How to Estimate Solar Charge Time
Unfortunately, solar charge time is not as simple as just dividing your battery capacity (measured in Watt hours) by the power of your solar panel (measured in Watts). Even in perfect conditions, you get loss due to:
- Voltage drop of solar panel or Maximum Power Point being lower than rated peak panel voltage
- Energy used by circuit to buck or boost voltage
- Energy used to convert electrical energy into stored chemical energy – with excess lost as heat
We go through two common battery chemistries and give you some rules of thumb for each.
Lithium Ion Battery Charge Time from Solar
Voltaic carries a full line of IoT Power Banks and small solar panels. Since the charge cycle slows down considerably after the battery reaches 90% capacity, this calculation assumes full is about 90% of complete capacity.
Battery Capacity (in Watt hours) / Panel Power (in Watts) X 2
We perform better than this on our own panels and battery combinations on clear sky days, but this is a more realistic estimate for less than perfect days.
Example: 6 Watt Solar Panel charging a 4,000mAh, 3.7V Battery – Time = 14.8Wh / 6 Watts X 2 = 4.9 hours
Tip: Get a “USB Multimeter” from Amazon to verify your charge rate.
If you are connecting to an off the shelf battery pack, there are a number of reasons that the charge rate could be worse. We look at those in our post: “Can Your Panel Charge My Battery Pack.” Adafruit and SparkFun both offer Lithium Ion charge controllers that can work well with solar panels.
Lead Acid Battery Charge Time from Solar
The effective capacity of a lead acid battery is about 50% of stated capacity, i.e. you shouldn’t discharge it past 50% full. They also have a lower charge efficiency than lithium ion. For the purpose of this calculation, we’ll assume that the battery capacity is the true capacity.
This is our favorite waterproof charge controller for lead acid batteries.
Battery Capacity (in Watt hours) X 2 / Rated Panel Power (in Watts)
Example: 10 Watt, 18 Volt Solar Panel charging a 12V, 10 Amp hour Lead Acid Battery (120Wh) from 50% full to Full – Time = 60Wh x 2 / 10 Watts = 12 hours
Environmental Factors Will Likely Increase Charge Time
The solar charge times above assume a 25 degree Celsius day with the panel pointed directly at the sun. Some quick rules for estimation:
Heat: Power output of a panel will decrease by about .005% per degree over 25 Celsius.
Angle: As the panel rotates away from the sun, power output drops.
| Angle to Sun | % Output |
| 90 | 100% |
| 75 | 97% |
| 60 | 88% |
| 45 | 77% |
| 30 | 61% |
| 15 | 32% |
| 0 | 3% |
Clouds: Any haze or clouds will slow down charge time, often significantly. See: Solar Performance in Cloudy Conditions.
Solar for IoT and Remote Sensors
Want to learn more about how Voltaic solar panels can work for your next IoT project? Contact our experienced team today to set up a consultation.
Hi,
I have a 5KWh 24V Lithium battery with 280Wp 24V solar panels. With 80% Depth of Discharge, 3 days autonomy, how many of these panels would be needed to charge the battery?
It sounds like your system uses ~1.3KWh (1,300 Watt hours) per day (5KWh * 0.8 / 3 days). It depends greatly on your location and time of year, but you would need approximately 9 hours of good sunlight from a 280 Watt panel to get that amount of power per day (1300 Watt hours / 280 Watt * 2). Our guess is you would need approximately 3 panels, but you would need to test and adjust for your location. Southern Colorado would perform better than Western Washington for example.
I have a load with 18 wh and a 3.7V + 3300 mAh. so what is the best solar panels to generate the load and charge the battery and how many hours will take to fully charge the Battery due to Florida weather.
Do you mean the load is drawing 18 Watts or consumes 18 Wh per day?
And your battery is 3.7V @ 3300mAh or 12.2Wh in capacity.
Assuming it is drawing 18Wh per day, the battery capacity is not enough. Industrial customers typically have batteries that last 5+ days with zero sun. Even if you went down to 3 days, you would want a battery 5 times the size of your current battery.
In terms of panel size, Florida provides about 3 hours of sun to a panel during the worst month of the year, so you would want about a 10 Watt panel (facing south, unshaded) to keep up with your power consumption.
Hi I have just purchased a 30 watt solar panel that comes with a compatible solar charge controller. I have a 12 volt 3.2 ah Motor cycle battery. This battery is brand new fully charged, came as a free gift when purchasing some biking stuff. I would like to use the combined kit and battery in my garden shed. Items that would be used are some LED strip lights, perhaps a small water pump (now and then), but a 12 volt security alarm and 12 volt CCTV camera.
The camera and alarm would be on standby unless motion detected, and obviously only when I was not in attendance.
My question is regard the battery. Would this be compatible with the Solar Kit ?
Would just like to know before I set it all up.
Kind regards
Mac
Malcolm, we are not lead acid battery experts, but you should be looking for a deep cycle battery vs a starter battery. A deep cycle battery can be discharged to up to 50% of its capacity, vs a starter battery which is designed to only discharge a little bit. If you use a standard battery, it will likely have a fairly short life.
I have 5 280 watt solar panels, and an mppt i plan to use at 24v charging. How much battery amp hours do I need to store all these panels are capable of ? My sun hours are:HIGH: 5.50
LOW: 3.97
AVERAGE: 4.73
Hi, I was wondering what is the charging voltage used in the equation above?
As of right now, I am doing my own solar battery charger project. I have a 12V 25W solar panel to charge a 12V 7.2Ah lead acid from 50% state of charge (12.20V) to fully charge.
The time I’ve calculated is 3.456 hours, but I need to know the charging voltage, as i’ll be comparing the rate of charge with different charging voltages.
We’re assuming that the panel voltage is slightly higher than the battery voltage or maximum power point. For example, we use a 6V panel to charge a 3.7V battery with a max power point around 4.9V. The equation above should be considered a rule of thumb. Actual charge times will vary on solar conditions (obviously), but also the charge circuit in your battery.
Note that most panels that say they are “12V” often have a peak voltage (Vp) closer to 18V. A panel that has an actual Vp of 12V would not charge a 12V lead acid battery well.
We definitely agree you should test to confirm performance of your system.
Yes, the output voltage of the panel is definitely higher, but it is connected to my charge circuit to limit the current flow so the battery is not damaged. That’s why I wanted to know the overall estimation of charging voltage and their difference in the rate of charge.
I have an 80watts solar panel and 12v battery ,33ah.how many ampere charge controller can I used? And how long can it take to fully charge in a bright sunlight of temperature 30°c?