Honda (or similar) good quality inverter genset to run during the day (light power tools, charging batteries, running lights, some music, etc.). And at night a 1,000 WH per day system (roughly 300 Watt maximum 12 VDC sine wave AC inverter for quite nights, battery bank to run an RV/Caravan 12/24 volt water pump, charge your cell phone and laptop).

    However, since you have a system you are looking at, we can extrapolate what such a system can do and see if it a "balanced" design.

    3,000 Watt AC inverter, 24 volt battery bank @ 600 AH, and 6x 275 Watt solar panels (guess will be 3 in series * 2 parallel connected strings). Such a system, using our (relatively conservative rules of thumbs for an off grid full time home).

    First the battery bank. For flooded cell lead acid batteries, suggest that they store 2 days of energy usage (no-sun) and maximum 50% discharge (for longer battery life). Note these batteries are rated at 600 AH @ 120 Hour discharge rate--In reality, a 20 Hour discharge rate is more like how "we" use them (i.e., 5 hours of usage in the evening, 2x nights, 50% capacity). Guessing, but these are probably around 400 AH @ 20 Hour discharge rate (you have to be very careful reading specifications--Many vendors will "error" on the nice marketing numbers side):

    • 24 volt battery bus * ~400 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 2,040 Watt*Hours per day

    If you have lots of sun, and not much cloudy weather, it may be "OK" for running a refrigerator plus some other relatively small loads (refrigerators in the US tend to be around 1,000 to 1,500 WH per day--As always, get the energy efficiency numbers from the store and/or use a Kill-a-Watt type meter to measure the actual power usage (fridge at home on grid, run a genset for a day or two, etc.). And on Day 2-3 of cloudy weather, fire up the genset, or turn off fridge (use ice, etc.).

    Next, FLA (flooded cell Lead Acid) batteries usually need 5% rate of charge minimum, and most manufacturers say they need 10% minimum rate of charge. For full time off grid, we suggest 10% to 13%+ rate of charge. For that battery bank:

    • 400 AH * 29.0 volts charging * 1/0.77 panels+controller deratings * 0.05 rate of charge = 753 Watt array minimum
    • 400 AH * 29.0 volts charging * 1/0.77 panels+controller deratings * 0.10 rate of charge = 1,506 Watt array nominal
    • 400 AH * 29.0 volts charging * 1/0.77 panels+controller deratings * 0.13 rate of charge = 1,958 Watt array "cost effective" maximum

    Array suggested:

    • 6 x 275 Watt panels = 1,650 Watt array

    Actually, that is not bad for a "kit system"--Good balance between battery bank capacity and size of solar array (>10% rate of charge).

    And then three is how much energy your system will generate. Select a city (Coimbra), facing (south), a tilt (50 degrees from vertical):

    And we see in winter 2.91 hours of sun (December) to ~3.4 hours of sun (November/January). Say you will use less power/use genset when needed, that February is your "break even" month with 4.06 hours of sun per day.

    If we use the 2,040 WH per day (matching your battery bank daily useful capacity), then it would supply in February:

    • 1,650 Watt array * 0.52 off grid AC solar system eff * 4.06 hours of sun = 3,483 WH per day (average Feb daily harvest)

    Yes, your solar panels are derated by almost 1/2 to account for the marketing numbers and other system losses. And you cannot use 100% of your daily harvest every day... I would suggest that your 'base loads' (minimum needed daily load) be around 50% to 65% of "predicted" harvest--Or you will be using the genset too much:

    • 3,483 WH per day (Feb ave) * 0.65 "fudge factor" = 2,264 WH per day "base load"

    Overall, that system is quite well sized to your region and usage (battery bank size, solar array). For a "400 AH" @ 24 volt battery bank, I would suggest a ~2,000 Watt AC inverter is a better size... To pull a true 3,000 Watts from your battery bank (and a 6,000 Watt surge--such as starting a good sized well pump), I would be suggesting closer to a 600 AH @ 24 volt FLA battery bank (rated at 20 Hour discharge rate).

    If the amount of sun is approximately correct (I don't know where your property is located, and we are assuming no shading from trees and hills/mountain ranges from ~9:00 to ~15:00 hours year round (any shading "kills" solar electric panel output).... It is a good start if you are very frugal in your energy usage or use a very very efficient refrigerator (or DC refrigerator with thick insulation).

    The kit uses a combination AC inverter + MPPT solar charge controller. All in one systems can work fine--But if one thing fails, then the other may be out of commission waiting for repairs/replacement (AC inverter fails, and the charge controller fails/removed for service).

