Help with small system design

DaramusDaramus Registered Users Posts: 2

I have been lurking for some time and I finally took the opportunity to register so I could post a question. I live in Phoenix, Arizona, and I want to buy a small solar setup to recharge my portable devices at night. In an effort to minimize our life and conserve electricity, we only use battery powered devices during the "peak" electrical hours to assist in keeping our summer electrical bill low. My problem, and where I need assistance is calculating battery and panel size for my needs. I do have a killawatt meter and here is the info I have gathered so far:

3 DS lite consoles - 4 watt / .07 amp (each)
Cell phone - 5 watt / .07 amp
Nook tablet - 6 watt / .09 amp
2 laptops - 40 watt each

I plan to use each item daily until the batteries are low. My goal is to completely recharge these items once throughout the day. I would like a one panel / one battery solution and the battery needs to be stored inside so I will need AGM battery. I am looking for optimal recharge rate and don't want to take the battery below 50%, etc. Any assistance would be appreciated. I just don't want to by something and then find out it doesn't meet my needs, so I am coming to the knowledgeable folks here for guidance. :)


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help with small system design

    Welcome to the forum.

    There's an important factor missing from your data: time. You need to know not only how many Watts each device takes but how much time it takes to recharge it.

    On the other hand since solar works only in daylight anyway you could just allocate for running everything off solar during whatever sun hours you get and let the batteries in the devices supply power in darkness for as long as they can. If, however, you want to extend run time into dark hours with external battery power the system becomes larger and more expensive.

    For instance one laptop @ 40 Watts could obviously be power by slightly more than 40 Watts of panel all day (about 5-6 hours where you are). It then has enough battery power built-in to run for another 2 or 3 hours. So you'd get up to 9 hours of run time with just a 60 Watt panel. If you can use the DC direct and avoid external battery & inverter.

    If you want to have the total power available 24 hours a day you have to add everything up (103 Watts as listed), factor the inverter efficiency (90% +/-) add in the inverter consumption (a small inverter ~ 6 Watts) multiply by 24 and get 2880 Watt hours. That will be much bigger than just supply power for recharging from solar, and you probably won't use it.

    So you also need to know for how long each device will draw its power. Otherwise you get a system that's too small or too expensive.
  • DaramusDaramus Registered Users Posts: 2
    Re: Help with small system design

    Thanks for the welcome and for your reply. At most, each device would be charging a maximum of 3 hours per day.
  • BB.BB. Super Moderators, Administrators Posts: 32,013 admin
    Re: Help with small system design

    Welcome to the forum Daramus.

    We are missing one piece of the puzzle here... How many hours per day for each appliance. I.e., a 100 watt light running 8 hours per day would be 800 Watt*Hours per day.

    The simplest way is to use the Kill-a-Watt Watt*Hour meter and measure your usage over one to several days. The real "killers" here in power usage will be the laptop computers if they are run for 10+ hours per day each.

    To give you an idea, I will do some sample calculators and you can plug your numbers into the equation. For example:
    • 2 * 40 Watts * 10 hours = 800 Watt*Hours per day
    • 6 watts * 4 hours per day = 24 Watt*Hours per day
    • 5 Watts * 8 hours per day = 40 WH per day
    • 3 * 4 Watts * 4 hours per day = 48 WH per day
    912 Watt*Hours per day

    That is actually very near the power usage of a single full sized Energy Star rated refrigerator (1-1.5 kWH per day).

    Before I "run the numbers", a little discussion about your power options. In general, if you want to save money, a Grid Tied Power system is the only way to go. A set of solar panels + Grid Tied Inverter tied into your main AC panel. A building permit, and an agreement with your utility company rounds out the system. Grid Tied power is lower in cost (probably 1/4 to 1/10th the the cost of an off grid system of equivalent power output) and much less maintenance (battery, cables, replacing battery bank every 3-7 years, etc.).

    In California where electric power during summer afternoons can cost ~$0.27 to $0.50 per kWH peak (noon to 6pm for me) and ~$0.10 per kWH off peak, a GT system can really save money. But this is because our PUC (public utility commission) has pushed our major utilities to allow GT systems and give us a 1 year net metering plan.

    Your utility may or may not allow GT systems, and in Arizona, there is talk by at least one utility to make GT systems much less cost effective (changes to billing plans). A system installed now may or may not be "grandfathered" with the older (and more advantages) rate plans.

    With an off grid system, you get one major advantage over GT based solar... And that is the ability to run the system during a power failure (solar+charge controller+battery bank+AC Inverter). A pure GT Solar system cannot run when the utility power is down.

    If you have utility power, you will almost never save money with an Off Grid power system... The typical cost of utility power is ~$0.10 to $0.20 per kWH. For typical off grid solar power with 20 year system life and replacing batteries every 3-7 years and electronics every 10+ years is in the range of $1.00 to $2.00+ per kWH (AGM batteries push power costs up because they cost more and typically do not last as long as flooded cell batteries). One person here has his Off Grid system costs down towards ~$0.30 per kWH--But that is pretty cheap.

