# Battery for ~360W/day

intel415
Solar Expert Posts:

**31**✭
Hi guys,

Before i purchase any batteries, i don't want to ruining anything or spend too much money on batteries that my solar can't recharge them. i didnt go with my old plan 1x 240W solar panel, 2 T-105 batteries, morningstar mppt 45A.....

Anyways my load will approximately 360W per day

I have purchased most of the stuff here on solar-electric

1x Morningstar mppt 15A

1x RM-1 meter display

1x morninstar 300W

1x 120W mono. panel

2x 15A DC circuit breakers

1x 50A fuse

thought i was going to go with 2x t-105, it would be no brainer if my 120w panel can;t fully charge them. I saw posts that talk about batteries can't discharged below 50%. My question is will a Everstart group 27 with 105Amp hour work for my setup? I do get a lot of sunlight during the day from 9AM - 5PM, hardly any cloudy day at noon time unless it rains chances are slim lol. What do you think? Thanks.

Before i purchase any batteries, i don't want to ruining anything or spend too much money on batteries that my solar can't recharge them. i didnt go with my old plan 1x 240W solar panel, 2 T-105 batteries, morningstar mppt 45A.....

Anyways my load will approximately 360W per day

I have purchased most of the stuff here on solar-electric

1x Morningstar mppt 15A

1x RM-1 meter display

1x morninstar 300W

1x 120W mono. panel

2x 15A DC circuit breakers

1x 50A fuse

thought i was going to go with 2x t-105, it would be no brainer if my 120w panel can;t fully charge them. I saw posts that talk about batteries can't discharged below 50%. My question is will a Everstart group 27 with 105Amp hour work for my setup? I do get a lot of sunlight during the day from 9AM - 5PM, hardly any cloudy day at noon time unless it rains chances are slim lol. What do you think? Thanks.

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## Comments

31✭oops wrong section can someone move this to beginner section please thanks

262✭✭The recommendation here that I think was originally suggested by neil, BB, and perhaps others is that you keep the charging amps of the system at the charging voltage between 5% and 13% of the battery amp-hour capacity (20 hour rate). This is after a .77 derating of the panel (since it will not usually put out its 'rated' wattage, even in full sun).

So if you have a 120 watt panel, 120 x .77 derating = 92 watts. 92/14.8 charging voltage = 6.2. So at the 10% charging rate, that would be about a 60 amp-hour battery, and at the 5% rate, 120 amp-hours. This is, at best, half of what you'd need for 2 t-105s, so they are out of the question.

I'm wondering why you have a very expensive little charge controller for only 120 watts of panel. Is that panel not a 12 volt type? If you're starting from scratch, for the cost of the Sunsaver MPPT ($222), you can get a 15 amp PWM controller (about $90), and apply the difference to a second 12 volt panel. I don't know what your panel cost, but it's likely that for not much more than you spent on this system, you could probably have twice the power (almost enough for a pair of t-105s). There's almost no gain from an MPPT controller for a system this small.

17,615✭✭No trouble at all to move the thread. 8)

This is what we call "designing a system backwards". You have need of 360 Watt hours per day, so the first thing you plan on is the battery that will supply that. From there you pick the panel & controller that will recharge it.

Without the conversion losses, 360 Watt hours on 12 VDC is 30 Amp hours. If you want everything to balance out, you pick a battery size where that will be less than 25% of the total capacity. That's 120 Amp hours or more. You should "round up" here to the nearest available size. This gets a bit tricky for FLA's which are usually cheaper than AGM's as there aren't that many small ones available. Crown has a 130 Amp hour unit: http://www.solar-electric.com/cr12repose12.html (The reason for the T105 recommendation is that these are readily available and very inexpensive per Watt hour).

So based on the 130 Amp hour 12 Volt Crown, you can apply the formula Eric L used and get:

10% peak charge current rate (keeps everything balanced nicely with a 25% DOD in most cases) = 13 Amps.

13 Amps * 12 Volts = 156 Watts. Less 77% efficiency derating = 203 Watt panel.

The 120 Watt panel isn't going to work for this, which can be seen by analyzing its potential "harvest" using the Icarus formula:

120 Watts * 4 hours equivalent good sun = 480 Watt hours * 50% over-all system efficiency = 240 Watt hours AC "out the door".

