3 ms·

This is what confuses many people, especially with solar and battery storage. "I've got 400 watts of solar feeding into my 400AH of batteries with a 3000w inver

by vondur 5y ago

This is what confuses many people, especially with solar and battery storage. "I've got 400 watts of solar feeding into my 400AH of batteries with a 3000w inverter" Most people will have no idea how much usable power they get and how much energy storage they need to meet their needs. (this example is mostly from an RV perspective)

danans 5y ago
In general, people are not used to formally separating power and energy as concepts, much less as units of measure. This makes perfect sense because most of the measurements of things we own or consume are counted in units like area, volume, and energy, which are inherently more tangible than an instantaneous rate like "power". Speed is a bit of an exception to this, but I think that has to do with the ubiquity of speedometers in cars. To the extent that people broadly understand power as distinct from energy they tend to do so in a mechanical, not an electrical context, and in less precise terms ("That's a powerful truck"). However, some exceptions to this are light bulbs and electric space heaters, where people generally understand power as being proportional to brightness and heat output. In a way, it's an incredible testament to the engineering of electrical systems that people have been able to largely ignore the difference between power and energy, or just need to flip a circuit breaker if something exceeds a limit.
Ajedi32 5y ago
Amp hours is particularly annoying IMO because you can't convert it to energy without knowing the voltage.
- imtringued 5y ago
  The reason that battery capacity is measured in amp hours is that you can't know the exact voltage of the battery as it drops with lower levels of charge.
  - danans 5y ago
    Newer large-capacity battery systems seem more likely to specify battery capacity in (k)Wh. I think this is because they come with sophisticated battery management systems that keep the battery within a narrow charge (20%-80%) range, so therefore they can be assured of the bounds of the voltage. Also, Li-ion and LiFePO4 batteries have a much different discharge curve than earlier chemistries [1], with much more constant voltage up to 80% discharge. 1. https://www.powertechsystems.eu/wp-content/uploads/sites/6/2018/11/LiFePO4-vs-Lead-Acid-Discharge-Curve-EN.png https://www.powertechsystems.eu/wp-content/uploads/sites/6/2...
  - Ajedi32 5y ago
    If voltage drops with lower levels of charge, then so does the current (assuming a load with a constant resistance). Isn't that just another reason to use Watt hours instead?
danans 5y ago
> "Ive got 400 watts of solar feeding into my 400AH of batteries with a 3000w inverter" Assuming 12V, that's about 400Ah*12V=4800Wh of storage. 4800Wh/400W = 12 hours to charge those, which seems reasonable for an RV. The thing that seems a bit oversized is the 3000W inverter.

by vondur 5y ago

danans 5y ago

In general, people are not used to formally separating power and energy as concepts, much less as units of measure. This makes perfect sense because most of the measurements of things we own or consume are counted in units like area, volume, and energy, which are inherently more tangible than an instantaneous rate like "power". Speed is a bit of an exception to this, but I think that has to do with the ubiquity of speedometers in cars. To the extent that people broadly understand power as distinct from energy they tend to do so in a mechanical, not an electrical context, and in less precise terms ("That's a powerful truck"). However, some exceptions to this are light bulbs and electric space heaters, where people generally understand power as being proportional to brightness and heat output. In a way, it's an incredible testament to the engineering of electrical systems that people have been able to largely ignore the difference between power and energy, or just need to flip a circuit breaker if something exceeds a limit.

Ajedi32 5y ago

Amp hours is particularly annoying IMO because you can't convert it to energy without knowing the voltage.

imtringued 5y ago
The reason that battery capacity is measured in amp hours is that you can't know the exact voltage of the battery as it drops with lower levels of charge.
- danans 5y ago
  Newer large-capacity battery systems seem more likely to specify battery capacity in (k)Wh. I think this is because they come with sophisticated battery management systems that keep the battery within a narrow charge (20%-80%) range, so therefore they can be assured of the bounds of the voltage. Also, Li-ion and LiFePO4 batteries have a much different discharge curve than earlier chemistries [1], with much more constant voltage up to 80% discharge. 1. https://www.powertechsystems.eu/wp-content/uploads/sites/6/2018/11/LiFePO4-vs-Lead-Acid-Discharge-Curve-EN.png https://www.powertechsystems.eu/wp-content/uploads/sites/6/2...
- Ajedi32 5y ago
  If voltage drops with lower levels of charge, then so does the current (assuming a load with a constant resistance). Isn't that just another reason to use Watt hours instead?

danans 5y ago

> "Ive got 400 watts of solar feeding into my 400AH of batteries with a 3000w inverter" Assuming 12V, that's about 400Ah*12V=4800Wh of storage. 4800Wh/400W = 12 hours to charge those, which seems reasonable for an RV. The thing that seems a bit oversized is the 3000W inverter.