Solar system design tips

I'm doing design checks on a small, intermittent use solar system. Application is to run a 115v, 1200w blower for an approximately 10min interval. In other words, someone bought all the parts then asked how well it would all work.

My main question is this- where do I want my battery voltages to stay? The question is two-fold, to determine allowable cycles on available storage and where to set lo-V protective measures.


Math Q-
1200w motor + 20% =1440w. At a nominal 12v that's a 120amp draw for about 10min. Or about 20ah. Is that the proper way to look at this?

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Here's a state of charge chart. You do not want to let the battery go below 11.96 volts. Below that voltage excessive sulfation will occur and can eventually damage the battery by buckling plates. Sulfation is normal during discharge but the plate spacing is only designed to tolerate so much buildup. Deep cycle batteries have wider spacing and can tolerate more discharge.



Hopefully the solar cells to be used have enough capacity to keep things charged during a period of several overcast days.

Cole2534 said:

I'm doing design checks on a small, intermittent use solar system. Application is to run a 115v, 1200w blower for an approximately 10min interval. In other words, someone bought all the parts then asked how well it would all work.

My main question is this- where do I want my battery voltages to stay? The question is two-fold, to determine allowable cycles on available storage and where to set lo-V protective measures.


Math Q-
1200w motor + 20% =1440w. At a nominal 12v that's a 120amp draw for about 10min. Or about 20ah. Is that the proper way to look at this?

Programmed via Mazatrol

Click to expand...

Yes, that way of looking at it is fine. Essentially, if you want that 20AH draw, the panels need to be able to replace about 24 AH before the next draw starts, in the worst solar conditions you expect.

based on drawing no lower than to a state of 80% charge, your minimum battery would be about 100AH. That is for ideal conditions, and one "cycle" per day. Adjusting for different conditions may be required.

As for "state of charge" charts...... They can be misleading. The chart has to be for a battery that has a particular specific gravity at "full charge". Voltage is dependent on acid concentration, which is measurable by specific gravity (SG).

So, if a battery has (by design) a lower SG at full charge, that will move the chart down.... 80% charge will be at a lower SG than a generic chart may show.

The SG chart shown by the other poster is for a Trojan T-105 battery, a good 225 AH deep cycle type. If that is not what you are working with, it may not apply directly.

Many long life deep cycle batteries have SG at full charge considerably lower than an automotive starting battery, which is sold on price, not performance long term. For those automotive batteries, the higher SG allows a smaller battery volume and price.

Cole2534 said:

I'm doing design checks on a small, intermittent use solar system. Application is to run a 115v, 1200w blower for an approximately 10min interval. In other words, someone bought all the parts then asked how well it would all work.

My main question is this- where do I want my battery voltages to stay? The question is two-fold, to determine allowable cycles on available storage and where to set lo-V protective measures.


Math Q-
1200w motor + 20% =1440w. At a nominal 12v that's a 120amp draw for about 10min. Or about 20ah. Is that the proper way to look at this?

Programmed via Mazatrol

Click to expand...

Don't forget to factor in the inverter that will be converting 12 volts to 120 volts. I'd think you will probably have at least a 2,000 watt inverter and it will likely be running 24/7 unless someone will be there to turn it on and off when not needed. Inverters use power whether they are running a load or not. I have a 2,800 watt inverter in our RV and it takes 4 amps at 12 volt just to keep it running with no loads on it.

And as DD says, you don't want to run your batteries below 12 volts and ideally not even nearly that low. Low voltages is the death of lead acid batteries.

Make sure that you use storage batteries like golf cart batteries and not starting batteries like you use in a car.

I'd probably start with a pair of golf cart batteries and if that isn't enough add two more at a time until it gets the job done.

And there is nothing cheap about going solar. I have over $5K into our system with me doing all of the work.

Trojan makes a number of batteries intended for RE, meaning they withstand periods of partial discharge better than golf cart types. Thicker plates than T-105s and more room below the plates to allow for sulphates to drop off.

