WiFi station Solar Power Calculations

sanitariu · August 2014

Hello,
i am building small wifi station consisting of 2 wifi devices (24 watt) and 1 router (16 watt).
Totally i have 40 watts. That usage is on max, i think it will be lower.
Can someone help me with calculations for panels,controller, battery.
Let's say we would like 1 day for reserve and 50% discharge.

stephendv · August 2014

Re: WiFi station Solar Power Calculations

I think those ratings are far too high. Ubiquiti wifi gear for example doesn't consume more than 10W for a 1km range base station.
Anyway, using 40W x 24 hours = 960Wh per day, and you want 2 days = 1920Wh. On a 12V battery that's 160Ah. You'd want to draw down to only 50%, so x2 = 320Ah battery.

How much solar you'll need will depend on where you are, and whether that load will change with the seasons or if it's constant year round.

sanitariu · August 2014

Re: WiFi station Solar Power Calculations

Yes both wifi are Ubiquiti and the router is small tp-link 740.
These 40 watt are max. I just do not know how much is the average and i gave the max values.
We can calculate for 5 hours sun per day.

BB. · August 2014

Re: WiFi station Solar Power Calculations

40 watts is just about the power requirement for a full sized refrigerator... Computers are really power hungry devices.

Anyway:

960 WH per day * 1/0.52 AC end to end system eff * 1/5 hours of sun (not winter?) = 369 Watt array based on sun+loads

Based on Battery bank AH and 5-13% rate of charge:

320 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 301 Watt array minimum
320 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 603 Watt array nominal
320 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 783 Watt array "cost effective" maximum

Assuming your loads draw 40 Watts during the day, when battery bank is also charging:

40 Watts * 1/0.77 panel+controller derating * 1/0.85 AC inverter eff = 61 Watt array to "run" loads

Minimum array to run loads plus minimum 5% battery charging:

61 Watts + 301 Watts = 362 Watt minimum array for loads+5% rate of charge

So, somewhere around 362 to 783 Watt array with ~603 to 664 Watt array (nominal charging + array load) for a relatively "healthy" sized array (10% rate of charge)... Would also give you more power during winter (when you have less than 5 hours of sun).

-Bill

westbranch · August 2014

Re: WiFi station Solar Power Calculations

Where will it be located? Mountain top ? In a house? Does it rain frequently or have days without full sunshine??? What is the daily ambient air temperature?
Will there be an inverter ? Size?
Is this for home use or commercial?

hth

sanitariu · August 2014

Re: WiFi station Solar Power Calculations

It's for home system. I will not use inverter but direct 12 volt from the battery.
40 watt is maximum power of the devices. More realistic is around 20-25watt.
It will be on 200 meters high. No frequent rains. Normally 250-270 sunny days in the year.
May be i must recalculate BB formula using 25 watts.
I do not understand "1/0.52 AC end to end system eft". I am not using AC .

BB. · August 2014

Re: WiFi station Solar Power Calculations

Sounds good--That is why I use the equations so that you (and others) can plug in the numbers that make sense for their needs.

Regarding the 0.52 derating--That is for a "typical" AC system:

0.77 panel+controller derate * 0.80 flooded cell losses * 0.85 AC inverter losses = 0.52

For a DC system:

0.77 panel+controller derate * 0.80 flooded cell = 0.61 end to end DC system derate

And if you have AGM, you can use 0.90 (or even higher) for the battery efficiency.

-Bill

sanitariu · August 2014

Re: WiFi station Solar Power Calculations

Thanks Bill !
As always you are comprehensive.

WiFi station Solar Power Calculations

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