First off, and I declare this openly, I don’t speak electricity.
Oh, I’ve tried.
But it feels like power, current, resistance, voltage, wattage, amperage, charge and time are all a giant conspiracy to make me sound like I just said “I like your mother for bedtime chocolate” in Norwegian.
I can’t tell you how many times I’ve done the “But I don’t understand – why can’t you tell me how long this 12 volt battery will last in my home?” question.
And the response I usually get is *eyes*
It’s VERY frustrating, but apparently, that’s the electrical equivalent of me asking:
“How much money can I spend on my credit card if I swipe it three times a day?”
It’s not a very good question. Some might even call it a bad question.
But even if I sometimes ask bad questions – I know enough about batteries and UPS/home-inverters to know that Elon Musk has just presented mutton as lamb.
For three reasons:
- The powerwall/powerpack/whatever that everyone is so excited about? It doesn’t come with a DC-to-AC inverter, nor does it come with installation included, nor those efficient solar panels that are meant to power it. It is just a prettily-packaged battery that needs lots of other expensive stuff before one can use it in one’s home. But it is very pretty, agreed.
- But also, it is not an especially excellent battery. It is not a fruit of new technology, nor of battery progress, nor of any other such thing. It is just a big lithium battery, packaged to be less “unattractive”.
- For the price in question, you can do better with this giant of a gas generator.
So for all the South Africans that are busily sharing the “good news” on Facebook about Tesla batteries being available at some point in 2016, I thought I’d try to explain why you shouldn’t wait until 2016 to get a battery system in your home.
All the W/V/A/VA/Ah/Wh/kWh/MWh/AARGH/F-ing symbols
Having spent some time with the internet, a green pen and a sheet of paper, here are the important things that I have learned (rightly, hopefully):
- When I need to power a device or a lightbulb, then the power that I’m looking for is measured in Watts. And because we’re into shorthand, we tend to talk about kiloWatts (kW), which are 1,000 watts each. And Eskom likes to talk about MegaWatt (MW) when they’re describing their loadshedding plans – and each MW is 1,000,000 Watts.
- The term “Watts” translates into “how much power I need to power something every second”. A 100 watt globe requires 100 watts of power to power it this second, and next second, and so on until you flip the switch.
- If you want to know about what you need over a period of time, then you take the required Watts, and you multiply them by your required hours, and you get Watt-hours (Wh)*.
*And again, because of the shorthand, we tend to talk about kiloWatt hours (kWh), which are 1,000 watt hours each.
- So if I wanted to have the 100 watt globe lighting my dining room for an hour, I’d be asking for 100 Wh – or 0.1kWh.
- People look at me at this point like it should have been so painfully obvious*.
*In my defence, the spoken word doesn’t distinguish between Watt-hours and “What hours?” – so I’ve always had questions, myself. After all, it’s not like I did physics at school or anything. *hangs head in shame*
- Annoyingly, Watts, kiloWatts and Watt-hours do not feature anywhere on the side of any battery or inverter unit that I have ever purchased.
- Admittedly, I’ve only ever purchased one inverter unit.
- But still.
- What I see on the side of the box are volts, amperes, and occasionally, a VA number (just in case I wasn’t confused enough already).
- It seems that there are a set of equations that engineers use that look a lot like various versions of: Watts (power) = Amperage (the electric current) x Voltage (electric potential/strength)
- So if I want to power a 60 watt bulb with a 12 volt battery, then I need to draw a 5 amp electrical current from said battery.
- Sometimes, people and the sides of inverter boxes talk interchangeably between Watts (W) and Volt-Amperes (VA). The first is a measure of actual power, the second measures apparent power; and sometimes, those are the same thing.
- When you’re talking about Direct Current (what comes out of a battery), 1 W ≈ 1 VA.
- But when you’re talking about Alternating Current (what runs your home), you’d apply a power factor to that (because apparent power isn’t quite equal to the actual power that you’ll get out). So, say: 0.9W of actual power for every 1 VA of apparent power. Because, um, “reactance” or something.
