Condo solutions

Mike at Energy CommissionMike at Energy Commission Solar Expert Posts: 50 ✭✭✭✭
Thanks for all the info guys, this is a great resource. I am in So CA and get lots of sunshine. For my first project I want to build a mobile stand alone array and battery just for my entertainment / computer area of my condo. This will be a portable device probably housed in something like a plastic stanley toolbox on wheels. I will want 2 batteries, use one while the other is being charged. The array (panel) will be mobile also, I can move it between my patio and my parking space. The Killawatt meter tells me the 120V load is 350 watts +- and 3 amps +-. I would be happy with 12V 85 AH AGM , or smaller batteries to swap out, the Samlex 600 watt RV inverter looks nice, I don't know for sure how to size the panel I saw a nice 250 watt 30v monocrystaline 8.22 amps I liked, but I don't want to buy more than what I need for this application. Would I be better off with a smaller 12v panel? (I do like monocrystaline panels though) The Blue Sky SC30 looks intriguing as it allows for "all charge settings are user adjustable." Wouldn't the expense of an mppt charge controller be a waste on project this small? I would use a basic shunt if it would work. I also assume the low voltage disconnect of the inverter would protect the battery from over discharge. I don't mind paying a few extra dollars for quality equipment, then on the other hand I don't want to buy bells and whistles I don't need for such a simple application. If any of you experienced guys could guide me along I would really appreciate it.

Thanks mw

vacant patio seeking a solar panel


  • simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
    Re: Condo solutions

    My reply is aimed at the "patio" situation. I also have my ""MOBILE" panels out on my deck. What I have done (and it works great) is this.. I placed them on outdoor swivel chairs. I placed wheels under the chairs to move them around the deck and of course it swivels. The panels follow the sun via me ( I'm the tracker). I have them sitting vertically on the chairs. I have a 1 inch square aluminum tube attached to the bottom of the panel with "outdoor" double faced tape. The length of this tube is just a bit longer than what is needed to allow the panel to rest on the arm rests of the chair ( as MY panels are narrower than the distance between the armrests. I also have little plastic pieces (at the shoulder area of the chair) which keeps the top of the panel from sliding off. Now to keep it from sliding off the arm rests, I have a 'good" black plastic clamp ( plastic as opposed to metal so no rust issue) on each armrest. By "easily" sliding these clamps for and aft, I can change the angle of the panel to the sun........ Even if you don't get swivel chairs, the wheels under the chair ( attached by you), allows the whole chair to move instead of a little swivel action
    I can change the angle (depending on time of the year) and I follow the sun as it crosses my deck during the day.....
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Condo solutions

    Here are a couple designs for small emergency power systems:
    BB. wrote: »
    Add this link to our Solar Beginner Post:

    Emergency Power

    Basically a very long thread that starts from the beginning with a few vague requirements through design and assembly for a "portable" solar RE off-grid power box.

    And here is another example by Mike90045 called the Solar Monolith:



    Update pictures/information here.


    The need for a MPPT type charge controller depends on the Vmp voltage of the solar panels. Vmp for a PWM controller should be around 17.5 to 18.6 volts or so... The higher the Vmp (voltage maximum power) rating above that value, the less efficient the solar panel will be.

    Roughly, below ~100 watts, there are many panels with Vmp~17.5 volts available. Over ~140 watts, most panels have Vmp>>17.5 volts and should use a MPPT charge controller so you get the most of of those panels. The larger panels are generally designed/used for Grid Tied solar power systems which "don't care" about specific Vmp values for battery charging.

    Most >>100 watt panels are much less expensive (on a $$$/watt basis) vs the smaller panels.

    Some other things to think about--140 watt and smaller mono/poly crystalline panels are small enough to ship via FedEx/UPS/etc... Larger panels usually have to ship by truck/pallet and can cost a lot more to ship (single panel/small lots can cost about as much to pack and ship as a full factory pallet of panels).

    And, very roughly, 140 watt and smaller panels are pretty easy for a single person to move around. And ~175 watt panels are about a big of panel that one person can move around and place on a roof. Over that, you should use two people to handle the panels.

    In the end, it is all about loads. What is the load you want to supply and for how long? A 250 watt panel on an off grid system will supply around 500 Watt*Hours of useful AC power per clear spring/fall day (more in summer). Or a ~250 watt AC load for two hours a day.

