# System sizing checkup

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Registered Users Posts: 12
I have a garage/shop/barn that currently doesn't have any electric wired to it. I have been using a 100ft ext cord to get some power for lights.

I am wanting to install a small solar system to be used for all my overhead lighting. My load I currently foresee is 10 - 13w CF over head lights of which I see using for about 5hrs per day, for half the week.

So, is my calculation correct that I will need 650w of capacity (13w*10ea*5hr)?

Then for battery capacity: 32XHS-12v has 105ah at 5-hr rating....This battery gives me 1260watts total over 5hrs? (am I reading the battery information correctly)?

As for panels, I already have 2-Grape solar 185w, 36wmpp....So to make a rough estimate of how much power these can produce I just took total rated power and halved it...so I can make about 185w/hr. I live in the pacific northwest so using peak sun of 3.5hrs per day, I can produce about 640w per day.

Questions:

Are my calculation remotely correct? Also, I am not sure if I have enough power production to recharge my batteries, especially in the winter when I get the least amount of sun?

I know I also will need an inverter and charge controller, but I just want to see if I was on the right path to start with?

Thx for the help?

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Re: System sizing checkup
I have a garage/shop/barn that currently doesn't have any electric wired to it. I have been using a 100ft ext cord to get some power for lights.
Honestly, I would dig a trench and bury some 3 conductor+ground 10 AWG or 8 AWG direct burial cable (I think code is 18" deep minimum) and bring 120/240 VAC to the shop.

Then look at solar for the education/backup power...
I am wanting to install a small solar system to be used for all my overhead lighting. My load I currently foresee is 10 - 13w CF over head lights of which I see using for about 5hrs per day, for half the week.

So, is my calculation correct that I will need 650w of capacity (13w*10ea*5hr)?
• 650 Watt*Hours per day.
Then for battery capacity: 32XHS-12v has 105ah at 5-hr rating....This battery gives me 1260watts total over 5hrs? (am I reading the battery information correctly)?

Nominally, recommend 1-3 days of "no sun" plus 50% maximum discharge for long life. For 2 days of power:
650 WH * 1/12 volt battery bank * 2 days no sun * 1/0.50 max discharge = 217 AH per day

You can go with 105 AH @ 12 volts, but you will only have one day of "no sun" carry through. And a bit shorter battery life--But it is certainly a valid configuration.
As for panels, I already have 2-Grape solar 185w, 36wmpp....So to make a rough estimate of how much power these can produce I just took total rated power and halved it...so I can make about 185w/hr. I live in the pacific northwest so using peak sun of 3.5hrs per day, I can produce about 640w per day.
2*185 watts * 0.52 system eff * 3.6 hours of sun per day = 692 WH per day of 120 VAC power (approximately)

Note that you should never plan on using 100% of your power every day--But if you go in the shop every other day, you will be OK.
Questions:

Are my calculation remotely correct? Also, I am not sure if I have enough power production to recharge my batteries, especially in the winter when I get the least amount of sun?

If you are on the cost, the amount of sun can be terrible. Using PV Watts fixed array tilted to latitude for Quillayute WA:
```Month    Solar Radiation
(kWh/m 2/day)
1      1.51
2      2.21
3      3.12
4      4.04
5      4.73
6      4.74
7      4.91
8      4.57
9      4.37
10      3.31
11      1.87
12      1.85
Year      3.44
```

So, yes, during winter you may need a lot more panel or use a lot less power. That Marine layer is a killer.

I know I also will need an inverter and charge controller, but I just want to see if I was on the right path to start with?

The Rogue 30 amp MPPT controller would be a great fit.

The MorningStar 300 watt 12 volt TSW inverter is the best small / 12 volt inverter out there right now.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 12
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Re: System sizing checkup
hen look at solar for the education/backup power...

I am wanting to install a small solar system to be used for all my overhead lighting. My load I currently foresee is 10 - 13w CF over head lights of which I see using for about 5hrs per day, for half the week.

So, is my calculation correct that I will need 650w of capacity (13w*10ea*5hr)?

650 Watt*Hours per day.

Basically, I am using this solar system more as an education lesson then to get power to the shop. The battery I quoted, I already have for a travel trailer, and I have the panels, so I am somewhat half way there with equipment.

As for your correction as quoted, you highlighted HOURS per day. With my plan, I would use 13w*10ea=130w per hour the lights are on. If I use them 5hr per day, isn't it a total of 650watts per day? Or are you correcting my mis-use of terminology?? Meaning 650watts total per days, should be stated as 650watt hours (wh) per day? As I have learned, using the correct terminology makes getting help so much easier.
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Re: System sizing checkup
Basically, I am using this solar system more as an education lesson then to get power to the shop. The battery I quoted, I already have for a travel trailer, and I have the panels, so I am somewhat half way there with equipment.

As for your correction as quoted, you highlighted HOURS per day. With my plan, I would use 13w*10ea=130w per hour the lights are on. If I use them 5hr per day, isn't it a total of 650watts per day? Or are you correcting my mis-use of terminology?? Meaning 650watts total per days, should be stated as 650watt hours (wh) per day? As I have learned, using the correct terminology makes getting help so much easier.

The confusing part is the Watts per Hour... Watts is already a Rate (like Miles per hour). There is not a Watts per Hour (--actually there is, it is the equivalent to acceleration or ft/sec2).

Multiply Watts * Hours gives you Watt*Hour which is equivalent to MPH * Hour = Miles driven.

I understand the desire to learn more about solar by starting with a small system... Your system will work fine (within its limitations).

-Bill

PS: Watts are already Joules Per Second (J/S)... And somebody decided to call the "derived unit" a "Watt".

Sort of like naming Miles per Hour a "Bill". And Bill*Hours would be miles driven.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,741 ✭✭✭✭
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Re: System sizing checkup

Power is measured in watts, and is a rate of energy usage. WattHours is an amount of energy.
Then for battery capacity: 32XHS-12v has 105ah at 5-hr rating....This battery gives me 1260watts total over 5hrs? (am I reading the battery information correctly)?

that's 1260 watthours or 1.26 kilowatthours
If I use them 5hr per day, isn't it a total of 650watts per day?
No, its 650 watthours.
My load I currently foresee is 10 - 13w CF over head lights

So your load is 130 watts. Your inverter must be sized larger than that because the power factor of CFs is less than 1. May I suggest the morningstar suresine 300. Also, no inverter is 100% efficient. Therefore you will be drawing more than 650 watthours from your battery every day. That's more than 50% of your battery's capacity and will shorten your battery's life.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i