Electricity usage (off-topic)

[Ryan Petris]

Your calculations and math is a bit off.

First, *watts* (w) is a continuous usage amount, *watt hours* (wh) is a specific amount. For instance, if you run an appliance that uses 100 watts, it's using 100 watts continuously. If you run said appliance for an hour, you've used 100 watt hours of electricity; if you run it for 5 hours, you've used 500 watt hours of electricity.

Based on that, I'm assuming you use 600,000 *watt hours* per month.

On top of that, the simple definition of watts is volts multiplied by amps; there's some additional nuance in there when dealing with AC circuits and power factors, but we'll leave that out to simplify it for now. So, it doesn't matter what specifically the voltage or amperage is, watts is watts.

Therefore, step #1 is unnecessary and throws off your calculation by a factor of 10. 600,000 watts at 120v is also 600,000 watts at 12v.

Step #2 should be 600,000 *watt hours *divided by 30, which gives you 20,000 *watt hours* on an average day.

Step #3 should then by 20,000 *watt hours* divided by 4 hours, which is 5,000 *watts* continuously over 4 hours.

So, you would need a solar system capable of at least 5,000 *watts*, and factor in how much additional capacity you need to account for loss due to age. You also need to factor in that 20,000 watts per day is an average and you may have more usage on some days and less on others. You'll want to see what your peak daily usage is if your goal is to not use grid power at all.

On top of all that, that's still not the whole story; to be completely off grid you'd also need some way to store that power, so you'd need batteries as well. The capacity of those batteries would need to be at least what your peak nightly power usage is, also factoring in loss due to age. 



On Tue, Nov 26, 2024, at 3:31 PM, Keith Smith via PLUG-discuss wrote:
> Hi,
> 
> I have been curious about solar for sometime.
> 
> I am looking at my electric bill and I use less than 600k watts a month 
> for 1/2 the year.
> 
> My math does not seem to work out.
> 
> 1) Convert 600000 watts at 120 volts * .1 = 60,000 watts at 12 volts.
> 2) 60,000 watts divided by 30 = 2000 watts at 12 volts per day.
> 3) If I can harvest solar for 4 hours a day that is 2000/4 = 500 watts 
> at 12 volts.
> 
> That means I only need 500 watts of solar.  This does not take into 
> consideration loss of power in the system.
> 
> If the system losses 30% then I need around 715 watts of solar panels to 
> meet my needs.
> 
> I was expecting a lot more.
> 
> Any help is much appreciated!!
> 
> Keith
> 
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