# Thread: Sizing Batteries for inverters

1. ## Sizing Batteries for inverters

How long will the batteries last when I am running off my inverter?

This is usually one of the first questions clients ask

Well that depends on numerous factors.
1) The Power or wattage you are using
2) The size of Battery (Amp hours)
3) The depth of discharge (DOD) of the batteries
4) The efficiency of the inverter/battery system.

To simplify this, I created an online calculator (View Here)
This can be viewed on a Desktop computer, a Tablet or Smartphone.

Simply input the following
1) no. of batteries you intend using
2) The battery voltage
3) The Amphour rating of the battery
4) The inverter efficiency (default is 80%)
5) The Depth of Discharge of the Batteries (default is 50%)
6) The wattage of appliances or circuits you intend using

The number of hours is then calculated based on all of the above.

The next question is

How many years will the batteries last?

Firstly, deep cycle batteries lifespan are measured in cycles & not in years.
(a cycle = 1 discharge & 1 charge cycle)

The manufacturer of the battery should provide you with a discharge graph
showing different cycle lives @ different depth of discharge percentages.

An example would be as follows
400 cycles @ 50% depth of discharge
200 cycles @ 70% depth of discharge
100 cycles @ 80% depth of discharge

As can be seen the cycle life is greatly affected by the depth of discharge
and it would be advisable to not discharge the batteries by more than 50% to ensure
a decent battery lifespan.

Lithium Iron Phosphate batteries (LiFePo4) are now available locally which guarantee
a 7000 cycle life @ 50% depth of discharge.
They are however frightfully expensive, ranging from R40000.00 - R460000
(View Specs Here)

Also bear in mind some other advantages of Lithium Iron Phosphate batteries versus Lead Acid.
An increase in Temperature of say 25 degrees C to 35 degrees C will halve the lifespan of a Lead Acid Battery.

Lithium Iron Phosphate batteries are unaffected by temperatures up to 45 degrees C.

Typical re-charge time of a lead acid battery is 8-9 hrs (irrespective of the depth of discharge)
The re-charge time of a lithium Iron phosphate battery is 30-60 minutes.

The next question is

How do I control the depth of discharge of the batteries?

Some inverters have a certain amount of control to turn the inverter off when the batteries reach a certain low voltage
(The voltage basically determines the state of discharge of the batteries)

However most inverters do not have a variable low battery control.

These inverters will drain the battery to +- 90% before the inverter shuts down.

The Lithiun Iron Phosphate batteries have a built in battery management system which is pre-programmed
at either 50 or 70% depth of discharge cutoff.

By interfacing the Battery management system with the output of the inverter, the inverter
will shutdown when the determined depth of discharge has been reached.

However most people do not wish to layout the capital for these batteries.

So how do we overcome this problem?

This can be achieved by using the calculator mentioned above & constantly monitoring the wattage usage.

The next obvious question is

How do I monitor the wattage?

Efergy have a wireless power monitor, which can be purchased at Builders Warehouse (Ellies)
(View Details Here)

Simply clip the supplied CT (current transformer) to the output live wire of the inverter.
Connect the CT to the wireless transmitter.
Turn on the Wireless monitor to pair with the transmitter.
You can now monitor your power consumption in real time.

The voltage has to be entered into the monitor for it to calculate the wattage.
Use the std voltage of 230v
If the voltage fluctuates then the monitor would not be 100% accurate.
However one can expect a 90% accuracy.

As batteries will be an ongoing expense, it is worthwhile installing one of these to ensure you get the maximum lifespan

I hope this helps & would appreciate any comments or suggestions.

2. ## Thanks given for this post:

mikilianis (24-Feb-16)

3. Hi Donny,

I am a bit lost regarding your question.

By using the online battery calculator (View Here)
you can determine how many batteries you require, based on power consumption & other criteria used on the calculator
to determine how many hours you can run off grid (shore power).
I see you are in Canada.
If the wattage is not shown on the appliance you wish to use, you can calculate the wattage by using
the formula Volts x Amps = watts
For you that would be 120v x amps (shown on the appliance)