# Calculating battery capacity

Registered Users Posts: 19
I'm trying to calculate how much battery backup to use with a solar power system. I need to power the load for 24 hours a day 365 days a year and its located near Tucson.
How do I figure this out? Do I just need to find out what's the max number of consecutive days with no sun? Or is there another way to calculate what my battery needs are?

Re: Calculating battery capacity

PV Watts has an Output Hourly Data option that gives "Typical" output (something like a 20 year average). You could look for the "worst case" day/series of days in the data...

However--I guess the question is what happens if you have a 1 in 50 year series of "dark days"... Do you size the array+battery bank for the worst case 50+ year weather (if known?). Do you put a small autostart genset (propane, etc.) out there? Do you on third day hope out in an car and drive to the site and setup a Honda eu2000i and DC battery charger?

Can you shed loads? Can the station go dark after 6 days? (three days to 50% state of charge, and 50% reserve capacity)? Replace battery bank after 4-6 days of no sun?

Is this a HAM repeater or do you have a contract for uptime to maintain?

Sorry--more questions than answers. Engineering frequently points to 10x safety factors--especially when dealing with random unknowns like weather.

For Tucson, it appears that you average over 5 hours of sun per day... Pick something like 2.5 hours of day of sun and 3 days of backup with 50% maximum discharge...

More than likely, that would give you more than a week to get out there with a genset to bring the unit back up...

I guess--if this is, for example, a HAM repeater--you should assume worst case transmitter operation during a worst case weather (or fire) event.

Given that a HAM transmitter can be upwards of 1,500 watts (one frequency) with something like 50% efficiency (>3,000 Watts worst case?)--you could end up with some very ugly power requirements.

On the other hand, if you can fall back to 150 watts during "emergency" operations, that is only one S meter unit down--probably would maintain usefulness as a repeater with reduced power.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,656 ✭✭✭✭✭
Re: Calculating battery capacity

It comes down to threat assesment, so it's a personal thing. Really!

If it's life support and there is no other backup. One number, likely involving 2 systems with multiple backups of things that are likely to fail. still have outages when a charge controller, inverter or battery dies, but with the flip of a switch your up and running.

If it's what it likely takes to have, sized for general use, with very few outages over a finite number of years, and you can tolerate a 12 hour period without if you had to, then a calculation of the number of days w/o sun (X) the use of your household (X) percent of maximum discharge.

If you have a willingness to do with out and cut back, when you're facing dark days, No laundry, less A/C,... and use what is generated. That's a different level.

I think most people new to solar are in the first group, feeling the electric should be there at the flip of a switch, people who have grown a system and lived with out, or have lived off grid for a while and have maintained battery banks, get farther down on the "needs".

And after almost 10 years off grid, I'm ready (and have prepared for) a transtion from the "use what's generated" to "sized for general use"
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 19
Re: Calculating battery capacity

Yea I knew I was asking a vague question.
Thanks for that link to PV Watts. I plotted it and according to that 365 days of data I can estimate that a "low" sunlight day is about 50% of normal. Also I can see that there were never 2 consecutive days of "low" sunlight in that data.
I think I can then safely assume 2 full days of battery backup is sufficient. (I can tolerate a 1-in-50 year freak weather event).

Now a question about batteries: If I have, for example, a 100Amp-Hr battery, does that mean that the battery is at 0% capacity after 100 Amp-Hours? Obviously I wouldn't want to drain it to 0% because that's bad for a battery, so I therefore I should add battery capacity? What if I calculated 2 days of battery backup to drain 50% of battery capacity. So I figure out the battery capacity I need and then double it. Does that sound reasonable or is there another commonly used formula out there?
• Banned Posts: 17,615 ✭✭
Re: Calculating battery capacity

50% is the normal maximum discharge for a deep cycle battery. A few can take more, but not many.
So if you need 100 Amp hours you should have at least a 200 Amp hour battery bank.
My rule-of-thumb for off-grid design is to plan on 25% DOD, know that you can go to 50% on that sun-less day, and then start the generator on the third day if the clouds are still there.
Some recommend a 3-day battery bank, but that gets to be expensive and redundant, as you need enough panel to recharge the whole thing in one day and that extra panel will be "wasted" on days when the sun shines.

