When installing batteries to solar systems or RV, we all hear the unit of amp-hours. You probably know it as a unit of capacity, but this is not entirely true. We cannot conclude the capacity of a battery based on a single parameter.
Keep reading, we will take 200Ah battery as example to explain but how does battery capacity relate to the number of hours of electricity used by household appliances and how to predict how long a battery will last meeting your home's electricity needs by its capacity.
Before giving an example to calculate the length of time the battery will work, It is also worth emphasizing that the type of battery is also an important factor that affects the working time of the battery.
There are two main kinds of solar batteries in this size range are lead acid and lithium phosphate. Each type has different characteristics, advantages and disadvantages.
Lithium batteries are more expensive, but they have several advantages, the biggest being that they are intrinsically deep-cycle.
They can be discharged to a much greater depth than lead-acid deep-cycle. This means that you can get more power out of it for the same rated capacity.
Most LiFeP04 batteries can be discharged 95%, but 80% is recommended to maximize battery life.
A 200Ah lithium (LiFeP04) battery connected to a 400 watt DC load with 80% Depth of Discharge will last for almost 5 hours. A 200Ah lithium phosphate (LiFeP04) will run a 400W rated fridge for about 55 hours at a rate of 40 watts per hour.
So, there are many aspects to consider when you want to buy a backup battery that can supply power for a long time.
Powmr's 48V 200Ah lithium battery is a good choice, although the depth of discharge does not reach the highest value, but this also means that it has a longer life and is also more efficient than lead-acid batteries.
Ampere-hours (abbreviation: Ah) is obvious from the name, it is "Current x Time". According to Charge Conservation, Ah can be interpreted as the amount of lithium ions available for charging and discharging in a lithium battery during a certain period of time.
Usually, the voltage and ampere-hours together indicate the capacity of the battery, such as 12V200AH, 12V100AH. The same voltage of the battery, the higher the amperage, the larger the capacity. Therefore, the battery life cannot be predicted simply by the Ampere-hours.
Before the explanation on how long will a 200Ah battery run, it’s necessary to clear one thing up: There is no way to calculate the running time of battery precisely, because it is influenced by many factors.
In general, the run time of a 200Ah battery depends on the battery chemistry and the power of the load connected to it. Battery temperature and discharge rate also impacts how long a battery will last, but it’s the load that drives it all.
In an ideal situation, to calculate how long a battery can supply power, we only need to know another three parameters, in addition to ampere-hours.
Battery voltage
The voltage in a battery refers to the potential difference between the positive and negative terminals of the battery. But the battery voltage will gradually decrease as the power passes, so that's why I said earlier that there is no way to accurately calculate the range of a 200Ah battery. However, here we still use the nominal voltage of the battery for calculation.
Battery depth of discharge (DoD)
Discharge depth is the ratio of the power released from the battery to the rated capacity of the battery.
For example, the rated capacity of the battery is 200Ah, the capacity remaining after discharge is 100Ah, the actual discharge capacity is 100Ah, then the depth of discharge of the battery is 50%.
Usually, the battery with the depth of discharge from 10% to 80% is regarded as shallow cycle discharge battery, and another with the 80% depth of discharge is deep cycle discharge battery.
Wattage of the loads
The higher the power of the loads, the higher the power consumption, which reduces the time that the battery power to be consumed. So, for a same battery, the lower the power of the load connected to it, the longer the battery will last.
Now, we can use these 3 metrics to calculate how long will a 200Ah battery last. Here is the 200Ah battery running time formula that we will use:
200Ah Battery Running Time = 200Ah × Voltage × DoD / Device Wattage
There are two basic types of loads:
DC, such as supplied by solar panels and solar batteries
AC, such as the power supplied by utility companies
In general, a 200Ah battery will run an AC load for less time than a DC load, simply because an inverter is needed to convert DC to AC.
The best quality inverters are about 95% efficient, so you lose 5% running time as extra power is needed to power the inverter.
A 200Ah lead-acid deep-cycle battery should only be discharged 50% in order to maximize battery life, so only 100Ah is available for use. (80% can be used now and again.)
100Ah converts to 1200 watt-hours. Dividing this by 400 watts load gives us the answer:
A 200Ah lead-acid deep-cycle battery with 50% Depth of Discharge will run a 400W AC appliance for about 3 hours.
If the load is DC, such as might be found on boats or RVs, then the calculation is pretty straight-forward:
200Ah = 200 x 12 volt = 2400 watt-hours
However, only 1200 watt-hours is available at the recommended 50% discharge so:
Run time for 200Ah battery running 200 watts DC = 1200 watt-hours/200 = 6 hours
For an AC load, it’s a little bit different. Let’s assume it’s a constant load and not a fridge, heat pump or air conditioner with compressor motors on board.
AC appliances use inverters which also need power to run i.e. they have losses. If running at full rated capacity, an inverter may have an efficiency of 95%, if it’s high quality.
The calculation becomes:
Run time for AC load = 1200 watt-hours/200 x 0.95 = 5.7 hours