Sizing power stations / inverter trolley solutions

What is a power station / inverter trolley / inverter battery box/ solar generator?

It is a portable enclosure that typically houses a battery, an inverter, and a charger.

It looks like loadshedding is here to stay and you may have decided that you would like a portable solution that would be able to supply power to some of your electrical appliances like a TV, a fan, a computer, a printer or a desk lamp etc. during a power outage. A power station is one potential solution. 

Not all power stations are created equally. There are better types of batteries, inverters and chargers and there are worse kinds of batteries, inverters and chargers.

What you should know:

Batteries:

Typically, three types of batteries are commonly used in power stations:

Lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LiFePO4/LFP) or lead acid (Flooded/SLA/AGM/Gel). Each battery chemistry has pros and cons, but one major difference is the amount of charge cycles that are supported by each type of battery.

Charge cycles refers to the process of discharging and then recharging a battery. A full cycle means discharging the battery from 100% to 0% and then charging it back up to 100%. Batteries are rated to support a certain number of charge cycles.

LFP batteries are currently the best option because they can typically do 2000-3000 charge cycles, where NCM does around 800-1000 cycles and Lead acid does around 150-200 cycles. 

LFP are also often marketed as “lead acid replacements” as their voltage is similar to lead acid.

When considering the life expectancy of a battery in terms of cycles, it’s important to keep in mind that the depth of discharge can have an impact. The less deeply a battery is discharged, the more cycles can be expected from it. To ensure a longer lifespan, it’s a good idea to select a power station with more watt hours of available power than what you typically use, so that you avoid completely depleting it before recharging. With lithium batteries, it’s recommended to aim for having around 20% battery left after use. While it’s also good practice not to charge the battery all the way to 100% (lead acid needs to be charged to 100%) this may be inconvenient for most users, and it’s not discussed here. This advice is especially relevant to NCM batteries, as LFP batteries have a significantly higher number of cycles.

Generally, lead acid batteries can only be discharged to 50% before permanent damage occurs. Lead acid batteries also take a long time to charge so you might not have enough time between sessions to fully recharge again.

Because of this, the weight, the very limited cycles and long charging time, I would avoid lead acid batteries if your budget allows it.

Inverters:

An inverter is an electronic device that converts direct current (DC) electricity into alternating current (AC) electricity like the electricity from your wall plug. The primary function of an inverter is to allow electrical devices that require AC power to be used with DC power sources, such as batteries. There are pure sine wave inverters and modified sine wave inverters on the market. Pure sine wave inverters are superior because they provide the same type of sine wave that you get from your wall socket. A modified sine wave will be bad for some electronics, and it won’t work well for some appliances like a fan. I recommend that you go for a solution that has a pure sine wave inverter. 

Chargers:

Once the power comes back on, the batteries inside of the power station will need to be recharged, this is done via a built-in charger which takes AC electricity from your wall and supplies it as DC power to your battery in order to charge it. Chargers are usually rated in Volts(V) and Amps(A). a 12v, 10a charger will provide 120w of power (Volts x amps = watts). The more watts that the charger is able output, the faster it would be able to charge the internal batteries. Check the charging speed of the power station that you are looking at and make sure that it meets your needs. Some power stations will recharge from 0% to 100% in just over an hour, while others may take more than 8 hours.

Some power stations can also charge via solar panels. If you want to use this feature you will have to look at the specifications of the power station as well as the solar panels to make sure that the solar panel is compatible and to see how fast you would be able to charge via solar, so that it meets your requirement.

Understand your needs. How much power? How long? 

Now that you know some of the basics, the next step is to understand how much power you need and for how long you would need it for. One of the easiest ways to find out how much power you need is to get a device that is commonly referred to as a “Kill a watt” digital watt meter.

Link to buy a Kill a watt meter from Geewiz here.

This is a device that you plug into your wall and then you plug your appliance into it, you then leave it plugged in for the time that you would like it to run (e.g. 2 hours) and at the end of the time that you would like it to run for, it will tell you how many watts per hour (Kilowatt hour/ kWh) that appliance drew as well as the peak/surge amount of watts that it drew. Multiply the killowat reading by 1000 to get to watt hour (Wh).

Regarding peak or surge usage wattage: Some appliances will draw a larger amount of power for a short time when it starts up, the inverter inside would need to be able to cope with the peak/surge requirement otherwise it would overload the system and it would trip the overload protection.

Overestimate your usage. There will be losses.

Inverters and batteries are not 100% efficient and there will be losses when you convert the DC power inside your battery into AC power for your appliances. There are also other components inside of the power station that will use some of the power that is stored in the battery.

Say that you have a power station that is rated for 1000Wh (1kWh), you may find that you are only able to get 700Wh usable power from the power station. If you only get 700Wh from your 1000Wh power station the power station has an efficiency of 70%.

Generally power stations have an efficiency of between 70% and 90%. So, assume that you’ll get between 70% and 90% of the rated capacity in terms of real-world usage.

Remember that for the sake of the battery’s longevity, it is also recommended not to drain it to 0% and ideally one should aim to have around 20% left after you have used it (not applicable to Lead acid).

Also oversize the inverter. If the power station says that it can provide 500w of power, it would be bad to constantly run it at the 500W limit for long periods of time because this will stress the inverter and it will probably shorten its life.

Some examples:

My freezer.

If I would like to run my freezer for 5 hours from a power station in case of an emergency.

My watt meter tells me that my freezer draws 70Wh on average, but when the compressor kicks in it surges to 650W for a second or two. 

I would need at least 350Wh (70w x 5 hours) of usable power. I would look for a power station that can supply roughly 500Wh to be safe. To get 350Wh out of a 500Wh power station it would need to have an efficiency of 70% or better. I would also need to make sure that it can output around 100W of power constantly and that it supports a surge output of at least 650W.

My TV.

Let’s say that I would like to run my TV for 4 hours. I plug it into the watt meter and set a timer for 4 hours. At the end of the 4 hours, I check to see the usage (You could also leave it plugged in for just 1 hour and multiply the result by 4 to get the usage for 4 hours.).

The watt meter tells me that it used 308Wh in 4 hours. This tells me that I used 77Wh per hour (308 / 4). It also tells me that the peak amount of power that my TV drew was 78W.

I’ll need to look for a power station that has 440Wh or more (assuming 70% efficiency). I would also need to make sure that it can output around 100W-200W of power constantly.

A hairdryer.

My wife would like to be able to use the hairdryer on occasion during a power outage. This is a very hard use case for a portable power station because producing a lot of heat requires a very high amount of wattage. Her hairdryer uses between 1500W – 2000W (low to high setting) and she would like to use it for 5-10 minutes. We would need to look for a power station that can supply around 1500W- 2200W of power constantly and has a capacity of around 240Wh – 500Wh. In this scenario it would be hard to find something with only 500Wh that outputs 2000W. As an example the Ecoflow Delta 2 can output 1800W (2200W with Xboost) and has a capacity of 1024Wh and while it may not be able to power the hairdryer at full speed it would still work good enough with Xboost turned on, in the Ecoflow app.