- Then also, for my sins, there is yet another symbol (Ah) that you find on the battery, which tells you how many ampere hours the battery will hypothetically deliver from full charge. And it’s hypothetical because the battery will rarely discharge down to zero – it’ll just stop giving you current at about 30% of its capacity (or so I understand).
So let me give you a real life example*.
*And bear with me – I am getting to Mr Musk’s battery life.
Behold my inverter:
- I have an inverter that says it’s a 24 volt system, and makes mention of 1,400 VA of apparent power.
- I have two 12 volt batteries, each capable of 200Ah of charge.
- So the system collectively involves 24 volts of electric potential, 1,400 VA of power, and 200Ah of charge.
What that means, in comparison to the Tesla Powerwall:
- Each battery can deliver 2.4 kWh of electricity.*
*12 volts x 200Ah = 2,400 Wh = 2.4 kWh
- By combining the two batteries together in a relay circuit (or whatever kind of circuit one uses to do such things), I can get 4.8 kWh of electricity out.
- Which you can compare to the entry-level 7 kWh home battery that Tesla presented to the world.
- Meaning that I’m one more battery away from matching Elon Musk’s one.
- Sure, mine aren’t as pretty. But each battery cost me about $275. If I were to buy three, then that would cost me $825. Which is a FAR CRY from the $3,000 Tesla Powerwall.
- That said: Tesla’s Powerwall is a Lithium ion battery (like a cellphone battery), while the ones I’m using are Lead Acid batteries (basically, truck batteries). So the Powerwall will last a bit longer and charge a bit quicker – even if using a giant iPhone battery to power your bedside light does seem a little…extravagant.
But knowing that a battery can give you 2.4 kWh isn’t that helpful for, like, real life. So here is what I am told that means, approximately*:
*Things are complicated by the fact that a battery loses voltage as it discharges, which drops the power output over time. Hence my use of “approximately” all over the place.
- A 2.4 kWh battery can deliver 2.4 kW for one hour, and then be completely drained.
- Or, a 2.4 kWh battery could deliver 1.2 kW for two hours, and then be completely drained.
- Or, a 2.4 kWh battery could deliver 0.8 kW for three hours, and then be completely drained.
- Or, a 2.4 kWh battery could deliver 0.6kW for four hours, and then be completely drained.
- Or, a 2.4 kWh battery could power my 100 W lightbulb for 24 hours, and then be completely drained.
- Or it could power two 100W lightbulbs for 12 hours.
- Or it could power three 100W lightbulbs for 8 hours.
- Which is exactly why the installation of an inverter means that you dash out and buy those 2 Watt LED light bulbs – because if all your lightbulbs were originally 100W globes, then you just increased your battery time by a factor of 50.
So far, so good. Only, I haven’t made mention yet of that 1,400 VA on the side of my inverter box.
And what that means: the inverter has a limit to how much power it can deliver (or convert from DC to AC) at any given point in time.
Given that 1,400VA limit, and the fact that you can only get about 90% of that in actual Watts, what we’re saying is that my inverter can convert a maximum of about 1.26 kW* of power in one go before it blows.
*1,400 VA x 0.9 = 1,260 W = 1.26 kW
Let me rephrase that: I can draw a maximum of 1,260 Watts for about 3 and a half hours before my batteries run completely dead.
The next question: what will 1,260 Watts get me?
Well here’s a list of appliances and such:
And here is a great screencap from the Tesla Powerwall homepage itself:
1,260 Watts will give me computer usage, and some lights for a bit. And maybe a fridge, if it’s a new energy efficient fridge and I don’t run everything all at once.
Which, when you think about it – that’s all you really need.
Because for Mr Musk’s batteries, you’d still need an inverter. The largest one that I managed to find was a 10 kW one (see here), which would let you draw the full 7kW out of that battery in an hour. And in that hour, you’d do what – run your geyser?
Seems like a bit of a waste.
And if you really wanted to run that geyser, you’d buy the 16 kW generator that costs a bit more than the Powerwall, but does oh-so-much more.