Size of Battery : Solar Panel Size Ratio Question

[SeaDooDave]

Hey Everyone,

I'm new to solar energy but have an electronics background and I've been doing a lot of research so I believe I have a decent understanding, but I got a question about batteries. Just for starting, I recently purchased a 30 watt solar panel. Still waiting for everything to show up in the mail (charge controller, inverter) but I will have it setup to charge an Werker Sealed 12V 18AH battery. What's the rule of thumb when selecting a correct battery size with the size of your solar panel? I've seen diagrams of batteries around my size for a 30 watt panel but I've seen pictures of same size panels attached to much larger batteries. Obviously it depends how much power you want to store but at a certain point I'd imagine it wouldn't be efficient anymore, say a 30 watt panel with a 200+ ah battery, if it takes you over a week or way longer just to get it back up to 100% and you factor in the natural discharge rate of the battery. In my research so far I haven't found much regarding this so I figured I'd ask....curious how you make sure you pick a battery that's not too large for your system. I'm just using this panel to keep lights on in case of an outage or charge my phone if I want, simple things like that.


Thanks guys

[Naptown]

The general rule of thumb with flooded lead acid batteries is 1 watt per amp hour of battery.
It appears you are using an AGM battery so the charge current can be much higher.
To refine a bit further a flooded battery needs a charge rate in amps of between c/8 and c/12 where C= the amp hour capacity of the battery at the 20 hour rate.
Some AGM batteries can take a c/1 or greater rate but always check the battery manual to verify the max charge rate.

[SeaDooDave]

Okay, thanks for the post. So it'd be even more ideal if I had a battery around 28ah - 30 ah then.

[Naptown]

Okay, thanks for the post. So it'd be even more ideal if I had a battery around 28ah - 30 ah then.

somewhere between 25 and 30 is good

[SeaDooDave]

If I'm using an inverter and hook up one 120v 13 watt CFL bulb, because of the change from 12VDC to 120VAC, at the battery isn't it actually using 130 watts to power that bulb? I thought I read this somewhere but can't seem to find it now and I'm just making sure I'm right before doing any further calculations.

[Naptown]

Best way to tell is to hook up an ammeter between the battery and the inverter. You are pulling from the battery a bit over 1 amp for the bulb + whatever the inverter consumes.
When doing this remember a watt is a watt is a watt. 12 Watts at 120V =12 Watts at 12V The difference is at 12V it takes 1 amp and at 120V it takes 1/10 of an amp

[Sunking]

If I'm using an inverter and hook up one 120v 13 watt CFL bulb, because of the change from 12VDC to 120VAC, at the battery isn't it actually using 130 watts to power that bulb?

Depends on the inverters efficiency at the power level you are operating at. For example if the inverter is say 75% efficient at the power level you are operating at like 13 watts, then you will pull 13 / .7 = 18.5 watts from the battery.

[PNjunction]

That Werker is a general purpose AGM - I use similar brands for other solar stuff. The typical maximum charge current is either specified as 0.25 to 0.3 C. Consult the manufacturer's rating, but I'll bet the max charge current is somewhere between 4.5 to 5.4 amps.

Your 30 watt panel will put out about 1.5a max under the best of conditions, so you have some room to grow if you want to charge faster, or your solar-insolation hours are short - like in winter.

If you want your battery to last more than perhaps 100 cycles, don't discharge it more than 50% DOD. This means that at say 18.5 watts of your CFL / Inverter load, you'll be able to run it about 5 to 6 hours or so before wanting to recharge it.