Uninterrupted and quiet power is crucial if you work remotely. The flexibility that working from home offers you means you can incorporate naps, late-night and wee hours into your work cycle. It’s good that you’re either considering or sticking to an inverter setup for backup power, but the jargon can feel overwhelming and you end up with either an underpowered or overly expensive setup.
My objective is to demystify the process so you end up with just the right setup for you.
Inverter
An inverter converts battery-stored DC power into AC power, which your household appliances use. It’s rated in kVA. The size of your inverter (Watts) determines how many appliances you can run at once. To calculate this, you multiply your kVA rating by the power factor (often 0.8).
A 3.5 kVA inverter, for example, would handle 3.5 * 0.8, which is 2.8kW or 2,800 Watts of load. It can handle a little above this load, but for a very short duration. Recall that devices often have a startup voltage that is higher than the operating voltage. Because the startup phase lasts only a few seconds, your inverter would supply the necessary power during this surge as long as it doesn’t exceed the inverter’s peak power supply.
What kVA rating do I need?
Three things you should take note of are:
- The wattage of the heaviest appliances you wish to power
- How your appliances are grouped, i.e., which appliances need to run at the same time
- The starting Wattage
Once you have the answers to these, you divide the total wattage rating by 0.8 to get your ideal kVA rating. For example:
Lets say I have the following applicances:
- 1 Ceiling fan (80W)
- 2 standing fans (70W)
- 1 Laptop (45W)
- 1 TV (90W)
- 1 Deep freezer (160W)
- 5 10W bulbs (50W)
- 1 Washing machine (600W)
The heaviest appliance is my washing machine at 600W. If I wish to power all at once, then the total load is 1095W. I could, on a tight budget, opt for a 1.5kVA inverter because 1.5kVA would be 1,200W, but I need to consider two things:
- Load on the inverter: If the rating is 1,200W and I’m constantly drawing 1,100W, then the inverter will constantly be under strain. You can tell this by the loud hum of the cooling fan.
- Starting voltage: If I turned on the inverter while all these appliances are plugged in, then I might get an overload warning, and the inverter could shut off. The deep freezer alone, for example, could draw as much as 480W to start.
For this reason, you might opt for a slightly higher kVA rating, say 2kVA, which could cost you more money.
Another approach is to peg the kVA rating around the heaviest appliance (600W in this case), meaning the inverter can power each appliance if they’re running alone. You could opt for a 1kVA or 1.2kVA inverter and stick to power-saving wash cycles. This means most appliances will be turned off while the washing machine is running. The idea is to group appliances such that the total load in Watts is always significantly lower than the inverter rating. This approach allows you to maximise your inverter while saving money.
What Voltage rating do I need?
Inverters also have voltage ratings, usually in multiples of 12, like 12V, 24V, 36V, 48V, etc. The Voltage rating tends to increase with the kVA rating. A 12V inverter, for example, would require at least one 12V battery. A 24V inverter would require at least two…and so on. You can connect several batteries for a larger battery bank, but the total voltage of your battery connection must match the inverter’s Voltage rating.
Why does the voltage rating matter?
Let’s say you have a battery bank of 200 amp-hours (Ah). This means you have 1 hour of power if you’re drawing 200A of current from it or 2 hours if you’re drawing 100A of current from it…and so on.
Now, let’s compare three identical inverters except for their Voltage ratings of 12V, 24V and 48V. Each, powering a load of 2000W.
Power (W) = Voltage (V) * Current (A)
For the 12V system, 2000W / 12V = 166.66A. This means that the 12V system would be drawing about 166.66A per hour from the 200Ah battery, so you have about 1 hour 12 minutes of power.
For the 24V system, 2000W / 24V = 83.33A. This means that the 24V system would be drawing about 83.33A per hour from the 200Ah battery, so you have about 2 hours and 24 minutes of power.
For the 48V system, 2000W / 24V = 41.66A. This means that the 48V system would be drawing about 41.66A per hour from the 200Ah battery, so you have about 4 hours and 48 minutes of power.
The higher the Voltage rating, the lower the Amps drawn from your battery to power the same load and the longer your uptime. On the flipside, the higher the Voltage rating of your inverter, the more expensive it gets.
A 12V system is cheap and ideal if you have minimal power needs (fans, lights, small TVs etc.) A 48V system is ideal if you’ll be powering heavy loads and want to get the most out of your battery, but it is quite expensive. A 24V system is a balance of cost and performance and is ideal for most homes.
Should I opt for a Transformer-based or Transformerless inverter?
Transformer-based inverters are also called low-frequency inverters, while Transformerless inverters are also called high-frequency inverters.
Transformer-based inverters are bulkier and more expensive than Transformerless inverters; this is for a reason. They include a large iron-core transformer to step up the DC voltage before converting it to AC. This provides galvanic isolation for safety(surge protection) and allows the inverter easily handle an inductive load, e.g., your water pump, fan, washing machine, refrigerator, etc. The trade-off here is that they’re less power efficient and consume more battery than Transformerless inverters.
Transformerless inverters, on the other hand, utilise electronic circuits for the DC-to-AC conversion and Surge Protective Devices (SPDs) for monitoring. They have been improving significantly over the years and are more efficient and cheaper than Transformer-based inverters.
If you’ll be running heavy, sensitive or inductive loads, then a Transformer-based is the way to go; otherwise, a Transformerless inverter with proper grounding and safety measures would do just fine.
Bonus Tip
It’s a good idea to oversize your inverter (i.e., get a kVA rating higher than what you currently need) because your power needs are very likely to grow once you start enjoying uninterrupted power. It’s cheaper to expand only your battery bank than have to upgrade your entire setup.