BTW, before a battery reaches that 100% discharge state the system will quit functioning as the Voltage falls below minimum before the battery runs out of potential.
• Registered Users Posts: 19
Re: Calculating battery capacity

My system will run at 24VDC and I've spent a lot of time searching for a large capacity 24V battery. (My definition of large capacity is >100A-Hrs.) But it seems like the selection of 24V batteries is very small compared to 12V (and 6V) large capacity batteries. Should I just put two 12V batteries in series or is there any advantage to using a single 24V battery?
What is anybody's recommendation for a high quality battery manufacturer? I was looking at PowerSonic, I heard they have a good reputation. They have a nice selection of 12V batteries but they don't make 24V batteries, that I could find.
• Banned Posts: 17,615 ✭✭
Re: Calculating battery capacity

The reason you don't find "large capacity" 24 Volt batteries is because it would be as big as two 12 Volt batteries anyway. So it would weigh 100+ lbs. They get a bit difficult to move about at that weight. :roll:

Some folks use forklift batteries for 48 Volt system, for example. You need a forklift to move them.

It's best to buy two 12 Volt or four 6 Volt batteries to make up the capacity you need. Start at the NAWS battery page: http://www.solar-electric.com/batteries.html

BTW, 100 Amp hours @ 24 Volts gives you about 1200 Watt hours capacity at most.
• Solar Expert Posts: 102 ✭✭
Re: Calculating battery capacity

Diego: What are you powering? Such as, how may amps does it draw? What is the size of your solar arrangement, panels, watts, etc? How many batteries are you presently using, and what are their capacities? The majority of your questions were answered in post #5 by a moderator. Please be more specific.
• Registered Users Posts: 19
Re: Calculating battery capacity

I'm powering 370W of 24V electronic equipment. So that's 15.4A. I don't have any solar panels or batteries yet, I'm in the process of trying to figure out how to design it.
I have a question about solar panels: I'm looking at a datasheet for a Kyocera KD235GX-LPB. It gives specs for Standard Test Conditions and Nominal Operating Cell Temperature Conditions. I'm not sure what the difference is. For example I(mp)=7.89A for Standard Test Conditions, and I(mp)=6.31A for NOCT. When I'm calculating how much current I will get from a solar panel, which number would I use?
• Banned Posts: 17,615 ✭✭
Re: Calculating battery capacity

Probably neither, as they are both lab figures. The first under "ideal" conditions where the panel is "flashed" with light, the second at a perfect operating temp.

What you want to do is take the panel's Watt rating, derate it by 23%, and divide by charging Voltage. Because what really matters is not what current the panel is capable of putting out, but what you are likely to see going in to the battery. The charge controller will use some of the power potential, and that's included in the 77% multiplier you often see used around here.

So a 235 Watt panel @ 77% efficiency is 180 Watts. Divided by 28.8 Volts (typical charging for 24 Volt system) is 6.2 Amps. (Notice how close that is to the NOCT figure).

In other news, you're planning on power 370 Watts @ 24 Volts? Roughly 15-16 Amps current. For how long? That is a very important part of the equation.
• Solar Expert Posts: 1,341 ✭✭✭
Re: Calculating battery capacity
diego96 wrote: »
I'm in the process of trying to figure out how to design it.

370w * 24 hr = 8880wh/day

Then you figure out how much battery you need to supply that load for two days:
8880wh x 2 = 17760wh
17760wh / 24v (nominal) = 740ah

Double that to keep the batteries above 50%:
740ah x 2 = 1480ah

So you need a ~1500ah 24v battery bank.

Then, you figure out how much solar you need to recharge the bank while also supplying the load - you need enough PV to carry the running load and have enough left over to charge the battery bank.

