# Battery Calculations

for use with

## Inverters & Portable Power Systems

If you are intent on buying a marine or deep cycle battery at some point as part of an inverter, battery and charger system to provide power for your equipment outdoors, then there are a few things you need to be aware of when calculating battery life, capacity, losses and discharge rates etc.

These issues are mostly covered in my sections on inverters, batteries and busking power, so have a read through them to get a better understanding of the different variables involved. You might then want to do a few calculations and see if this sort of set up is the right option for you.

In this section we’ll go through the basic things you need to consider and then work through some simple calculations to help you get an idea of how to go about choosing your system.

Here are some units of measurement you might need to know before you start.

Battery Capacity in Amp hour (Ah)
Current drawn in Amps (I)
Time taken in Hours (H)
Voltage of power source in Volts (V)
Power in Watts (W)

## Battery Ratings, Variables and Losses

An approximate measurement of a battery’s ability to provide energy i.e it’s charge capacity, is it’s rating in ampere hours (Ah) or amp hours.

So a 100 Ah battery will produce 100 amps for 1 hour.

This capacity can be divided up any way you choose.

100 Ah could produce 1 amp for 100 hours, or 50 amps for 2 hours, 4 amps for 25 hours or 25 amps for 4 hours etc.

Battery capacity (Ah) = Current drawn (I) x Time (H)

or you could cross multiply and get..

Time = Battery capacity / Current drawn

Current drawn = Battery capacity / Time

So when choosing a battery for your set up, you need to know how many amps you will be drawing and for how long.

There are other things that also need to be taken into consideration when calculating a battery’s useable capacity.

When you discharge a battery by consuming the power stored in it, you should never discharge it beyond a certain point, otherwise you risk damaging it’s future storage capacity.

The maximum discharge level depends on the type and quality of the battery.

A true deep cycle battery from an RV, golf cart or forklift truck etc. can be expensive but should have a discharge capacity of up to 80%. Leisure batteries and marine batteries will be much less at around 50%, but no matter how good the battery is, if you want it to last and not pack up after a few months of use, it is generally accepted that you should never discharge a battery by more than 50% of it’s capacity.

You must also take into account, the battery’s listed ‘amp hour rating time’ when you buy it, and the speed that you intend to drain the battery when you use it. These two factors can dramatically affect the battery’s capacity. These are covered in my page on batteries.

Inverter losses also come into the equation when selecting batteries. This is also covered in my section on inverters and worked through in our calculations below.

Batteries are usually sold in voltages of 6v, 12v or 24v

To work out how much power you can get out of a battery you can use the equation..

P = V I

Power (Watts) = Voltage of battery (V) x Current drawn in amps (I)

The world’s first manned plane powered by 160 AA conventional dry-cell batteries. Produced by Japan’s Matsushita Electric Industrial Co.

## Worked Example

Say we were in need of a power set up for a small outdoor gig and we needed enough power to enable us to play for at least 4 hours.

Firstly we work out how much power we will be using at our gig by adding together the power consumption used by each piece of equipment.

I have given an example of how to do this in my inverters guide.

Say the power consumption of all our gear put together is 400 watts rms (per hour)

Batteries become less efficient as they get older and the level of inefficiency really depends on the way you look after them. If you choose to take this into account you can always add an extra percentage (15 – 35 %) to your power consumption needs for a future buffer.

In this example we won’t worry about that too much, purely for the reason that we are not going to be drawing 400w constantly for the whole 4 hour event. As we are dealing with live music, the power consumption will vary greatly with volumes and song breaks etc. and as our batteries will be newly purchased, they should perform well.

As stated previously, for longevity it is unwise to discharge any battery to below 50% of its full capacity, even if it is a deep cycle battery capable of being discharged by up to 80%

So if we need to get 400 w of power using only half of our battery’s stored energy, it means the full capability of our battery bank should be able to supply at least 800 watts an hour over that 4 hour period. This would leave us with half a tank by the end of the event.

Needing 800 watts capacity to run our gear, and using a battery – inverter set up, we must also to take into consideration the inefficiencies of the inverter, this is explained in my inverter calculations guide.

Allowing for inverter efficiency of 90%, therefore using an inefficiency factor of 1.1 we would need to put

800 x 1.1 = 880 w

into the inverter to get 800 w out the other side.

So all in all, we need to buy a battery that can deliver 880 watts per hour for 4 hours. This will allow us to compensate for inverter losses and to only have to discharge the battery to 50% every time we use it to power our gig rig.

If we purchase 12 volt batteries and need 880 watts for 4 hours, then we can work out the Ah rating of the batteries we need to buy.

Battery Capacity (Ah) = Current in Amps (I) x Time (H)

We need to establish the current drawn (I) by our system.

