How Much Solar Do You Need ?

Lithium Battery & Solar Power Calculator

We are often asked “how much solar do you need for lithium batteries to stay charged ?”. So we created this handy calculator to assist as a guide when purchasing solar panels and lithium batteries.

The answer to the question really lies in your daily usage of power measured in either Watt-hours or Amp-Hours. If you don’t already have a battery monitor that shows this information, you will need to examine the labels on each of the appliances and accessories you intend to use on a daily basis and obtain the Watts or Amps rating from the appliance or device and add them all up for an estimated hourly usage figure and multiply that number by 24 hours. Then enter that figure in this calculator.

Note: Our calculator assumes that the lithium battery allows for an 80% depth of discharge (DoD). We recommend adding at least a 20% buffer over your daily usage to allow for fluctuations in usage or less than optimal charging conditions.

Our calculator also assumes that there are 6 hours of optimum sunlight per day to charge your batteries. This will obviously vary based on location, season and weather patterns.

We always recommend installing more solar (Watts) than you require as conditions are not always perfect.

Calculate How Much Solar I Need:

So How Much Solar Do I Need ?

This is how to calculate how much solar you need:

Example

Noting that most lithium phosphate batteries (LifePo4) have an actual capacity of 80% of their rated value. We refer to this as the maximum depth of discharge and consider this to be a flat battery. It is therefore important to allow for this and add a minimum of 20% of your maximum daily usage to any lithium battery system you install.

The biggest consumers of power in any off-grid system is usually the fridge. In this example we show you how quickly your usage can add up over 24 hours and what it takes to recharge your lithium batteries each day.

Caravan Fridge: 4A x 24h = 96Ah

LED Lights: 2A x 4h = 8Ah

Phone/Tablet charger: 2A x 4h = 8Ah

Water Pump: 6A x 0.5h = 3Ah

Stereo: 5A x 1h = 5Ah

Total usage over 24 hours: 120Ah.

The Equation (Ohms Law)

Volts x Amps = Watts

Example: You have a 12v system and you need to replace 120Ah of charge back into your batteries each day. You have 6 hours of optimal sunlight each day.

You will need:

12V x 120Ah = 1440Wh

1440Wh/6h = 240W needed per hour of solar to recharge
120Ah/6h = 20Ah per hour

The above calculations assume there is no current or voltage drop between your solar panel and lithium battery and the panels are producing 100% of their capacity for 6hrs. However, we know that is rarely the case and real world factors need to be taken into account.

Whilst there are several factors that influence lithium battery charging and solar panels, we will aim to keep it simple.

Simple example
Example – A caravan fridge, a 200Ah lithium battery and a solar panel.

Battery: 160Ah usable (200Ah lithium battery 80% depth of discharge DoD)

Power Consumer: 12V caravan fridge, drawing an average 5 amps/hour = 120Ah

Solar panel: assuming a solar panel is only 70% efficient, as it will rarely output 100% of its capacity constantly. In the real world scenario, we would calculate 400W of solar as 400 x 0.7 = 280Wh or 300W of solar as 300 x 0.07 = 210Wh and so on.

We know from our calculations above that we require a minimum of 240W per hour over 6 hours to charge our battery completely, however we need to factor in the inefficiency of solar panels. So the calculation looks like this: 240 ÷ 0.7 = 342.86W.

In other words, you will need a single 350W solar panel or 2x 190W solar panels to ensure you can more than meet the daily needs of your 120Ah power usage.

This Battery & Solar calculator is a buying guide only. Please use it for estimated information purposes only.

If you are unsure of your requirements please give our team a call, we are always happy to give you great advice.