So you need 8880wh/day to carry the load (370ah/day) and also need to replenish up to 740ah/day into the battery bank if you want to top the bank off in a single day.

So that's 1110ah/day you need to generate. Mulitply by charge voltage of 28v to get the watts:
1110ah x 28v = 31080w

Divide that by hours per day of "good" sun (I'll use worst case and say 4 hours):
31080w / 4hr = 7770 watts per hour.

So you'd need a 7.8kw PV array.

BUT...the PV only puts out around 80% of rating on average, so add 20%:
7800w x 1.2 = 9360w

9360w x 1.2 = 11232w

So you'd really need a 11.2kw PV array.

That's pretty much how it's done.

There are some fudge factors...

For instance you could plan on recharging the battery bank over the course of two days instead of doing a full recharge in one day - so you could possibly cut the PV array in half.

You might get more sun on average than I used in my example. If you average 6 hours a day of good sun instead of 4 then you might be able to cut the PV array by 1/3 - as long as it didn't create a significant shortfall in winter.

You could plan on one day's backup, and cut the battery bank in half.

There are various ways to play with the numbers, but the reality is that a 370w continuous load is a bloody big load to be running from PV and batteries.

At the very least, you'll need to supply 9kwh/day just to run the loads, so with say 6 hours/day of good sun, you'd need a bare minimum of about 2kw of PV + whatever you need to recharge the batteries when they need it.
• Registered Users Posts: 19
Re: Calculating battery capacity

Thank you very much dwh, that was invaluable information.
I will probably choose to let depleted batteries recharge over a period of 2 days, to reduce the number of solar panels.
I'm not sure what you mean by "lead-acid batteries are only around 80% efficient". Do you mean if a solar panel inputs X watt-hrs into a battery, I will only get X*0.8 watt-hrs out?
Re: Calculating battery capacity

For a first level approximation--yes, that is correct. Flooded Cell Lead acid batteries run about 80-90% efficient. It depends on how you charge (i.e., charging 85-100% is less efficient than charging 75%-90% range). Also, older batteries tend to be less efficient than new batteries.

The rule of thumb for energy efficiency is 80-90% for flooded cell and 90-98% for AGM. I like to use the smaller numbers to give "worst case" results.

When you take all loses into account:
• System Eff ~ 0.81 panel derating * 0.95 charge controller eff * 0.80 flooded cell eff * 0.85% inverter eff = 0.52 end to end efficiency for off-grid solar
Can you do better... Somewhat. Sub freezing weather will increase solar panel output with MPPT controllers. Use AGM batteries (more expensive). Size the inverter to the loads (very light and very heavy loads are less efficient on inverter, less "on time" with zero loads wastes less, etc.).

More or less, we try to error on the conservative side so that people can run their systems without having to use a stop watch and battery monitor 24x7 to not deficit charge the battery bank (use more power, on average, than gets returned to the batteries).

Add other variables--Bad weather, possible shading, dirty panels, less than idea wiring, somebody leaving the lights on, etc... You usually don't want to plan on using 100% of your "predicted power" every day with an off-grid system.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 9,362 ✭✭✭✭✭
Re: Calculating battery capacity
diego96 wrote: »
.....I will probably choose to let depleted batteries recharge over a period of 2 days, to reduce the number of solar panels.......

That is a bad idea for battery life. When the battery is below 75-80% charged, the sulfate deposits on the plates begin to harden. After 24 hours, they become permanent. This then reduces your battery capacity a little bit each time it happens
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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• Solar Expert Posts: 192 ✭✭
Re: Calculating battery capacity
mike90045 wrote: »
That is a bad idea for battery life. When the battery is below 75-80% charged, the sulfate deposits on the plates begin to harden. After 24 hours, they become permanent. This then reduces your battery capacity a little bit each time it happens

wow, I'm shocked and saddened that it happens so fast. I've probably shortened the life of my trainer set a bit just to recharging over 2 days on occasion. bummer...