Properly sizing the battery bank is important to the operation of the solar system. Often I will see designs were the battery bank is made too small and as a result the system will fail. The batteries store all the energy for the system without this storage your load loses the ability to operate when you want it to rather than when the sun is shining.
The basic calculation for this step is: Calculation 1 (Load WH/day) / Base Battery Voltage = (Load AH/day) Calculation 2 (Load AH/day) x (Days of Autonomy) / (Max DOD) / (Cold Temp Factor) = (AH Battery Bank Size Needed) Base Battery Voltage The base battery voltage is the nominal battery bank voltage, 12V, 24V or 48V.
While there are many more possibilities of battery voltages if you use a different voltage you lose the ability to use standard off the shelf charge controllers and inverters. The higher the voltage the lower the current. Lower current and higher voltages will result in less voltage loss and typically a less expensive solar system.
50A x 12V = 600W 50A x 24V = 1200W 50A x 48V = 2400W
Here are some general rules to help you select the best base battery voltage for your off grid solar electric system. If you are powering AC loads on large inverters 4000W and above, use a 48V battery bank to eliminate voltage loss. Small AC Loads or just a few hundred watts, 12V might be best.
- As you design the system, start with 12V and if you find the result is a minimum of two batteries then you have the option of 24V.
- But if the results are for a single standard battery size, stay at 12V.
- Mid size loads 1000W to 2500W, 24V is typical as you can easily find inverters in this size range at these wattage ratings.
In this example lets use 24V so I can also talk about series and parallel connections of the batteries. Days of Autonomy The days of autonomy is the number of days the load can operate without any charging from the sun.This is an important number because if you have bad weather today, you still need to make sure you have power for tomorrow stored in the battery.
Consider a week of solid rain, will your system contine to operate? The number of days used is highly dependant on the location of the system. Also consider if you have an alternative way to charge the batteries. For example if you have a cabin equipped with a generator than 2 or 3 days of battery autonomy is plenty.
In general if you have a low critical load that can fail, 2 or 3 days of autonomy is ok. My default is 5 days of battery autonomy as I find at 5 days produces a very reliable system design and allows the batteries to have a nice long life as they are not cycled too deeply.
If you have a critical load that can’t fail increase the days of autonomy to 8 to 10 days. Max DOD Max DOD stands for Maximum Depth of discharge. As you discharge and recharge a battery this is referred to as 1 cycle. The deep you discharge a battery the less life it will have and the life is referred to as cycles.
For example (based on a specific battery data sheet I’m looking at for this write up, your battery will be different) if you discharge a battery just 10%, leaving 90% of the power in the battery, it will last 5000 cycles. If this happens every day this equals (5000 cycles / 365 cycles/year) 13.7 years.
But is you discharge the same battery 50% every day you will see a life of just 1250 cycles or 3.4 years. For solar, without using computer simulation software it is hard to estimate the average cycle depth of discharge. In general what I’ve found is that a good quality battery with 5 days autonomy and an 80% max depth of discharge will last 5 to 8 years.
For this calculation 80% depth of discharge is considered dead for a battery and you will find the battery voltage approximately 10.5 volts. In this calculation I would use 80% Max DOD or 0.80 in the calculation. If you want a longer life from the battery use a smaller number.
80F = 1.0 60F = 0.95 40F = 0.88 32F = 0.80 20F = 0.77 0F = 0.60 -20F = 0.40
Below -20F you need to find a specific chart for your battery. Most batteries will operate from -40F to 160F. If the battery operates above 100F it will lead to a shorter life. In this example, let’s use 0.80. Putting it all together: Calculation 1 (Load WH/day) / Base Battery Voltage = (Load AH/day) 985 WH/day / 24V = 41.04 AH/D @ 24V.
Calculation 2 (Load AH/day) x (Days of Autonomy) / (Max DOD) / (Cold Temp Factor) = (AH Battery Bank Size Needed) (41.04 AH/D) x (5 Days of Autonomy) / (0.80 Max DOD) / (0.80 Cold Temp Factor) = (320.6 AH @ 24V of Battery Needed ) Let’s say the battery you selected is 120 AH @ 12V and 100HR Discharge Rate you will need 320.6 AH / 120 AH = 2.67 batteries.
Round this to 3 batteries in parallel. Because this is a 12V battery and you have a 24V battery bank, multiply the 3 batteries in parallel by 2 to create a 24V battery bank that is rated 360 AH and used 6 batteries total. As a side note, the 100HR discharge rate has to do with how fast you discharge the battery.
The quicker you discharge the battery the less AH you will receive from it. Most batteries are generically rated at the 20HR discharge rate but when you look at the datasheet you can find a more solar friendly rating. In this case we have 360 AH of battery and are only using 41.4 AH/D of load.360 AH / 41.4 AH/D X 24 H/D = a discharge rate of 208 HR.
This far exceeds the 100HR rating of the batteries.
