Hi again! I’d like to preface this post by writing, as I sit on the bed, that Freddie is providing assistance and that his and my training in remote power systems are exactly the same. Anything written here is drawn from our experience and reading and nowhere else. In other words neither me nor the cat are qualified experts, rather our knowledge is applicable to us and may or may not be to your situation. Now we have the obligatory rider out of the way, let’s get on with the post.
Figuring out what you need for off grid solar hurts! If you can get five different designers to call you back you’ll probably get five different approaches!
Don’t panic, the key to figuring out what should work is figuring out what you actually need. Think carefully about what you can actually do without and what’s vital for you. Some things to bear in mind here;
- Peukert’s law – the amount your batteries discharge does NOT have a linear relationship with the load placed on them. High loads cause batteries to discharge disproportionately fast. Long light load = good, high instantaneous load = bad. Toasters, non-heatpump clothes dryers, sandwich presses, high load electric motors like pressure pumps are all problematic.
- Can you shift around the times for what you do day to day? e.g. Can you do clothes washing during the day when your panels are maxing? Or can you have showers in the early evening?
- Can you find ways to do your night time activities that use no electricity? For example exhaust fans chew up quite a lot of power if used for long periods – could you use mechanical ventilation instead?
Don’t get crazy and think you can happily live like you’re in a cave! Just look for alternatives. For example can you use a 6 star fridge? Will your TV, if you are having one, be super efficient? Can you live with a little stove top toaster rather than an electrical unit? Can you budget to use an induction water pump rather than a conventional water pump (that actually makes a huge difference!!)
We’ll cover some of the solutions we’ve come up with in greater detail in future posts, but for now have a think about what you might need. If you click on the image below you can download an Excel spreadsheet that will help you do some calculations.You can use this one and add in whatever you need or make your own using the ideas – whichever
Just some explanations first. If you are pretty familiar with electricity and units of load then skip on…if not…
Everything electrical draws load to do it’s work. The amount of load (for whatever appliance we’re talking about) draws at any moment can be measured in watts. Don’t get too hung up on how watts (w) or wattage is calculated because pretty much everything electrical these days has a published value for its draw in watts (or Kilowatts i.e. 1000 watts). A hairdryer might draw at any one moment 1000 watts (=1 Kw) or more of electricity and that’s quite an astounding amount really! Equivalent to running fifty 20w globes at once.
The difference being, presumably, you don’t run your hairdryer for hours on end(??) but the same is not going to be true for your lights. We’ve seen from Peukert’s Law that load matters a lot and therefore hairdryers are bad, but that’s not the whole story. Load over time is also very important. If you doubt that try doing a quick plank position…easy huh? Now try it for an hour…not so easy! So the watts per hour is one of the key factors used to determine overall draw on your batteries and solar cells. That is to say the instantaneous wattage draw of the appliance times the number of hours it runs for. This is called KwH and it’s how your on grid electricity is billed.
To use the spreadsheet you’ll need to know the wattage draw for each electrical appliance in your house and how long you are likely to use it for. In the sample one we’ve supplied, we’ve doubled or more the projected usage times for each appliance. Why? Because it’s our only source of power. There’s no grid to rely on so we’ve chosen to work with some figures with pretty large safety margins. The cats might not care, but the humans don’t want to live in the dark…ever! Now you’ll also see it asks you to divide night and day usage. We’ll talk more about sizing your solar panels later but suffice to say if you’ve got a decent sized array your daytime usage is mostly irrelevant (Super high loads as the exception). Ah but what about cloudy days you say? Well our experience with our current solar equipped on grid house with 4.6Kw of panels facing North is that they never really drop much below 17% output unless the weather is really stormy. So if you have 10Kw of panels sitting on your roof, angled ideally, you’re probably still going to be getting better than 1Kw of output. If you are careful (and smart) you can probably run your house happily off that during the daytime. So long as you don’t decide that’s the moment to vacuum, do the washing or dry that hair you’ll be OK. You could certainly run a laptop, a router and a few lights from that. Again, to some extent it’s about adapting your life style to availability.
In summer, spring and autumn if your panels are orientated correctly and you have 10Kw on the roof then go mad! You’re probably going to be producing much more electricity than you can use – you could probably vacuum the washing machine with a hairdryer…or maybe not. The night time is the key because that is the amount of energy you are going to have save up over and above what you use during the day. In winter, in the the South, that’s just not always going to be possible unless you are particularly frugal. So we’ve opted for a diesel back up generator. We don’t expect we’ll need it very often and it should return the batteries to charge in around 2 hours and be fully automatic. We’d expect it will run less than 20 hours a year. So if you think about the carbon output of 20 hours of generator use next to 8736 hours of coal fired on grid electricity it’s negligible.
Now you can see from the example provided the daily requirement is just under 9 KwH of electricity. In reality it will closer to 5 kwH or less. In our current horribly inefficient old weather board home, even in the depths of winter, with a way less than optimal solar citing, we only use 3KwH more electricity than we generate. Our planned array on Langi Billin is 8Kw and even if it only sees 2 hours of usable sunlight a day that will way more more than satisfy our daily needs. Which brings us to autonomy. i.e. How many days of reserve power do you need? The generally accepted figure is 3 days but this a little misleading. It assumes a. that you incapable of making behavioural changes to accommodate the shortage of solar generation and that b. your panel output will drop to zero. I’d argue that needing 27KwH hours of reserve battery power (9KwH of daily demand x 3) is excessive and that the strain on environmental systems of incidental generator use is lower than adding on more and more battery packs.
We’ve opted for a 13.8 KwH B-Box battery pack to meet our needs and as written earlier 8Kw of panels. There’s a bit more to this decision than just the data we generated above. To misquote that old motor oil advert …batteries ain’t batteries. We’ll get into the mysteries of energy density, charge rate and depth of discharge in the next post and hopefully our choices will make sense after that.
I’ve just broken the news to Freddie he can’t have an electric blanket.
Just a final note. If you are putting up new house and your connection is going to be expensive you’d be mad not to go off grid if you can. Unless of course you enjoy exorbitant electricity bills! Expect though a system like the one we’ll be describing to you will cost close to $40K. But you only pay it once!