Objective: Today I power my tools (drill, circular-saw, reciprocal saw etc.) in my shed by running a power line from my house. I wish to install solar power panels on my shed; however, before I do so, I wish to make sure everything is theoretically and practically correct.
Prototype: My thoughts and calculation:
- A circular saw needs the most power compared to other tools (~13 amps); hence I used it for my baseline worst-case scenario. i.e. the circular saw needs 13 amps * 120 volts = 1,560 watts
- With a 12v DC battery, 1560 watts would imply that I would need 130 amps
- A normal car battery at full charge stores about 48 amp-hours
- This implies that a fully charged car battery would run my circular-saw for about 48 amp-hours / 130 amps = 0.36 hours = 0.36 * 60 ~ 22 minutes
- I plan on converting the 12v DC to 120-volt AC using a standard OTS inverter (~$20)
Questions:
- Is my (sizing) calculation correct?
- Is my prototype’s design reasonable?
- Any suggestions?
Best Answer
I think your math all checks out and you have good command of the concepts here.
You need a few more concepts though.
Lead-acid batteries are bizarre. While they are cheap, they do not work the way they obviously ought to work. The #1 thing they don't do is deep-discharge well. If you run a lead-acid battery from 90% to 10%, you will get three to 100 of these full cycles before the battery loses capacity dramatically and is kaput.
People who depend on lead-acid batteries for deep-cycling find they can only use the top 30% of capacity on a regular basis or they will get very short life out of their battery.
Further, the type of battery matters a great deal. Car starting batteries are made for delivering a big one-shot impulse, but do particularly badly in deep-cycling. Deep-cycle batteries do well with deep cycling, but have too much internal resistance to start an engine in adverse conditions.
They are a big pain in the butt, but you like the price.
If you want a battery that actually behaves like it says on the tin, then look at lithium (which has different kinds of feisty) - or the perfect large battery is nickel-cadmium or nickel-iron. I have a set of NiCds that came to us as scrap in 1986, and they're still going. Nickel-iron is impervious to almost any operational abuse, and last 40 years, but have quite high internal resistance so are not good at starting engines.