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How much energy do I need for my off-grid home?

Each home has its own unique energy requirement so there is no such thing as a one size fits all off-grid solar power system. As a rule of thumb most suburban homes connected to the energy grid use between 12 and 15kWhs of electricity per day (excluding hot water and swimming pools) however grid-connected homes are not particularly energy efficient and considerable energy savings can be made by switching to LED lights, ensuring lights and appliances are switched off when not in use, using gas for cooking and solar hot water instead of electric hot water systems for your hot water needs.

Try to avoid energy hungry appliances when considering your off-grid needs, these include dishwashers, tumble dryers, electric ovens, electric hot water system, hairdryers, curling tongs, electric kettles and so on.

By doing that you will be able to reduce your daily energy requirements to around 8-10kWhs however we strongly recommend discussing your energy requirements with us so we can calculate how much you will require.

Batteries

Batteries are, without doubt, the most critical part of any off-grid solar power system and the biggest regret we come across from people looking to upgrade their existing off-grid solar power system is under-sizing the battery bank which results in increased generator use and early failure of the batteries.

The term “Lead-Acid” covers a wide range of batteries such as Tubular Gel, AGM (absorbed glass mat) and flooded and the good thing about lead-acid batteries is that that they have been in existence for more than 100 years and are extremely reliable.
In the future we are sure that lead acid batteries will be replaced by a new technology such as Lithium however this technology is still in its infancy and if you depend on batteries to supply your home with power we recommend sticking with lead-acid for the time being.

Total energy in kWhs vs usable energy (apologies if this gets a bit technical)

Calculating the total amount of kWhs in a battery bank is basic maths, simply multiply the Ah rating of the battery (usually C100 – more C ratings below) by the system voltage for example:

4 x 12V 200Ah batteries connected together in series gives a total system voltage of 48V.  The total energy stored on the batteries is therefore 200 x 48 = 9,600watts or 9.6kWhs.

If you have 2 banks of 4 x 12V 200Ah connected in series and then in parallel you have a total of 400Ah at 48V or 19.2kWhs

So if you want to have 9.6kWhs of energy per day you may think that all you need are 4 x 200Ah batteries. Unfortunately, that is not the case, as you need to consider the following:

Battery Ah Ratings

All batteries have a C rating and this is usually displayed as 200Ah at C100. The C rating is the amount of energy that the battery can discharge over a set period, so a battery with a C100 rating of 200Ah can discharge a total of 200Ah over a period of 100 hours which is not much use when you want to use the battery daily.
The C10 rating is of far more use and this will be lower than the C100 rating, for example a 200Ah battery at C100 will have a C10 rating of around 186Ah.

If we recalculate the total energy storage in 4 x 200Ah batteries connected in series using the C10 rating you end up with:

186Ah x 48V = 8.28kWhs

So the actual energy storage applicable for an Off-Grid solar power system is 1.32kWhs less than the headline total which you might think is OK however we haven’t taking into account the depth of discharge.

Depth of discharge

Depth of discharge is an indicator of how empty a battery is.
If the battery is full it is 0% discharged. If the battery is discharged only a small amount it might have an 20% depth of discharge. If the battery is discharged a lot, it might have an 80% depth of discharge.
The deeper your battery is discharged the less time it will last for this reason SunWorks Solar recommends a maximum Depth of Discharge of 50%

If we recalculate the total energy storage in 4 x 200Ah batteries connected in series using a maximum discharge of 50% you end up with:

8.28kWhs x 50% = 4.14kWhs

Now we need to consider the number of days autonomy you require for your system.

Days of Autonomy

When calculating the size of your battery bank you need to consider the number of days of autonomy. This is the number of days the battery bank can supply the required amount of kWhs for without being recharged by the solar panels or a generator. Or to put it more simply how many days will the batteries provide power for if it is raining without dropping below the recommended Depth of Discharge.

Most quality Off-Grid solar power systems have 3 or preferably 4 days autonomy.

If we recalculate the total energy storage in 4 x 200Ah batteries connected in series using with 4 days autonomy you end up with:

4.14kWhs / 4 = 1.035kWhs

As you can see the useable battery storage in 4 x 200Ah batteries is only 1.035kWhs which is a long way from the total energy of 9.6kWhs.

Total energy vs usable energy calculation
Lets recap the calculation to determine the amount of usable energy vs total energy

Total Energy
C100 Ah rating of the battery x System Voltage / 1000

200Ah x 48V = 9,600watts / 1,000 = 9.6kWhs

Usable Energy
C10 Ah rating of the battery x System Voltage x 1-DoD% / number of days autonomy / 1,000

186Ah x 48V = 8,928watts x (1-50%) = 4,464 watts / 4 days = 1,116 watts / 1,000 = 1.116kWhs

If you know you require 9kWhs of daily energy storage you can calculate the required Ah rating of your battery bank by following this calculation:

kWhs required x Days Autonomy / (1-DoD) / System Voltage x 1000

9kWhs x 4 days = 36 / (1-50%) = 72 / 48V x 1,000 = 1,500Ah @ C10

For more information on Off-Grid Solar power contact Sunworks Solar today.