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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:

  1. 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
  2. With a 12v DC battery, 1560 watts would imply that I would need 130 amps
  3. A normal car battery at full charge stores about 48 amp-hours
  4. 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
  5. I plan on converting the 12v DC to 120-volt AC using a standard OTS inverter (~$20)

Questions:

  1. Is my (sizing) calculation correct?
  2. Is my prototype’s design reasonable?
  3. Any suggestions?
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    Is there a reason you're using a car (SLI) battery for deep-cycle service? – ThreePhaseEel Sep 13 '20 at 0:39
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    1) You’re not going to get a real 1500W inverter for $20. 2) Motors generally take a lot more than their rated power to start (like 3X). Circuit breakers and wires are fine with momentary overloads, inverters are generally less tolerant. 3) Never discharge a lead acid battery 100%. – nobody Sep 13 '20 at 1:18
  • ThreePhaseEel: No particular reason why I chose a car battery; I just needed an electric store and thought that a car-battery would do it. Would appreciate your thoughts and recommendations for improvement. – user97485 Sep 13 '20 at 1:59
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    @nobody I think by "OTS", OP means the eBay/Amazon Marketplace/AliExpress/Banggood Chinese junkstream. After all when stuff is on Amazon why would a consumer think it's bad? – Harper - Reinstate Monica Sep 13 '20 at 1:59
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    @Harper-ReinstateMonica I did check Amazon before posting - the absolute cheapest and sketchiest-looking “1500W” (yeah right) inverter there is $60. – nobody Sep 13 '20 at 3:26
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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.

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  • Harper: Thank you SO very much for your educational and insightful comments; When you say "deep-cycle" battery, I suppose you mean the type of battery used in backup (battery) sump-pump. I will do some research/investigation into "deep-cycle" battery....I have heard about them but don't know the details. As for NiCd, NiFe batteries, what should looking for on (any) eMarketPlace. I am asking as the ones that I (just) saw on Amazon had low amp-hours (max=3 Amp-hours). Guess I need a lot more storage capacity. – user97485 Sep 13 '20 at 2:22
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    @user97485 -- there is no good reason to be shopping for this sort of stuff online, especially not on Amazon Marketplace...if you want deep-cycle lead-acid, head to a place like Interstate Battery or Batteries+Bulbs, I reckon. (If you really want to shop for it online, I'd try a MRO supplier like McMaster-Carr, MSC, Grainger/Zoro, or such) – ThreePhaseEel Sep 13 '20 at 2:33
  • Thank you, Sir!! Will follow your advice and check at Grainger, B+B since they are in my area. – user97485 Sep 13 '20 at 3:03
  • Harper: I checked B&B's catalog and I found a 33AH, Deep-Cycle, 12v battery that seemed to fit my need. However, they were all lead-acid type and NOT Lithium/NiCd/NiMH as you mentioned. Thoughts? – user97485 Sep 14 '20 at 14:33
  • You're gonna need some more concepts in the area of battery procurement :) Lead-acid deep-cycle batteries are best bought the same place you buy car batteries, or alternately, 2 golf-cart batteries. For instance CostCo has some deals (to convert from Reserve Minutes to AH, multiply by 0.416). Optimize for AH per dollar. Remember you can only count on using about 30% of published AH. Never mail-order lead-acid; they're too heavy! – Harper - Reinstate Monica Sep 14 '20 at 18:21

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