Renewable Energy – RE | Trojan Battery Company

But the biggest factor in buying FLA batteries is transportation. They're heavy, and expensive to ship. I've had good luck with Trojans, but Surrette is a possibility, too. Whichever has a delivery point nearest you.

But the system is a lot more complicated than batteries, panels, solar charge control, inverter ?, load. Combiners, disconnects, fusing, circuit breakers that meet everchanging code are important, too as are battery box venting systems.....

FWIW: Outback makes some of the best standalone inverters (if needed) and the best charge controls. Midnite Solar is good, too. Welding cable makes for the easiest low voltage wiring, Square D QO breakers work on DC as well as AC, run + and - battery cables to opposite corners of the bank, run a separate + cable from the solar charger.

And consider what could happen when there isn't enough sun for the load. A battery bank sized for 3 days of no sun is the rule of thumb. Generator? Can the load be switched off for a few days?

The blower on period is useful info, but how many times a day does it come on? Through the night?

Watt-hrs/day requirements are a bit more useful for system sizing than just AH, as is peak draw (for inverter sizing, if needed) for when the fan motor starts. Or is this a pure DC application?

One other thing to consider is the starting current for the motor.
Might not be an issue for a small, DC motor, but for my solar inverter selection,
I had to consider the LRA (Locked Rotor Amps) of the AC well pump motor, which were 6x more than the running current.

Cole2534 said:

My main question is this- where do I want my battery voltages to stay? The question is two-fold, to determine allowable cycles on available storage and where to set lo-V protective measures.

Click to expand...

Depends on the battery.

Lead or lithium?

Stationary PV chemistry or big box store deep cycle?

Rule of thumb with lead-acid cells is >50% state of charge at all times except for brief dips below that. Unless the battery is identified otherwise, assume it's chemistry is such that it will begin to sulfate below 50% state of charge. Allowing it to remain sulfated for any appreciable length of time will drastically shorten your cyclic life. Photovoltaic batteries (e.g. some of Rolls Surette's product lines) will handle deeper discharges better than ordinary run-of-the-mill deep cycle batteries but the problem is more or less intrinsic to lead-acid cells to varying degrees.

With lithium cells, rule of thumb is 10%-90% state of charge. Lithium iron phosphate cells run this way (and within their 'C' ratings) will easily last thousands of cycles - several times longer than lead acid in many cases.

Of course the up-front cost between the two is much different, hence "which is better" remains a subject of never-ending debate. Bottom line it comes down to your scope of use, up front budget and preferred maintenance interval - bearing in mind that recycling batteries more frequently is less ecologically responsible as there is always significant wastage and energy cost involved. 3.5 years vs 19 years on paper for 50% depth of discharge at <0.25 C.

If there is no strict need for remote power, grid-interactive solar is a much cheaper and more eco-friendly option. Selling power to the grid during the day helps to reduce demand seen by natural gas peaking stations and you'll never have to worry about dead batteries. There are few transmission losses since most of the power sold that way ends up going to your neighbors. Of course when the grid fails...

Hence hybrid PV systems that utilize the grid most of the time but are equipped with small standby battery banks to either ride out brief outages or allow time for controlled shut-down similar to a UPS. Since these only discharge during an outage, they can be sized for much greater depth of discharge, lowering expense considerably.

Lead acid:

Lithium iron phosphate:

Also consider that, especially for lead acid, battery capacities get significantly lower as you raise the discharge rate. A typical battery will have rated amp-hours based on a 10 or 20 hour discharge. You're looking at discharging it in about an hour (based on 1/6th capacity per interval and 10 minute intervals). Starting load will be even worse.

How often does this fan need to run, and is it only during the day? I would investigate running a fan directly off the PV with a VFD or a DC fan. This eliminates the battery maintenance requirements and you might get away with a smaller fan if you have continuous airflow.

If you have enough space, go for nickel-iron accumulators.

Good thing about batteries is the ability to have full current on demand. Direct from solar panels is great on a sunny day. Pretty crummy with heavy overcast, you'd need massively oversized arrays to make it work.