P = V I

Power (watts) = Voltage (V) x Current drawn in Amps (I)

I = P/V

880/12 = 73.333 amps

So our battery capacity in Ah would need to be

Battery Capacity (Ah) = Current drawn (A) x Time (H)

4 x 73.33 = 293 Ah

In this instance, you would need to have around 300 Ah of battery power to do the job, cover losses and take care of your battery life.

You could achieve this with 3 x 100 Ah 12v batteries connected in parallel, giving you 300 Ah at 12v.

See my page on connecting batteries in series and parallel to find out how.

You might find in reality this gives you a much greater capacity than you actually need as we’ve originally calculated for a constant supply of 400w. The noise levels and power used in a live music situation is going to vary greatly over time so your average draw is probably going to be less than you have designed for.

In a live situation though, having a greater battery capacity than you need is never a bad thing.

## More Compensations to Watch Out For

One thing you may have to watch out for is your ‘amp hour rating time’. This subject has been covered on my batteries page. We will assume the capacity figures we are given for the batteries we buy will be rated under the 8 hour amp rating time and not the 100 hour rating time. This means our batteries’ labeled capacities are accurate for our needs as they were tested and rated in similar circumstances to how we intend to use them.

(Had they been rated under the 100 hour amp rating and not the 8 hour time, then we might have had to compensate our final capacity by up to 20 %)

Also, as our batteries are to be drained to half their capacity over a 4 hour period (simulating an 8 hour full discharge) we don’t have to compensate for an excessive or quick current drain. Peukert’s law suggests that consuming a battery’s charge at very high current rates (i.e draining the whole cell in 1 hour) can reduce the capability of a battery by anything up to 50%.

In our case, none of the above conditions need to be considered. As long as you are aware of them, you can take steps to compensate if they arise in future.

## The Easy Method

Let’s not forget the easy method of doing things. Using a clamp on ammeter, which you should be able to get for around £20, set your equipment up in your own home and measure the current draw on a normal household supply. Use the results and a few power equations to calculate the correct sized batteries for your needs.

Now you know what sort of battery bank you’ll need, go and check out my inverter page. You’ll also need to find a good charger to keep your batteries topped up.

If you need to step up your power requirements to greater levels, then increase the amount of batteries you use or if you’ve got the cash, go out and buy your self a honda gas generator.

By the end of my power guides, you should have a good idea of what will best suit your needs.

For time and legal reasons I don’t answer reader’s questions on calculations or individual set up’s, but if you need to know anything about creating your own power set up or need advice on calculations, make sure you fully read through my battery guide and posts on inverters, battery calculations and battery connections.

There you should find all you need to know to help you get started.

Other Posts of Interest

### 30 comments to Battery Calculations – Battery Life Span – Current Draw & Amp Hour Rating

• S.Banerjee

I would like to ask that how much current doeas a 150 amp battery consumes . The battery is connected to a 650va inverter and its on 24 hrs.

• It depends what the load on the system is. If you read my guides on inverters and battery calculations you should get a pretty good idea of how to calculate any figures you might need.

• Thanks for the post. This post was very useful in calculating battery calculations.

• Gerrie

If 0.27 amps is drawn from a 12v 80 a/h batt how many hours before batt is flat?

• Hi Gerrie, sorry, I don’t do calculations for readers, read my battery guide and posts on inverters and battery calculations. By the end of that you should be pretty clued up on how to work things out for yourself.

• Helen

For a Seyvlor raft, can I substitute a 12v, 7 amp battery with a 12v, 15 amp battery? Bought two 12v about three years ago and never used them, not they won’t charge since they’ve sat around too long. Thank you.

• Sandeep Malik

Which is a better 12 Volt battery to be used for a heavy vehicle application:-
120 Ah @ 5 hr rating
or
140 AH @ 20 hour rating?
How does one compare the two?

• mamun

Dear Sir,
Please inform me if i use 4 pieces of 200 amps (Deep cycle)batteries in series and consumption of 1200 watt, how long i will get backup from it?
Sincerely,

Mamun

• amruth

21plates batterys in engines how much power it consumes , give the details with brief.

• Udoka Ukeje

IAM DOING A DESIGN NOW ON HYBRID SYSTEM . I WANT TO KNOW BATTERY CAPACITY TO USE TO ACHIEVE 12HRS BACK UP TIME WITH 5KW LOAD.