    So--If it is to save money, Off Grid power is not going to do it. If it is for a week or so of emergency power, a generator plus some stored fuel is probably a better choice.

    Assuming 1kW per day * 30 days = 30 kWH per day or ~$10-$20 in power usage (assuming $0.10 to $0.20 per kWH) per month.

    If you want to save money, use your Kill-a-Watt meter to measure your major appliance usage (fridge, computers, lighting, Entertainment System, etc.) and see if you need to replace any of them with newer, more efficient, appliances.

    Also, with older homes, insulation, double pane windows, new heat pump based water heater (if you have electric hot water), upgrade to your heating/cooling system with modern heat pump based unit (central or new mini-split types) can also save lots of money.

    To give you an idea on energy usage:
    • 1kWH per day / 30 kWH per month is an off grid cabin with propane refrigerator (typical usage)
    • 3.3 kWH per day / 100 kWH per month is a very efficient off grid home with refrigerator/washer/well pump/lights/laptop
    • 33 kWH per day / 1,000 kWH per month is typical power usage for North American home (natural gas heating/hot water)
    • 33-100 kWH per day / 1,000-3,000 kWH per month is typical usage for home with heavy AC or electric heat usage

    Ok--off soap box and back to your questions. Sizing a battery bank for 912 Watt*Hours per day with 1 day storage (1-3 days recommended) and 50% maximum discharge would be:
    • 912 WH per day load * 1/12 volt battery bank * 1/0.85 Inverter Efficiency * 1 day storage * 1/0.50 maximum discharge = 179 AH battery @ 12 volts minimum

    The solar array needs to be designed to meet two needs. The battery charging current and the amount of power used/available sun per day. First based on battery size.

    We recommend ~5% to 13% as a good range for solar power, with 10-13% being recommended for most people that will be using power during the day (5% is the minimum power needed to just recharge the battery bank):
    • 179 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 169 Watt minimum array
    • 179 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 337 Watt array nominal
    • 179 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 438 Watt array "cost effective" maximum

    For the sizing of the solar array for your loads, we need to know how much sun per day (on average) you will get. Using PV Watts for Pheonix Az, fixed array tilted to latitude, the "hours of noontime sun" per day would be:
    Month    Solar Radiation (kWh/m 2/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 are in a very sunny region, so we can use 4.88 hours of sun minimum (remember, you should make the system array ~ 1/0.75 to 1/0.66 times larger to account for cloudy weather if you don't use a generator/utility power for backup--or cut power usage during poor weather):
    • 912 Watt*Hours * 1/0.52 system efficiency * 1/4.88 hours of sun per day = 365 Watt Array Minimum

    So, the "optimal" solar array based on my above guesses would be ~365 to 438 Watts (round numbers).

    A MorningStar 300 Watt TSW 12 volt AC inverter would be a great fit here. A 30 amp minimum MPPT charge controller (Rogue 30 amp MPPT--New model due to ship very soon, or MorningStar 45 amp MPPT). Two * Concorde PVX-2240T 6 volt @ 224 AH AGM Batteries (or one * Universal 200 AH @ 12 volt AGM) batteries.

    I would also highly suggest getting a Battery Monitor (Trimetric or Victron). AGM batteries are sealed and you cannot measure their speciific gravity and a Battery Monitor is the nearest you can get to adding an "fuel gauge" to your battery system.

    And finally, a pair of 180 watt or larger solar panels. Note, most large format solar panels are not "12 volt" based (Vmp ~18 volts) and need a more expensive MPPT type charge controller. There are 140 watt or smaller "12 volt" solar panels that can use a less expensive PWM charge controller, but usually the panels cost more $$$/Watt than the larger panels. (shipping costs for 140 watt or smaller panels may be much less--So that can be an issue too--Shipping costs for a few solar panels can be very expensive).

    I have used equipment links to our host, Northern Arizona Wind & Sun--And they are pretty close to you in Flagstaff--But you are welcome to get your equipment anywhere.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help with small system design
    Daramus wrote: »
    Thanks for the welcome and for your reply. At most, each device would be charging a maximum of 3 hours per day.

    So a 40 Watt laptop then becomes 120 Watt hours. X2 = 240 Watt hours. The tablet 6 * 3 = 18 Watt hours. Cell 15 Watt hours. DS consoles 36 Watt hours. Grand total: 309 Watt hours. Factor the conversion of a good inverter (I'm going to assume it's impractical to try and power them DC direct) and you get 343 Watt hours. Add the inverter's power over that time and it's about 360 Watt hours.

    This is a much more manageable number, isn't it? :D
    That's roughly 30 Amp hours @ 12 Volts used. A 120 Amp hour battery would give you this at about 25% DOD (giving you an extra day of power if needed). You want AGM so something like this Concorde:
    Pretty pricey, but that's a problem with that type of battery.

    To recharge it you'll not need too much. Example you only need about 10 Amps and probably could get away with a bit less. A single 130/140 Watt panel will provide a bit over 7 Amps for example, and a small SunSaver charge controller would handle the regulating.
Sign In or Register to comment.