Contents
How big should my solar battery be?
The average household uses between 8-10 kWh of electricity per day. Home storage batteries start at around 2.5-5 kWh in capacity for small systems, up to the larger systems which offer around 13-15 kWh of energy storage.
What size solar panel do I need to charge a 200Ah lithium battery?
Summary –
- You need around 300-600 watts of solar panels to charge common 24V lithium battery sizes from 100% depth of discharge in 5 peak sun hours with an MPPT charge controller.
- You need around 200-450 watts of solar panels to charge common 24V lead acid battery sizes from 50% depth of discharge in 5 peak sun hours with an MPPT charge controller.
What size solar panel do I need to charge a 100Ah lithium battery?
Summary. You need a 5-10 watt solar panel to maintain a 100Ah lithium battery.
How long will a 100Ah solar battery last?
100Ah batteries are quite big. They can be used for RV, as solar batteries, or even car batteries. You can imagine that one of the most frequent questions regarding the 100 amp hours batteries is this one: “How long will a 100Ah battery last?” This can be quite easily calculated if you understand the basic electric power law: Power (W) = Current (I) × Voltage (V) A 100Ah battery can last anywhere from 120 hours (running a 10W appliance) to 36 minutes (running a 2,000W appliance).100Ah 12V battery has a capacity of 1.2 kWh; that’s more than 2% of the capacity of the Tesla Model 3 car battery.
You can check here how long does charging Tesla cars with much bigger batteries last here, As you can see, how long will a 100 amp hour battery last depends primarily on how powerful the appliance you’re running. To fully answer how long will a 100Ah battery last, we will first look at how much capacity (or juice; in terms of Wh or Watt-hours) 100Ah 12V battery has.
We will also illustrate how you can calculate how long will a 100Ah battery run any appliance. On top of that, you will also find a ‘ 100Ah Battery Life Calculator ‘ further on that makes all these calculations effortless. 
Screenshot of the calculator: You just insert wattage and the calculator returns how many hours will a 100Ah battery last. You can use the calculator yourself further on. Near the end, we also solve two real-life examples just to illustrate how you can use the calculator. These are:
- Example 1: How long will a 100Ah battery run an appliance that requires 400W?
- Example 2: How long will a 100Ah battery run an appliance that requires 100W?
The goal here is that once you finish this article, you will be able to determine how long will a 100Ah battery last yourself for any appliance. Let’s start with looking at how much juice the 100Ah battery has:
Can I connect 200Ah with 100Ah battery?
Yes, you can for as long as the internal resistance of each battery contributes to a drooping, Voltage/battery current characteristics so that the voltage of each battery will settle to 12 volts.
How long will a fridge run on a 100Ah battery?
How do you work out how long a 100Ah battery will run a fridge? In this post I look at the question: How Long will a 100 amp-hour battery run a fridge? Like many complex questions, the answer depends on these smaller questions:
- How much energy does the fridge consume?
- What kind of battery is used, regular or deep-cycle?
- What battery technology, lead-acid or lithium?
A 100Ah lead-acid deep-cycle battery will run a fridge using 630kWh/year for 13.3 hours.80% discharge is assumed, but at the recommended DoD of 50% the same 100Ah battery will run the fridge for 8.3 hours. A lithium iron phosphate 100Ah battery will run the fridge for 15.8 hours at 95% discharge.
Fridges are a special case. Like heat-pumps, freezers and air-conditioners, they have compressors with motors which start and stop. When an electrical motor starts up it draws a surge current from the supply 3 times greater than normal running current. At other times the motor is not running, and sometimes it runs.
This means the power needed by a fridge changes over time, unlike a purely resistive load. For this reason it’s difficult to say exactly what power a fridge takes. The easiest way to work out how long a battery will run a fridge is to calculate the fridge energy required over time in kilowatt-hours (kWh).
How do you calculate how many solar panels and batteries do I need?
Sizing solar panels, batteries and inverter for a solar system – A true off-grid solar power system includes solar panels, a bank of batteries for energy storage and one or more inverters. This kind of system has no connection to the utility grid. It is possible to have home battery storage, even when normally using the utility company’s grid connection.
- Size the solar panels according to energy consumption
- Size the inverter according to the solar panel system power rating
- Size the battery bank according to how many hours you need it to run i.e. autonomy
Solar panel size is found by dividing daily load kWh by the location’s irradiance to give solar kW rating. Inverter size is equal to solar panel rating. Battery size is found by multiplying the daily load by the number of days autonomy required, and dividing by system volts to give amp-hours.
How many watts solar do I need to charge a 12V 200Ah battery?
How many solar panels are needed to charge 4 numbers of 12V, 200Ah batteries? – To charge 4 numbers of 12V, 200Ah battery in 5 hours of sunshine you will require a minimum of 8 numbers of 325 Watt of solar panels with MPPT based charge controller and seasonal structure.