As is so often the case, you need more panel.

29,994adminI will argue a bit with the use of "12 volts" here... For PWM controllers, remember that they take 100% of the current from the panel (no more, and only a couple percent less) of the current and pass it on to the battery bank.

And most "12 volt" solar panels are actually Imp rated at Vmp~17.5 volts... So a better formula for rating the panel with PWM based controllers would be:

13 amps * 17.5 volts = 227 watt solar panel

If this was an MPPT type charge controller, then I would use ~14.5 volts for battery charging--Since if you are discharging a battery by only ~25% maximum on a day to day average--Then you will rarely be charging it at ~12.0 volt.

The 0.77 derating we use for panels+charge controller may be a bit on the conservative side (for example, we use it to account for "hot panels" having lower Vmp--When for PWM controllers, the lower Vmp has no effect on PWM controller output and Imp gain with hot panels is very small so we ignore it)--that in the end, for rule of thumb calculations if we use

0.77 deratingand14.5 volt battery chargingwe sort of average out the functional differences (in most situations--0.77 derating over estimates PWM losses, and 14.5 volts underestimates PWM losses; 0.77 and 14.5 volts are a good estimate for average MPPT based system losses) between PWM and MPPT controllers and can use the same formula to estimate panel size for both types of charge controllers.-Bill

17,615✭✭Guidelines, Bill, guidelines.

If we start changing the rules-of-thumb mid-stream it will only lead to confusion. Then we'll have to make every ballpark suggestion based on all the fine details of each individual system, like elevation, ambient temperature, et cetera. I really ought to mention more often how these guidelines are arrived at. Like the 77% "typical efficiency" rating being the combination of average panel output plus controller (or GT inverter) loss. Even so, that isn't carved in stone.

As far as getting close to the final solution goes, using the formuli mentioned in my previous post works fairly well under most conditions for either type of controller.

Besides which he was thinking of using MS's 15 Amp MPPT controller, which would actually work out with that battery and inverter if only there were enough panel to supply it. 120 Watts isn't going to do it.

And I was assuming the "360W/day" meant 360 Watt

hoursper day. Perhaps I shouldn't have?Mia culpa!

29,994adminMarc,

I think you are correct on 360 WH per day--I just thought the "12 volts" for calculating charging voltage was low for estimating either PWM or MPPT controller/array sizes.

-Bill

17,615✭✭Depends on whether your looking for expected output from a panel or sizing a panel for current potential kind-of-thing.

(I know Bill already knows this stuff; the following explanation is for anyone reading who doesn't.)

Yes "12 Volt" systems get Absorb charge at 14+ Volts, but the current won't be maximum at that point as the Bulk stage (when current draw is at its peak) is over. So by using the minimum system Voltage (as in; you don't want to draw the battery below that) times 10% of the Amp hour capacity less typical panel & controller loss you get a reasonable expectation of panel size.

Keep in mind that this peak current potential is almost never seen in operation. In order to achieve it, the panels would have to be in maximum insolation and the battery at minimum state of charge. Since the former occurs about midday after hours of charging have already passed and the latter is usually just before recharging begins the "two planets rarely align". This is one of the great inefficiencies of off-grid systems, and why we are always looking for clever ways to utilize the full-sun Watt hour "harvest" that would otherwise be lost because the batteries are already fully charged. This is what is known as "load shifting"; deliberately turning on loads when the batteries are charged to make use of the power potential still available from the panels.

Thus you have: system Voltage * 10% battery bank capacity / 0.77 efficiency = array size.

This should be checked against the Icarus formula for daily Watt hour "harvest": array size * hours of equivalent good sun * 0.50 over-all system efficiency = daily Watt hour needs.

Usually if you use the first formula and limit depth of discharge to 25% the second formula will fall into place even for a minimal 4 hours of sun.