    Most people underestimate their loads and over estimate how much energy a solar power system can supply.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mike at Energy CommissionMike at Energy Commission Solar Expert Posts: 50 ✭✭✭✭
    Re: Condo solutions

    for my first project I want to power my entertainment / computer area with a portable battery bank, something on wheels. The KillaWatt meter reads this load at 355 watts +- and 3 amps +-, I would like to get 4 hours run time, so I'll step up to a 600 watt RV inverter the Samlex looks good. Looking at shipping costs now, probably go with 140 watt panels. Trying to understand battery charge rates 12V vs 24V and 8 amps vs 16 amps and how that affects the battery charge rate. I assume amps are amps, 12V or otherwise so why use 24V panels? I assume once the battery bank is determined you use the .5 and .13 charge rate multipliers of the batteries Amp Hr rating and size to that, otherwise you can put too many panels on and since you need to protect the batteries from charging too fast it's a waste of resources, conversely try match charge capacity with enough panels. Trying to figure out the size of the battery bank and once again 12V vs 24V?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Condo solutions

    Couple of battery charging basics.

    1). You have to have higher than battery Voltage to charge. So if you have a 24 Volt system you need array Voltage above about 30 Volts. Hence 12 Volt panels ( Vmp 17-18 ) for 12 Volt systems and 24 Volt panels (Vmp 35-36) for 24 Volt systems.

    2). You need to have enough current available to affect the battery. Putting 1 Amp to 400 Amp hours of battery is insignificant and will not charge the battery. What's more, you're trying to charge it in the limited window of opportunity of good available sun. This is usually 4 to 6 hours maximum. So we have the rule-of-thumb about trying for a peak charge current of 10% of battery capacity.

    Some essential reading:
    Battery system Voltages:
    Battery FAQ's:
  • Mike at Energy CommissionMike at Energy Commission Solar Expert Posts: 50 ✭✭✭✭
    Re: Condo solutions

    Thanks Cariboocoot will look at that.
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Condo solutions

    355 Watts is actually quite a bit of power for a "small" solar power system. My first suggestion would be to go with an LED TV (from 150-300 watts to 19 watts) and a laptop computer vs desktop (20-30 watts vs 100-200 watts).

    Anyway, just to give you an idea what you are looking at.

    First, 12 or 24 volt battery bank does not matter when looking at power usage/generation... You should think about going to 24 volt if your inverter is >1,200 watts continuous power output--That is over 100 Amps of load current at 12 VDC--takes a lot of copper to bus around those levels of current. At under 400 watts, 12 volts is OK--and can be very nice if you have 12 VDC devices (CB, HAM radios, etc.).

    So--Just start with the basics to support your loads as defined. 355 Watts * 4 hour per day (note, you should use the kill-a-watt meter's kWH measurement over your 4 hours/full day of power usage-much more accurate than looking at "average watts * time"), sunny southern California with > 4 hours of noon time equivalent sun for ~9 months of the year (the other 3 months, use grid power or backup genset). The battery bank should be 1-3 days of storage and plan on a maximum of 50% discharge in normal operation. Pick 2 days and 50% max discharge as a nice balance:
    • 355 watt load * 4 hours * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days of storage * 1/0.50 max discharge = 557 Amp*Hour @ 12 volt battery bank

    That is around 6x 6 volt 220 AH batteries (12 volt @ 660 AH). Connect two batteries in series for 12 volts, and connect the three parallel strings together.

    Next, there are two ways of calculating the size of the solar array... One is based on the 5-13% rule of thumb for recharging the battery bank, and the second is based on hours of sun per day for your region.

    First, 5-13% rate of charge:
    • 660 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 621 watt array minimum
    • 660 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,243 watt array nominal
    • 660 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,616 watt array "cost effective maximum"

    And second, based on the hours of sun per day--pick 4 hours for a sunny south western US region minimum:
    • 355 watts * 4 hours of use * 1/0.52 off grid system efficiency * 1/4 hours of sun per day = 683 Watt Array Minimum

    So, for your system, you are looking at a reasonable sized array of around 683 watts minimum to 1,616 watts maximum... And 1,243 watts as being a healthy balanced system.

    Note, the numbers are not "that exact"--Just using enough digits for you to reproduce my math (hopefully) and follow were everything is coming from.

    So, that is what a "rule of thumbs" system would look like that would meet the needs you mentioned.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mike at Energy CommissionMike at Energy Commission Solar Expert Posts: 50 ✭✭✭✭
    Re: Condo solutions

    Thanks Bill,
    I didn't realize that old 50" TV and DVD player was such a watt monster, that is a lot. Thanks for laying that out, I got the main idea and have been doing some of the math, when you line it out like that it sinks in, appreciate that. I'll look at some new appliances.
  • BB.BB. Super Moderators, Administrators Posts: 32,018 admin
    Re: Condo solutions

    You are very welcome Mike... By the way, those are just wild guesses of mine--So using your Kill-a-Watt type meter and the Energy Star website can make conservation a lot easier these days--Plus the serious energy reduction that has been done in many of today's appliances.

    You can see if you can reduce your power need by a factor of 10x, then the size of off grid power system is dramatically reduced too.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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