Yes, discharge rate affects A-H capacity. The T-105 is 225 AH at 20 hour rate, but 185 AH at the 5 hour rate. At 75A, about 140 AH, which is just under the 2 hour rate.

Nickel-iron can be run down flat, and left for months, and then charged up and almost as good as new. Expensive, and really crappy at holding a charge, but damn near indestructible without explosives or hammers. Also not prone to exploding or burning up.

Might actually be a very good choice for unattended systems.

Put the sun in the middle. [emoji16]

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JST said:

Good thing about batteries is the ability to have full current on demand. Direct from solar panels is great on a sunny day. Pretty crummy with heavy overcast, you'd need massively oversized arrays to make it work.

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True, but it depends on what the fan is for. If it's a cooling fan, demand is going to scale pretty well with the available power. And running a fan slower for longer tends to be *vastly* more efficient - something like the 3rd power IIRC.

Unless you size the battery to run with a week of minimal sun, you're going to need to oversize the array a lot anyway. That's assuming there's no need to supply excess starting current - that's what you really can't do with a direct solar system. It needs to be brushed DC, BLDC, or VFD. Inverting to drive a DoL AC fan is daft.

You might also want to look up a few (very few.....effective censor)...videos of "Tesla Battery burning out"......they burn out because you cant spray water on them......now if the burning out battery has your house wrapped around it .....then tough......Incidentally,the worlds largest Tesla battery ,under construction has had four of the shipping container size units burn out......before its even connected......Quote from the fire service'.."We attend a Li battery fire every day ,many times with a house involved."

Guys, thanks for the info. Itll be a minute before I digest it all.

As far as panel sizing, well Im using the juice to operate pneumatic boat lifts on my dock. Generally speaking, if there's no sun hitting the panel we probably wont be hitting the water.

Here's the fan-
$125.49 Ametek 116474-37 Blower / Vacuum Motor 4M983

They get used once or twice per day per boat on the weekends, and that is normally only saturday, during the summer. This is a community dock with 5 boats in it.

Currently we use a little Honda EU2000i and that works very well and that unit will be there as backup.

The parasitic drain of the inverter is good to know. Sounds like one of those timed switches would be helpful to prevent it from being left on.

Unfortunately grid power isn't economical. We are hundreds of yards from the nearest pole.

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john.k said:

Quote from the fire service'.."We attend a Li battery fire every day ,many times with a house involved."

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Uhh, yeah, whatever you say


A pair of big 6 volt deep cycles ran my inverter and electric chain saw long enough to drop limb and mostly section an average 8-10 inch tree.

300 bucks or more worth of batteries and more duration than you probably need, dependant on charging

52 Ford said:

Put the sun in the middle. [emoji16]

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Yeah, I saw the title and my first thought was he wanted to know about planet masses and orbits and stuff.

52 Ford said:

Put the sun in the middle. [emoji16]

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Click to expand...

Clever.

VRLA batteries are not that complicated but first thing is to get a data sheet and read it.

Most lead acid batteries are rated for expected 1000 charge and discharge cycles.

Full discharge is 1.7 volts per cell.

You can improve by using 1.8.

1.75 is 10.5 volts on a 12 volt unit.

Only discharging to 12 or 11.5 requires more batteries that may last longer but at the end your cost is same as the 2 times the battery cost may have lasted 1.5 times longer.

Pay attention to the load ratings.

A 100 Amp hour battery is rated at 8 hour load.

Some cheat and will state a larger output but 20 hour rate.

Calculate your estimated run current then look up on sheet.

If it draws 10 amps at 120 it will be 120 plus inverter overhead at 12.

If using inverter look for unit that operates on higher input voltage.

Many available with 48 volt input.

Reduces input current and easier to build.

Whatever battery you choose to use, download the data sheet and understand how the battery works before you buy one.

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Before we go any further - what parts do you already have on hand?

Do we understand the GRAVITY of this project? A WHOLE solar system?! [emoji4] I'll see myself out.

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