How much power does a man require? A shit tonne! That’s how much.
Let’s be honest, if I had limitless free electricity, there would be no end to my innovation and creativity. But here we are… saving money and the environment are most people’s incentive to get Off the Grid, while spending money and a lack of technology impair our ability to do so.
On our journey to get off the power grid, one key question had to be answered… How much battery power do we need? A question far more difficult to answer than you think. Add in the expectation of moving house, changing appliances and entirely changing your lifestyle and the answer to that question becomes borderline impossible.
Here’s the process we took:
- Inventory all electrical equipment in the house and estimate their usage over time:
- Read the back of each appliance for the rated power usage.
- Some gadgets have multiple rates of draw depending on their cycle. E.g. our fridge had a standard “max” draw of 320 watts, a defrost function at 480 and then the unpredictable idle time once it reaches temperature. For these we used a cheap power metre from eBay to measure a few days of data to average out. I since question the accuracy of said device…
- Understand your expected new lifestyle:
- Will it be a colder or hotter climate?
- More or less efficient building?
- More efficient appliances?
- Additional appliances such as water pumps?
- When will most of the power be used?
- Day or night?
- Will there be a difference to consider between summer and winter?
- Don’t forget to account for the surge loading when motors start. This is very critical for sizing the maximum surge availability of your system. Not all batteries and inverters are capable of delivering the same power.
- List out everything in a spreadsheet to review continuously over a few months.
I’ve included our Off Grid Load Calculator. This is a minimal and simple calculator but would be the easiest way to get started. Just fill in the appliance name, power rating as written on the back of it and how long per day/night you intend to use it.
Power Ratings are Completely Inaccurate
OK I know, I just said to use the written values on the back of your appliances, but that’s really just a guess. The truth is, that some appliances run at that rating full time, others have a cycle time like an oven heating up, then turning off as it reaches the temperature. Worst of all, some appliances like one of our reverse cycle aircons have rating specified, but they draw something totally different.
We learned the last point the hard way. Our brand new 5.5 star reverse cycle was rated at 800W. My calculations made it a great option for our use, but the moment we installed and plugged it in I found it drawing over 2Kw during the heating cycle and 1Kw during at all other times. That wasn’t in the brochure…!!
So the most accurate way to do this is just to buy a reliable power metre to measure the usage. Plug it into your appliance and leave it on for a full week. Work out an average and a max usage rate per day. You’ll have to make some judgement calls on how realistic that measurement is according to your new lifestyle…
Batteries are for night time
Remember there is a significant difference between day and night. During the day, generally, you can use the power directly off your solar panels. So you don’t need to ensure the batteries have that available in storage. Well… not on sunny days anyway. Night time load is when you need the batteries. In our case we find on a reasonably sunny day we have more power than we can possibly use. The batteries fill up, washing and dishwasher is done, water is hot and heater is on all day long to a temperature my father-in-law would be satisfied with (he’s from the tropics…), yet a significant load of solar power gets wasted to the ether! Such a shame… I hate waste! Note to self: I need to automate appliances, to maximise power usage during the day.
Winter is cold and dark!
Well at the least it’s horrible where we are. It’s like Game of Thrones winter for 6 months of the year. Something I hadn’t planned for as well as I should have. This winter we’ve only had around 10 hours of daily sunlight and most days that’s been covered up by fog, thick fog. Account for winters in your night time hours or your family may turn into mushrooms…
3 days of autonomy
General rule of thumb is to plan for 3 days of autonomy. That is, battery power for 3 days of cloudy days before the need to start up the generator. This one is very hard to meet. Especially if you’re already planning for a fairly heavy night time load. Imagine if you’re covered with dark rain clouds for 3 straight days. That’s asking for all your night time draw and most of your day time draw. My experience is that those dark rain clouds almost entirely cut off our power supply. We had 1.7Kw generated all day long the other day from a system that can usually generate 25Kw+. That’s the effect of those rain clouds.
3 days of autonomy for us would mean commissioning a massive industrial battery bank. So we really had to think about compromises… No induction cooker, no dryer, smaller fridge, addition of a stove top kettle, etc. and then consider making it a 2-day bank instead of 3. The problem is, those rainy days are usually the days we need the most power. This is where the old lead acid types come into their own. They have tonnes of surplus power you don’t normally have left over with lithium.
So, once you have a comfortable measurement of how much power you need, consider adding a little contingency… maybe 10-15% extra for things you didn’t consider.
Next, it’s time to put it all together… said batteries need generators, solar panels, wind and other turbines, etc. I’ll soon post a shorty on how we went about combining our batteries and power generation thingamabobs…
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