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# Solar Lights: How Many Solar Panels Do I Need To Power Up My Office? (8/19/2012)

Anyone knows what kind of solar panels (wattage , ambers, Volts )should I purchase to have sufficient power for a Laptop , A modem/router And a fax machine?

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Player1 August 19, 2012 at 1:47 am

Alot…sadly solar panels collect limited spectrums of light, limiting their power, but they can help…not sure what you’d need though…

Ecko August 19, 2012 at 2:14 am

If it is only a laptop. a modem/router and a fax machine the power consumed is perhaps:

80W for the laptop
5W in sleep and 80W in standby and 800W max for FAX
20W for the modem/router.

This adds up to a peak power around 1KW for everything, and an idle power of 105W. This varies depending on your work cycle and the actual specifications and settings for the machines. Let’s say that over a 24 hour period the average is 80W, assuming you are using the stuff about 12h a day, but leave it on overnight in sleep mode. Maybe you can save more power by having a more rigorous routine for turning things off. You can make your own figures and get a closer estimate.

The calculations for solar panels and battery size follow:
The inverter may be 80% efficient, so you need 100W to get 80W.
The battery wastes about 40% of its energy input in chemical losses, so you need 1.4 x 100W = 140W. This is average power over the day, the full 24 hours. Yes you may not use much at night, so the battery losses could be lower, but use this figure anyway.

The energy needed over a day is 24h x 140W = 3360Wh (watt hours).

Determining the battery size:
Because of the maximum power load you need at least a 24V battery to reduce the current. With a 24V battery the capacity used is 3360/24V = 140 Amp hours per day. Allowing for a number of overcast days, say 3 in a row, the battery should be 420 Ampere hours. The next nearest may be 500Ah. Yes it is huge. This will have no problems working with the peak load which is about 1.2KW allowing for losses, and that would be 52 Amps. That determines distribution wire size from the battery to the inverter.

Determining the current from the panels:
They charge a 24V battery through a controller. We need 140Ah per day, so that is spread over the hours of sunlight. There is a figure for your region that is called the “full sun equivalent”. Thus the sun may shine for 12 hours in the tropics, but this is the equivalent of only about 5 hours with full sun square on to the panels. Thus 5 equivalent full sun hours a day. In a temperate zone it may only be 2 hours. Look it up on the internet. Divide the Ah per day by full sun equivalent hours to get the current from the panels to charge the battery. That is 140Ah/5h for the tropics giving 28 Amps. So we need 28 Amps from 24V panels. The larger panels tend to be 12V ones at 8 Amps, rated at about 140W. You would need 2 sets in series to get 24V, and 4 per set to get at least 28A. This slight oversizing leaves a margin for aging, and is also makes it work better.. You need 8 large panels.

The inverter – only consider a true sine wave inverter continuously rated at 1000W. This produces normal AC power.

Other ideas..
You could charge the laptop directly from the battery using a 24V DC charger for the laptop. I haven’t considered using th laptop in a power saving mode, just on and off. Depends on your work patterns.

You could try to eliminate the FAX machine by using email and an external web-site maybe. This would make a worth-while difference.

You could use a grid connect system to supplement your mains power instead. This eliminates the batteries. You would need about the same number of panels, though they would be configured differently. You have no power when the mains fails.

TonyM August 19, 2012 at 2:48 am

You need to work out first how much power you will need . The power consumption will be clearly statted on the equipment. Usually on a label on the underside. So add up the power ratings on all the appliances you will want on together. Including lights, heating, tools etc. You won’t need everything on at once so look to decide you maximum power requirement.
Next you need to set an amount of hours per day you will want to run the power. So power (in KW- 1000 watts) X hours per day = total KW/H needed. That tells you what you need.
You will need to know the power rating for your solar panels i.e 50W and multiply that by the average hours of daily sunlight for your area. i.e. (50W X 4hrs Sun)/1000 = panel KW/H. Remember sunlight hours can dramatically vary depending on winter/summer so make sensible allowance for your needs in estimating the daily hours of sun.
Finally all you need to do is divide the KW/H needed by the solar panel KW/H to get the number of panels you need. i.e. NEED 2KW/H by PANEL 0.2KW/H = 10 solar panels.
Depending on the system you create. i.e. battery quality, inverter efficiency etc there will be inherent losses in the charge/use cycle. You will need to allow 50%-70% additional panels to account for losses.
Best not try and ‘give it a go’ if you are not 100% sure what you are doing. get some expert advice and help. It’s not exepensive.
Checkout http://www.greenpowereasy-review.com/