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electrical:12v:dailypowerrequirements [2022/10/14 13:53]
frater_secessus [inverter losses] 		electrical:12v:dailypowerrequirements [2024/02/08 11:22] (current)
frater_secessus [now what?] power mix
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 +DRAFT
 +
 ====== How much power do I need to run stuff? ====== ====== How much power do I need to run stuff? ======
  
-It's not hard, but it does require paying attention and being realistic about what you will actually power in the van.+It may seem overwhelming at first, but all we really need are 
 + 
 +  - a list of things you intend to run in the van.  Be //specific// and //thorough//.  <- the hardest part.  
 +  - reading the product labels on the above items  
 +  - some math you already know from when you were a kid (addition, multiplication, maybe some division)
  
-The primary goal is how to figure out your **daily power requirement**.  We will do this step by step and end up with watt-hours (Wh, see below).((Ah or other units work, but Wh is easiest to understand))+The primary goal is how to figure out how much power you will require on a daily basis;  this is called the **daily power requirement**.  We will work through simple examples  step by step and end up with watt-hours (Wh, see below).((Ah or other units work, but Wh may be easiest to understand as it doesn't get tripped up by voltage differences.))
  
 ===== look at the labels ===== ===== look at the labels =====
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   * Most are in watts, which we will use directly for our math.     * Most are in watts, which we will use directly for our math.  
   * Sometimes they don't show watts but only Amps and Volts (example: **120vac, 2A**).  **Amps x volts = watts**, so our example would consume **240w**.  Easy peasy.   * Sometimes they don't show watts but only Amps and Volts (example: **120vac, 2A**).  **Amps x volts = watts**, so our example would consume **240w**.  Easy peasy.
 +  * the rated power is often the //max// power the device can consume.  They may use less over time (a laptop with a 90w adapter may pull 90w only when running at 100%); to know for sure use [[electrical:inverter#kill-a-watt|a kill-a-watt]] to measure actual energy use over a full day or more. 
  
 ===== how long will you run each load? ===== ===== how long will you run each load? =====
  
-Watts are only half the story.  We need to know //how long// the doodad will be run.+Watts are only half the story.  We need to know //how long// the you will run the doodad on average each day.
  
   * if we run the example doodad for 1.5 hours, the energy required is **360Wh** (240W x 1.5 hours).  Easy peasy.   * if we run the example doodad for 1.5 hours, the energy required is **360Wh** (240W x 1.5 hours).  Easy peasy.
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 The numbers in green were calculated automagically by the spreadsheet. The numbers in green were calculated automagically by the spreadsheet.
 +
 +Aside:  if you haven't made or used a spreadsheet before, here is [[https://www.youtube.com/watch?v=7Q2C9MB-YH4|a beginner's intro]] on youtube. Many computers come with a spreadsheet program (like Excel) and there are free ones like [[https://www.libreoffice.org/discover/calc/|LibreOffice Calc]] and free online ones like [[https://docs.google.com/spreadsheets/u/0/|Google Sheets]], etc. Google Sheets was used for the screenshots in this article.
  
  
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 The number is yellow is the inverter efficiency.  I typed that in one spot so the spreadsheet could refer to it as often as it needs to.  The number is yellow is the inverter efficiency.  I typed that in one spot so the spreadsheet could refer to it as often as it needs to. 
 +
 +A couple observations:
 +
 +  * we could save ~53Wh by running the fridge off DC rather than AC, if the fridge has that capability
 +  * this is a giant PITA for only two loads.  But when you get to ten or twenty (or more) loads the spreadsheet starts to shine.  It also allows you to easily see the increase/decrease from using, say, a 45w or 90w fridge instead of a 60w one. 
 +
 +===== also consider your highest-current loads =====
 +
 +Sometimes a load doesn't use much energy (Wh) because runtimes are short, but still consumes a lot of power (W).  An example might be an electric coffeemaker.  
 +
 +It might run for only 5 minutes a day (1200w / 60 minutes in an hour * 5 minutes) = **100Wh**.  But during that run it is pulling ~118A (1200w / 0.85 inverter losses / 12v).  **So the entire system (batteries, wiring, inverter) has to be sized to that 118A load** thar runs only 5 minutes a day.  Do you love that Keurig enough to buy extra batteries, fatter wire, and a bigger inverter to run it?
  
  
  
 +===== now what? =====
  
 +Now that we assessed our daily power and max current requirements we can think about
 + 
 +  * assessing our [[12v:bank-sizing|power storage needs]]
 +  * and [[electrical:12v:power_mix|power generation options]] ([[electrical:solar:sizing|solar]], [[electrical:12v:alternator|alternator]], [[electrical:generator|generator]], [[electrical:shore_power|shore power]], etc)
electrical/12v/dailypowerrequirements.1665770005.txt.gz · Last modified: 2022/10/14 13:53 by frater_secessus

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