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Whether you are tied to the grid or off, having backup power is essential to run basic appliances. Many consider a 150ah battery as large enough. But is this sufficient as backup? And how long is it going to last?
A 12 150ah backup battery is going to last 5 to 6 hours before it runs out of power, provided the battery is fully charged and runs a load of 300 watts an hour. The lower the hourly watt load, the longer the battery will run.
How to Calculate 150ah Battery Backup Hours
The following uses 150ah batteries as examples. But you can apply these principles with any battery size. If you are looking for one, we strongly recommend the Eco Worthy 12V LiFEPO4.
Bottom line: add up the watts of each appliance you want to run. Use the formula below to check if the battery can last for the runtime you need.
Battery voltage x amp hours / hourly watt load = battery backup runtime
Suppose you need to load 400 watts on a 12V 150ah battery. How long before the battery runs out?
12 x 150 = 1800
1800 / 400 = 4.5
A 12V 150ah battery can run a 400 watt load for 4.5 hours before running out of energy.
Another example. You want to run a TV, Blu-ray, a fan and lights for a 3 hours. Will a 150ah be enough?
Assume the TV is 200 watts, the fan 100 watts, Blu-ray player 50 watts and the lights 100 watts. These are just examples as TV power consumption depends on size.
That is 450 watts total. Using the same calculations as above, the battery will be good for about 4 hours.
A 24V 150ah battery holds twice as many watts as a 12V. So you can load up to 3600 watts of appliances and the battery will last for 4.5 hours, same as a 12V.
You can connect two 12V batteries in a series to get 24V. This will not increase the amps though. To do that you have to connect the batteries in parallel. If one 150ah battery is not enough, add another using either a parallel or series configuration.
Biggest Factors That Affect 150ah Battery Runtime
The calculation steps are correct, but the runtimes are estimates. It is difficult to give an exact number for two reasons: batteries lose charge with heavy use, and the depth discharge varies.
Deep cycle batteries are rated by amps drawn. Typically this is 5 amps per 20 hours. So if a load pulls 5 amps per hour, the battery is good for 20 hours. But due to Peukert’s Law , a battery discharges faster when more amps are drawn.
A 150ah battery with a 1800 watt load should in theory, last an hour. But in reality the battery will run out of amps before that.
So if you are going to place a full load on the 150ah battery, do not expect it to last for an hour. Be conservative in your estimate and assume the battery runs out of juice before that.
This is not an exact science. 5 amps and the battery is good for 20 hours. But increase that to 10 or 20 amps and the battery discharge rate quickens. By how much will depend on other factors like the weather, internal circuitry, design and so on.
That is one factor. The other is battery depth discharge.
What is Battery Depth Discharge?
DOD is depth of discharge, which is the level when the battery has to be recharged. The DOD determines how much of the amp hours you can use with each charge.
If you have a lead acid 150ah battery, you should only use 50% of the capacity. Once the capacity drops to 75ah, you should recharge the battery.
You can run the battery down until it is empty. But doing that will wear the battery out quickly. It is better to buy another 150ah battery so you can increase the available backup power. Or you can buy a deep cycle lithium battery that has a 100% discharge rate.
Lithium batteries cost more, but you do get almost full usage. It really depends on your budget and how much power you need. Your inverter and solar power setup are also going to determine how much power is needed.
These are the two factors that affect battery capacity and runtime. But we also need to talk about inverters.
Inverter Watts Usage and Effect on Battery Life
If you are going to run any AC appliances on the battery, you need an inverter. Solar panels produce direct current and this must be turned into alternating current before it is passed onto appliances for use.
Inverters use watts even without any load. It is only a small amount though. The bigger issue is how much energy is lost during the DC to AC conversion.
The more energy lost during DC to AC conversion, the more watts an inverter will use apart from its load. An inverter labeled 85% efficient means 15% energy is lost, or 15% additional watts are consumed.
So apart from the depth discharge you have to consider the inverter efficiency too. If you have a 12V 150ah battery with a 50% DOD, there is 75ah amps available.
75ah is 900 watts. So if you have a 450 watt load, the battery will last two hours. But this does not account for inverter efficiency.
If you have an 85% efficient inverter, that 450 hourly watt load becomes 517 watts.
450 x 115% = 517
Spread over two hours the inverter consumes about 1034 watts, more than the 75ah can handle. So instead of two hours the battery will cease to work in less than that, maybe an hour and 45 minutes.
This rule applies to any battery inverter system no matter the manufacturer. So it is imperative that you allocate more battery power (increase the capacity) or reduce the appliance load.
Is a 150ah Battery Enough?
A 150ah battery is not enough to back up all appliances in house. But if you only need a few -and for a limited time – it might be sufficient.
Assuming you have two 12V 150ah batteries, you have 3600 watts in total. 1800 watts is usable if the battery has a 50% depth discharge rate. 1800 watts is enough to run a typical laptop, a chest freezer, TV, lights and fans for several hours. The same battery can run:
- A coffee maker for about 2 hours
- A garbage disposal for 2 to 3 hours
- A dishwasher for an hour
- Clothes washer for 2 and half hours
- Dehumidifier for 2 hours (to run on solar panels, check this guide)
- Furnace fan for 2 hours
- Electric heater for 1 hour
These runtimes are estimates based on typical power consumption. Your appliances may use more power depending on what settings you have chosen.
To find out if 150ah is sufficient, list all the appliances you will use and for how long. Make allowances for inverter efficiency issues and drops in voltage. Round off the results for easier computations.
How to Make a 150ah Battery Backup Last Longer
Do not discharge lead acid batteries below 50%.
Do not top off batteries at 100%. 85% to 95% is acceptable.
Buy the highest efficiency rated inverter you can afford.
Do not load the inverter at maximum capacity. Leave some power as reserve.
Perform a proper maintenance for the battery.
Make sure the battery is installed correctly and in a well ventilated spot.
Not all batteries are made equal. Two 150ah units might perform differently depending on the design quality. This is why it is important that you buy only from well known brands.
There will be times when you might have to use a lead acid battery beyond its 50% DOD rate. It is all right provided this does not happen often. If you find yourself using more than 50% frequently, it is time to upgrade to a higher capacity battery.
The same applies to your inverter. If you need to run a lot of appliances, a 1000 watt inverter may be insufficient. Consider doubling the capacity and adding more battery capacity along with it.
As with all battery backup systems, it comes down to what you need. Power requirements vary so consider only what appliances you have to use. Having enough reserve power in case of emergency is also recommended.