    If the above numbers are within your needs, could be a good system (I am not in the solar business, and have no knowledge of the brands listed). Note that their 8,100 WH per day (I think that is what they are saying) is much different than my suggested Battery daily storage capacity and solar harvest. We try to be conservative--But "close enough" that there are no surprises either way. And the system will perform at rated capacity for decades to come (batteries may last 3-8 years typically, warm/hot batteries age faster, use distilled water and NOT well water when adding to electrolyte).

    Other suggestions--The Solar+controller+battery+inverters be in their own building--Not in the home or basement. And the genset+fuel be "elsewhere" too. Genset fires to happen and you don't want a fire taking out both your batteries and array, and your home.

    Some notes: I am using lots of "accuracy" in my math (3+ digits) so you can follow the numbers. In reality, in solar power systems--Anything within 10% of predicted/measured value--Is "dead on" accuracy. Today, batteries tend to be expensive (and have a relatively short life). Solar panels are generally inexpensive and have a long life. Having more solar panels can reduce genset usage and help keep your batteries "happy" (avoid going below 50% state of charge too often, get them >90% state of charge at least once per week, typically equalize charge once per month). And plan on having a cash balance to maintain your system. Batteries typically last 3-7 years, electronics 10+ years, solar panels 25+ years. And poor maintenance, or somebody visiting your home when you are not there--Leaves all of the lights on, uses a hair dryer, TV on, hose running in the yard--And they can "murder" your battery bank in a few days of such usage. You are your own "utility company" and you need to have a cash cushion so you can keep your system running. Solar is not cheap--So any conservation you do (very efficient appliances, turn stuff off when not used, use the washer, electric hot plate, etc. when the sun is shining, etc.) will save you money in the long term (smaller solar system, cost less up front, cost less to repair and maintain).

    What do you think of my guesses/suggestions?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanSel
    DanSel Registered Users Posts: 5

    Hi Bill,

    So first off, Thanks! again!

    It's the end of my day here so I'll go over your comments, 2nd time, tomorrow.

    I really liked the link you sent, though I'm near Castelo Branco, which is considerably sunnier then Coimbra.

    Till then - Muito obrigado!


  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin

    Very welcome Dan.

    You might also try Spain you are close enough to the boarder you may find a similar climate and latitude.

    Obviously, we only have the information supplied by you (and us). Please feel free to direct us to where we can be the most help.

    There are a ton of assumptions in our "rule of thumb" guides we use... But for a "typical" off grid home, they work pretty well.

    If you have other needs (irrigation for orchards, home office running a couple desktop workstations 10 hours per day, etc.)--Let us know and we can help adjust the system to better meet your needs.

    Come back when you are ready... We will be here.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanSel
    DanSel Registered Users Posts: 5

    Hi Bill!

    So let's see a basic list of uses, tell me if I'm anywhere near what I thought would be enough with this kit:

    Fridge (A+++), a few lights, 2 laptops (2-4 hours per day), 2 iphones, a small music sys(?), washing machine (twice a week), small water pump (undecided yet - any ideas? perhaps with a 12v with an independent sys just with panels)

    on the side, I have basic Makita tools, with two 4.0ah 18v batteries - which I will charge often, at least a the first few months.

    I would like to add a generator of about 2000-3000k to the mix.

    Now, did I understand you correctly - that you recommend to change the above kits' inverter/charger from 3000k to 2000k?

    And again - this is very helpful - Thanks so much!


  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin
    edited March 2019 #10


    Generally, adding a refrigerator to an off grid system--moves the system from "small solar" (~500-1,000 WH per day) to a medium size system (~3,300 WH per day).

    I don't know much about the "European" standard refrigerators (tend to be smaller than US versions, and should be pretty energy efficient)... But you need actual numbers. For a very efficient USA refrigerator, they take around 1,000 to 1,500+ Watt*Hours per day. And need a minimum of 1,200-1,500 Watt AC inverter to run them (and some LED lighting and other small loads).

    In many other countries (and few in the US) are Inverter/Linear Refrigerators--These use about the same amount of energy per day, but have very low starting surge and can "live" on a smaller AC inverter (1,000 Watt max, or a bit less).

    Inverter specifications are sort of fuzzy too... The 3kW unit in your link has a 4.8 kWatt surge capability (if the website is correct--And I read the translation correctly). Normally, we assume inverters have 2x rated capacity for surge (your 3kW would be expected to have a 6kW surge support). So, it is a bit smaller any way than our rules of thumbs assume. Need to know the Tare Loss for that inverter-charger to know if it is a good fit or not (your ~400 AH @ 24 volt battery bank would support ~4,000 Watt surge--Not a bad match to the "3kW" inverter in the kit (these numbers are fuzzy anyway--We are trying to have a reliable system over time and state of charge using these rules of thumbs).

