Absolute Beginner; Looking for advice on a back up system

nb12345
nb12345 Registered Users Posts: 2
Hello Everyone,

Right I am currently living in southern Africa in a particularly nice area prone to load shedding specifically the block/street I live on. I occurs very least once a week generally the power outage is for approx 4-6 hours in the evening perfectly timed when I arrive from work and want to put my feet up. So I've had enough of sitting in the dark somewhat bored and wish to start a little solar project, as a back up when these blackouts occur. When these happen I only want a few things to work during this phase essentially 2-3 lights, TV, PVR and potentially my router as well.

32" TV ~ 80w
PVR ~ 30w
Lights ~ 60w (just normal bulbs though Im going to switch them out for more energy efficient ones)

So I am currently thinking of just employing a small off-grid system to cover this. My biggest question though comes at connecting the output from an inverter to the loads listed above. In my mind I picture PV system and the AC mains supply wired to the main panel via a transfer switch (automatic though depending on costs maybe manual). That way in the event of a power cut I can separate my home from the mains and not cause any risk to someone working on the lines whilst still enjoying my quality TV. (I feel I should say that the power derived from the pv system would be wired to the specific panel(s) needed).

Is this a correct line of thought? I understand I could run an extension cable from the inverter though I don't particularly wish to do that unless its the only option available.

Currently I've bought nothing in terms of hardware I am in a process of pure pen and paper planning out the details (well learning pv systems). Any help would be greatly appreciated, not only on this post but once I've finalized plans any help/corrections are greatly welcome before I start spending.

If I've missed anything or its not clear please say and I'll fill in the voids or explain myself in a different manner

Just thanks again :)

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Absolute Beginner; Looking for advice on a back up system

    welcome to the forum.
    yes, i think i agree that a backups type thing is what you want. i don't know the frequency and voltage for your utility, but i'm sure there's something made to cover your needs in that respect. you have even gone through your load requirements, which is a good thing, but you failed to state if you want all of those loads going for the whole outage period of up to 6hrs.

    making the worst case assumption we get the 170 total watts x 6hrs to be 1020wh. for a 12v battery this is 85ah used. now this does not account for efficiencies so adding another 20% or so for that gives us about 102ah. now you don't want to go below 50% dod on your batteries so at least doubling the capacity to 204ah minimally is warranted. this works out well for many 6v type golf cart batteries as many are in the 210ah to 230ah class. you could also expand this to account for future loads and i noted you did not include a refrigerator to be running.

    yes, the loads can be switched from an automatic transfer switch and there are many inverters with this feature built into them, but your loads are light at this point and you will need to weigh if the smaller sine wave inverter without bells and whistles with external transfer switch is the better deal or not. btw, we do recommend the sine wave type of inverters over the modified type, but you may get away with a modsine inverter with your presented loads.

    the easiest way here is to feed the utility power through an inverter with a built in ats that then feeds the ac circuits you want backed up. this may need to have a sub breaker box for more than one circuit. when the utility ac fails the circuits automatically switch on from the ats and activate the inverter to keep an almost seamless continuation of power to those circuits.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Absolute Beginner; Looking for advice on a back up system

    And to continue with Niel's train of thought... Setup the battery bank + AC Transfer switch + AC Battery charger to run your loads for the length of your "normal" blackouts (or at least your evening power needs), and recharge the battery bank when the AC power comes back on.

    You now have a nice UPS system (uninterruptable power supply) built with components that meet your needs using locally available components.

    The addition of solar panels is purely your choice. The problem with "backup UPS type solar power systems" is that they almost never make economic sense.

    You need power at night--So it does not really extend your run time. And if the power only goes out once a week and you should recharge the battery bank in ~2 days -- The solar power may cost more than just using AC power. And using the batteries every day may tend to "wear them out faster"--So you end up with more battery costs over time--again, making the solar power system "expensive" compared with utility power.

    But in any... To give you some ideas. Two 6 volt @ 220 AH batteries (in series) connected to your AC Inverter (~ few hundred watt TSW inverter would be very nice). I will link to this inverter (it is very nice and about perfect for your needs)--but you will have to look and see what makes sense for your region of the world (Botswana?). In the ideal world, I would highly suggest using a TSW (True Sine Wave) AC Inverter.... MSW (Modified Square/Sine Wave) inverters work OK for ~80% of the devices out there, but for ~10-20%, it can cause them to over heat (some motors, small AC power supplies/wall mounted transformers, etc.)--Some devices will have a shorter than normal life, a few can die in days or minutes... It is very difficult to know.

    For an Automatic Transfer Switch, you might be able to purchase something locally (or over the Internet) like this...

    wind-sun_2249_13338696PowerMax PMTS-30 Automatic Transfer Switch

    However, you can also purchase a simple two pole (contact) Normally Open / Normally Closed relay. Power it from the AC Utility power, and when the power fails, the relay transfers over to the AC Inverter's output (the relay keeps the Grid separate from the AC utility--and keeps the magic smoke inside the inverter where is belongs). You do not want too big of AC inverter, because that can waste power (AC inverters can draw 6-20 watts, or more, just by being "turned on").

    So, with a 12 volt 220 AH battery bank. We recommend around a 5% to 13% rate of charge (to upwards of 25% rate of charge with an AC battery charger). So, for your AC Charger, a 10%-25% rate of charge would be:
    • 220 AH * 10% = 22 Amps "nominal"
    • 220 AH * 25% = 55 Amps "not to exceed"

    For solar panels (if you want to use them still), the "useful" charge rate is around 5% to 13% rate of charge, with deratings and losses:
    • 220 AH * 14.5 volts batt charging voltage * 1/0.77 panel+controller deratings * 0.05 rate of charge = 207 Watt array "minimum"
    • 220 AH * 14.5 volts batt charging voltage * 1/0.77 panel+controller deratings * 0.10 rate of charge = 414 Watt array "nominal"
    • 220 AH * 14.5 volts batt charging voltage * 1/0.77 panel+controller deratings * 0.13 rate of charge = 539 Watt array "cost effective maximum"

    Keep an eye on the battery bank when charging. Normally, the maximum charge rate is around C/8 (~12.5%) rate of charge. Higher charging rates can make the battery bank run hot/overheat. Many solar charge controllers (and fewer AC battery chargers) do have remote battery temperature sensor options that help reduce the charging voltage/current if the battery bank runs "hot" (hot batteries do not last as long as "cool" batteries).

    And in the case of AC power + Solar Charging--You can get fairly high charging currents.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Absolute Beginner; Looking for advice on a back up system

    hmmm. i forgot to mention the charger and thanks for mentioning this bill, but many of the inverters that have a built in ats do have a built in charger and of course we are talking more of the quality sine wave type inverters with all of this.
  • nb12345
    nb12345 Registered Users Posts: 2
    Re: Absolute Beginner; Looking for advice on a back up system

    Thanks greatly Niel & BB, I currently in the desert ATM on a site project just quickly scanned over your answers on my phone. When I get back to my PC I'll continue in further details but HUGE THANKS again