Now some people might argue that doing so means there is too much panel, but such is not the case. It is not a precise calculation by any means, but rather a shortcut that allows you to get a good balance of load/capacity/recharge that will keep the system working well for years. Some systems will be more efficient, others less. But by using the shortcut (and erring on the side of caution) you can quickly determine component sizing that will probably work well under most circumstances.

I probably need to add more caveats to that. :roll:

31✭sorry for the confusion guys, i need 360Whr a day total to run my 15W rated electronic

15W x 24hr = 360Whr

I'm waiting to get a sharp 240W soon, there are posts people running 300W+ on 12v configuration without problems.

120W + 240W = 360W

Using the formula you guys come up with

360 x .77 derating = 277.2W

277.2/14.8 charging voltage = 18.73

If i get 2 t-105 and add a 240w pv will it do it for my setup? thanks again guys

17,615✭✭Whoops. No.

The Watt hours is the load, in this case in AC Watts @ 120 Volts I presume. There's about a 10% addition for conversion between the DC supply and the AC load. So that's really 396 Watt hours DC.

First up; that doesn't include the inverter. The MS 300 uses about 6 Watts, times 24 hours is another 144 Watt hours.

Grand total: 540 Watt hours DC (see how fast it runs up?).

On 12 Volts, that's roughly 45 Amp hours. For 25% DOD that would be a 180 Amp hour battery.

When you start sizing panels you look at two separate but related items: being able to provide a peak charge current which will properly recharge the battery in the usual amount of time available and being able to "replace the used Watt hours".

For the first, you get: 10% of 180 Amp hours = 18 Amps @ 12 Volts = 216 Watts. Then you have to allow for the efficiency derating. Thus you get: 216 / 0.77 = 280 Watt panel.

For the second you plug that number in to the Icarus formula and see how many Watt hours you can expect that panel to harvest in a day: 280 * 4 hours equivalent good sun = 1120 * 0.50 over-all efficiency yields 560 Watt hours.

So you see if you calculate the loads right and use the "10% rule" with the "25% DOD rule" you get a system that should work under typical conditions.

You want me to start listing all the factors that throw "typical conditions" for a loop? :roll:

As you suspect, you have to round the battery size

upto the nearest available capacity and then adjust the panel sizing accordingly. The T105's are 225 Amp hours (or 220 for some) so you get this:22 Amps * 12 Volts = 264 Watts, less 77% derating = 343 Watt panel. Again you should round up to the nearest available configuration. This might be two 180 Watt units.

Now there are those who will point out that the minimum charge rate can be 5% of the battery capacity. In some systems this will work fine. But if you're drawing while charging (as most of us do) you have to remember that the charge rate is

net: in the last example 11 Amps would be 5%, but if your loads are drawing 2 Amps at the same time your net charging is 9 Amps which is only 4%. At that point it gets a bit dicey.262✭✭intel,

The Sunsaver MPPT has an output current limit of 15 amps. So it can't take as much as 360 watts for a 12 volt system. Rather more like 14.5 (top of bulk charging range) x 15 amps = 217.5 watts. The 240 watt panel alone is a decent match for it after you factor in some panel derating.

By the way it won't hurt the Sunsaver to give it more than 217 watts, it just won't use it. (Also, you need to make sure the 120 and 240 watt panels are sufficiently similar to use them on one controller, which they may not be).

I think the 240 panel with the Sunsaver is enough for two T-105s if you keep the loads light, as it sounds like you will. It's not optimal as Cariboocoot says, but it's roughly within the 5% range. I used to use the MPPT 15 with 270 watts of panel and a pair of T-105s and it worked pretty well (and yes, I did hit the 15 amp limit in good conditions). Maybe get a hydrometer to make sure the batteries are getting a full charge from time to time (and measure the specific gravity with it after the first full charge of the new batteries for a baseline).

Since you already have the 120 watt panel, my suggestion would be to find a cheap PWM controller for it (I'm still assuming it's a 12 volt panel here), and use that plus the Sunsaver for the 240 (so you'd have two controllers/panels on the bank). At that point I think you'd be in good shape from a charging current perspective for T-105s, given your 15 watt load, IMO. After that it would be up to the sun conditions whether the system could sustain that load 24/7.