    And the inverter-solar charger has a 50 amp charging limit--Which limits the "cost effective" maximum array to ~1,883 Watts... If you went with a larger battery bank (600 AH or larger), you would probably want a larger solar charge controller (or 2nd controller) to use a larger array (the problem with "all in one units"--The size of the inverter vs the size of the charge controller don't always match).

    Another inverter question is its "tare" loss... Basically, how many watts does the inverter take just being "turned on". An inverter for your needs may have 10-20 Watts tare loss, but some "less than ideal" inverters have 20-40+ Watts tare losses--Which becomes significant for smaller systems (your refrigerator may take, on average 60 Watts average power, and a 40+ Watt tare loss is almost like adding a second refrigerator). Larger inverters naturally have higher tare loss, and why we try to not "oversize" the AC inverter (smaller inverter that still meets your needs, the less tare loss).

    And, you can find refrigerators that use less power... A popular conversion is to take a middle size chest freezer and replace the thermostat for refrigerator temperatures... People get down towards 250 WH per day (1/4 that of a refrigerator). Down sides include no freezer section and having to lift and move baskets around to get at your food. Still need the 1,200 to 1,500 Watt minimum inverter because cheap chest freezers still use standard induction motor compressors, which still have the high starting surge.

    Assuming you are already in Europe/Portugal--Setting up your present home/apartment with the refrigerator and appliances you plan on using, and get a Kill-a-Watt type meter to measure each--And get a profile of your energy usage. That makes your system design much more accurate.

    Regarding the water pump. Again, details matter. Many folks (and well drillers) simply put a large pump in the hole and a pressure tank to give you stable pressurized water to your home. These well pump motors are many times over 1kWatt and need a pretty large solar power system to manage the starting surge and power to keep the house with water.

    Another way to do this is use a "slow" (low power) pump in the well and pump to a cistern. Then use a 12 or 24 volt RV (Caravan) water pump from the cistern to pressurize the home. In some places, elevated water tanks (on roof) are common. Or if your property has a hill where you can pump water to is another option.

    There are also in well pumps that are very "solar friendly" (some can even run directly from any power source, solar panels, battery bank, or genset directly). However these pumps are not cheap. But they are usually worth the expense when compared to building a larger solar power system to run the large/cheap pumps.

    Grundfos has a very broad range of pumps that are high quality (and not cheap) water pumps that will let you do just about anything you want:

    Energy usage is a highly personal set of choices. What works for me may not work for you. To build out a "cost effective" solar power system... I suggest aiming at 100 kWH per month or 3.3 kWH per day (3,300 WH) for your energy usage (some days will be more, others will be less). The choice to install more solar panels than you "need" is a tradeoff with almost never needing a backup genset (save in fuel and maintenance). These are choices you have to make--And you really need to know your loads to make these choices. Will a 3.3 kWH per day system be right for you? It is not my decision, it has to be yours. At that level with a refrigerator, you will still have to run lots of conservation and turning stuff off when not in use, but it can be a 'near normal' electrical life.

    Regarding gensets--Again, knowing your loads. For a genset, people generally use 10 to 20% rate of charge (~25% maximum for FLA batteries). As an example of the math:

    • 400 AH battery bank * 29 volts charging * 0.10 rate of charge * 1/0.8 charger losses/Power Factor * 1/0.8 genset derating = 1,812.5 Watt (VA -- Another discussion) minimum suggested rating for genset with 40 Amp @ 24 VDC charger

    You probably could get away with a 5% rate of charge in a sunny condition. Choosing a good battery charger (efficient and power factor corrected) will help keep the generator size smaller.

    The other option, a small/efficient genset (like a Honda eu20i or similar inverter-genset--quiet and efficient at lower power levels--probably too small for a 40 amp @ 24 volt AC charger). And get a cheap/noisy 3-5 kWatt genset for backup and running the occasional large power tools/fast charging your battery bank.

    Fuel choices are gasoline and diesel--Both need fuel preservative. Diesels are noisy and smelly, but can have a longer life. Propane is a great fuel for storage (keeps forever) and burns clean in a genset (gasoline genset propane conversions are available too). And if you already have propane for cooking and heating--You may already a genset fuel supply (just get a larger tank if you plan on running the genset for longer periods).

    Size matters with gensets too... Amount of fuel stored, propane vaporization issues (gas from tank, small tanks get cold and frosty. Larger propane engines take liquid propane and have on-board vaporizer using engine heat).

    Lots of research and decisions to make. 😃


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanSel
    DanSel Registered Users Posts: 5

    Wow Bill,

    I'll have to think of this some more, get my numbers right and get back to you......

    meantime - Thank you ever so much 👏🙏