Backup Power

ptrubey

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Use this thread to discuss backup power generation.
 

GGG

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Thanks for this helpful overview. Can you please explain why it's necessary to multiple the peak power usage by 4 to calculate peak load? Is that particular to your situation?
 

ptrubey

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Thanks for this helpful overview. Can you please explain why it's necessary to multiple the peak power usage by 4 to calculate peak load? Is that particular to your situation?
No, not particular to me. Let me explain by working it backwards. Remember that kW is the amount of power being used instantaneously (as in right at this instant), while kWh is the amount of energy used over a period of time (not necessarily per hour, but just over any unit of time, even as it has the word "hour" in the unit).

Let's say you were using 24 kW of power continuously and very evenly for an entire hour. That would mean you've used 24 kWh of energy. 24 kW of power used for one hour is 24 kWh. Now, how much energy would you have used in just 1/4 of an hour (or fifteen minutes)? That would be 24 kWh times 1/4 = 6 kWh.

So, if my SDG&E bill says I've used 6 kWh of total energy over 1/4 of an hour (15 minutes), then I must have been using an average of 24 kW of power during that 1/4 of an hour, correct?
 

ptrubey

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How much energy would you have used in just 1/4 of an hour?
The bill says I used 6 kWh of energy over that 15 minute period (1/4 of an hour).

Energy is defined as power used over time. Power is defined as the amount of work being done at a moment in time.

SDGE bills you for energy (kWh) since how much power you’ve used over time is what matters to them.

But when sizing a generator, you need to find out your maximum instantaneous power usage so that your generator doesn’t get overloaded. The SDGE billing portal doesn’t provide that information, it only gives you energy used over 15 minute increments, from which you can estimate instantaneous power usage if you make some assumptions like your load isn’t too spikey. The best way is to use the first method of turn on as many high power appliances and AC units as is reasonable and get read your meter.

By the way, I glossed over the calculations for sizing battery backup since that is much more complicated. Similar to a generator, your solar array ideally will generate more kW power than your house peak needs, but that isn’t always the case. Obviously it generates zero at night, and could easily generate 1/4 or less of rated output during a cloudy or rainy day. So the battery is used in those cases. But now you have to size the battery for both instantaneous power output (kW) and total energy stored (kWh) since, unlike a generator, you don’t have effectively infinite fuel supply. The battery has to have enough energy capacity for a long winter night until the solar panels start generating electricity again the next day. That could be up to 18 hours, or 3/4 of your daily electricity consumption.

For my large house, the bill shows one of my biggest days was using 320 kWh of energy. But some of that was my well pump, and charging my EV car, both of which I would set up to not work during a power outage. So say my max. total energy use on any given day was a more reasonable 240 kWh. Three quarters of that is 180 kWh. One Tesla Powerwall battery holds 13.5 kWh, so I’d need 180/13.5 = 14 Tesla Powerwalls. And an absolutely huge solar panel array to charge that. Impractical. That’s why the only real choice for large houses is a generator.
 
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