• Uttam barfa

How i can calculate the AH of battery if this parameter is given min.VAH:3456

• What battery and inverter do I need to power a Roland AC-60 (60 WATT) acoustic amp for 4 hours?? (power consumption= 68 watts) I just use it for one guitar and one vocal.

thanks

• Elango

We are planning to install a low voltage (approximately 0 to 5-10v) battery tester/cycler with a minimum of 8 channels to test and determine the charge/discharge characteristics on Li-ion, Li-polymer, Ni-cad, Ni-MH, Lead-acid, and ultra-capacitors. Specifically, we wish to test the maximum safe charge and discharge rates of button cell type samples of the above batteries.
We need to be able to remotely control and receive data from your device through our own proprietary software. The user should be able to program and control each channel independently to determine the charge & discharge curves and other characteristics on the above mentioned batteries. Temperature monitoring on each channel should also be included.
What are the maximum charge/discharge currents that our equipment can continuously handle?
what should be current range for (0-10)v Batteries that we use

Thank you.

• Ziggy Tomcich

Kier,

Thank you very much for posting this very useful information. I find this very helpful.

Frankly, many of the posts left here are an embarrassment. A surprising number of you are asking questions that he has already answered in this page. Please read this entire webpage carefully before posting a question. Many of you who are asking calculation questions have clearly not read the entire webpage. When you post a question that has the answer for in the very same webpage, you come across as a complete idiot. Thank you.

-Ziggy Tomcich

• Thanks for your input Ziggy, much appreciated.

• TNT

Hey nice page. Great info.

So your formula for a 400 watt system running on 300ah Is exactly what I’m running.
Do you have any idea how many watts of solar panels I would need to keep 300ah battery bank charged getting 5 hrs of light per day?

• enriv

Hi, Im still confused on how to compute the needed power for my inverter. I have two – 12V battery charged from a solar panel, the battery has indicated as 12AH/12V, since I’ve two units, it is 12AH*2=24AH, is this correct.Then my inverter wattage is only 150W,how long can I use this using my 48 W laptop.

Tnx a lot for your help

• Nirav

I just want to know that what would be the power consumption of the 14.4V battery if its capacity is 2.4Ah?

• Derek

What battery size do i need to hold up a 760ma load for 1 hour which also has 45 metres of 1.5m cable to the load???

• Derek

i have a load of 760ma which is fed by a 1.5m cable that is 45 metres long what battery AH would i need to maintain the load for 1 hour

• Baris

Pls I need to be clear on something:
what happens if my inverter of 1.4KVA/24V is connected with just 12V battery?
And:

I want to connect four 200AH/12V batteries in parallel so that I can have max capacity (800AH) but obviously by that, I would be getting only 12V entering my 1.4KVA/24V inverter…. anything wrong in that?
In summary:

what is the disadvantage of installing battery banks with voltages less than the rated voltages for the inverters?

• Baris

• Sam

@ Baris – 1. You can not connect 12 V battery to 24 V inverter. Battery will get damaged.

2. You can connect two 12 Volts in series to obtain 24 volt and you can obtain same power as in parallel.

• shunmuga

i bought an inverter battery named sonic & sf ppx 880 is put in that battery. how to i calculate the amp or ahp ?

• shunmuga

i bought an inverter battery

• esraa

thanks very much for this information its very usefull
Iwant ask you Ihave battery have capacity 100AH and when I charg it drown 0.27Amp its normally operated or not please help me

• bk

we have built a battery but how can we measure its Amp-Hr ratting. And how can we know that which sise of electrode will give how much of Amp-hr.

• Suresh

If i am playing music or executing some application. I want to know How can I calculate when will battery die.

• bl

So based on your calculations the current draw on your system has to be 400watts/12v = 33.33Ah so in 4 hrs it would consumed 33.33Ah x 4 hrs = 133.32Ah to meet the 50% battery capacity requirements. So the point is your system has to operate at or below 33.33Ah am I right?

• Peter Jacobs

Hi, very useful site. Thank you. Still confused. Can someone help.

I need to by a Pure Sine Wave inverter and prefer also to buy a 12V Deep Cycle battery. I shall soon start to do 3 hour busking gigs in Melbourne.

My equipment consists of: An AER Compact 60 Watt Acoustic Amp. Consumes 120 Watts.
My effects pedal board has:
- One Voodoo Labs Pedal POwer 2 12 Volts 8 gang power supply.
- One mains voltage (240V) TC Electronic Stereo Chorus (The only mains Fx pedal). Draws 100 ma current.
- One Rothwell Lovesqueeze Compressor Pedal. Draws 50 ma current.
- One ElectroHarmonix Memory Man Fx pedal draws 41 ma.
- One Peterson Strobostomp Tuner draws 80ma
- One Dunlop Custom Wah draws 75 ma.
-One TS808 Tubescreamer draws 7.5ma

What wattage Pure Sinewave Inverter ? over sized to take into consideration efficiency losses and battery capacity ?
What AH capacity battery )so that it retains after 3 hours at least 50% of its charge ?
What equivalent battery charger ?

I have sensitive equipment and so price not an issue.

Thanks