# Sizing up a small pv system



## john10001 (Mar 20, 2013)

I just wondered if there is a general rule of thumb on what the correct ratio of solar power to battery capacity would be able to fully charge a battery on an average day during the year for a location quite far North?

So for example if you live pretty much on a similar latitude as Edmonton in Canada, what size of solar panel would you need to say fully charge a 50ah 12v battery on average day? 75w? 85w? 100w?

I have also read somewhere that Monocrystaline solar panels are better for northern climes than Polycrystalline?

I am looking to have a small backup system for emergencies in case of power outages or worse WTSHTF situations with power down for days, weeks even months. Just mostly to run lights and power or recharge small devices like phone, radio, AA rechargeable batteries etc.

I'm planning on having just a small system but one that is dynamic and adaptable. So I will use it to keep a deep cycle battery topped up for use in emergencies (at night) but for normal non-emergency SHTF times besides keeping that topped up I will use a grid tie inverter to put electricity back into the grid and lower my electric bills.


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## rickkyw1720pf (Nov 17, 2012)

50AH X12V=600 WATTS FOR ONE HOUR or 60 watts for 10 hours under perfect conditions a 60 watt solar panel should take 10 hours to completely charge. With all the losses in reality my guess is you probably would need a 120 watt 12 volt panel to charge in normal conditions in 10 hours if the battery was discharged.

This is my system and it will do everything I would need electrically and is completely portable. A 52 watt folding solar panel with 14 watt back up and 2 lithium 145 and 150 watt hour, batteries battery charger and small 75 watt inverter.


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## Ripon (Dec 22, 2012)

Excellent portable system - nice. Your advise is right except in Edmonton or anything that far north I'd assume no more than few hours of prime exposure. You might even need 180 watts of production - two 90 watt pannels or three sixties.


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## Montana Rancher (Mar 4, 2013)

Hello
For a system that is just lights and recharging I'm not sure why you would need a 12v battery. There are plenty of setups out there that allow you to recharge batteries directly from the solar panels. With all the LED lamps out there it would be the best way.

The only reason to do the big batteries is if you want to run appliances or maybe a ham radio system.


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## rickkyw1720pf (Nov 17, 2012)

Montana Rancher said:


> Hello
> For a system that is just lights and recharging I'm not sure why you would need a 12v battery. There are plenty of setups out there that allow you to recharge batteries directly from the solar panels. With all the LED lamps out there it would be the best way.
> 
> The only reason to do the big batteries is if you want to run appliances or maybe a ham radio system.


The reasons I have the two lithium batteries is solar panels usually unregulated, it may be a 12 volt panel but if you put a volt meter to one their voltage may fluctuate between 14 to 28 volts which could damage some electrical gear. Batteries by their very nature regulate voltage to some degree. There are battery chargers for AA-AAA batteries but they have small solar panels and require hours to charge. With a much larger panel and storage battery it can be charged and used by itself or used to charge AA-AAA batteries when ever needed without using the panel. It gives you so much more flexibility. The lithium batteries storage batteries I have weighs 4 lbs and 2.3 lbs and each are rated at 13000ma @ 12 volts so each would be equivalent to 50 rechargeable NiMh AA batteries (1.2v @ 2400ma).


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## john10001 (Mar 20, 2013)

rickkyw1720pf said:


> 50AH X12V=600 WATTS FOR ONE HOUR or 60 watts for 10 hours under perfect conditions a 60 watt solar panel should take 10 hours to completely charge. With all the losses in reality my guess is you probably would need a 120 watt 12 volt panel to charge in normal conditions in 10 hours if the battery was discharged.
> 
> This is my system and it will do everything I would need electrically and is completely portable. A 52 watt folding solar panel with 14 watt back up and 2 lithium 145 and 150 watt hour, batteries battery charger and small 75 watt inverter.


Fantastic lightweight kit! You can go anywhere with that. I had looked at folding solar panels but they are very pricey.

Thanks very much for the advice. This gives me a lot to think about.


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## HuntingHawk (Dec 16, 2012)

If you run batteries below 50% of rated charge it kills the life cycle of them. So the 50AH battery is only 25AH. Probably better off to get a marine/deep cycle battery around 125AH. That would give you 62.5 which is closer to the 50AH you mentioned. Keep in mind weather can easily prevent any recharge to the battery for a day. So rule of thumb is you want enough stored power for minimum 48 hours.

Solar should be calculated for winter as that is the time of least sun, maybe as low as 4 hours where you mentioned. The smaller the system the more critical it is to be able to adjust the angle of the panels. I adjust the angle of my panels monthly & it makes a difference.

Don't get caught up in the wattage of the panels. Look at the rating of the amps which makes calculations easier. A 100watt panel might be rated at 5amps but that is under ideal conditions. Normal is figuring 80% of rated amperage. So you can figure 3amps X 4 hours would only be 12amps to the battery per day in the winter. But twice that in the summer.


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