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Home Forums General Discussion How to measure power.

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    One of the ongoing difficulties encountered by off-grid users is how to determine the amount of power used or required.

    For AC power measurements a relatively cheap device is something called “kill-a-watt’ which you can purchase from Amazon for around $20. Unfortunately a similar meter for DC measurements is somewhat more expensive at $200 and upwards to $600.

    In addition power measurements is something involving calculations which seem to scare off many people who think it’s too complex. It is not, and can be handled by a basic 4 function calculator

    The low priced multi meters seldom go beyond 10 amps and many DC systems have current levels above that. Any good solar panel of 130 watts and greater feeding a 12V panel will deliver in excess of 10 amps.

    What is needed for DC is a “meter shunt” If you lack the money you can make your own using heavy copper. Professionally made shunts use a special alloy but pure copper wire will work although it has a 5% variance as temperature changes.

    For technical details google shunts and various DIY sites will show you how. Don’t take my word for it; google it and see for yourself. The expensive DC power meters that count accumulated use have a micro-processor but you can usae a watch to manually do the same job.

    Commercially made shunts cost $20 – $50 each so when I needed quantity 10 to measure individual battery currents in one installation I fabricated shunts using copper bar stock. The important aspect was that all of them read simultaneously so the variance due to temp changes was not as critical.

    Its important to know how much power (watts) you need in order to select the solar panels or wind generator for your system. Going by the rating plate doesn’t really help because the rating plate only indicates maximum power draw not actual running watts. If a motor is involved with the appliance there is starting amps and running amps. Resistive devices like a lamp bulb or heater ther is cold inrush and hot running amps.

    It has been my experience that newcomers to off-grid quite often get this aspect wrong. Either they overspend and buy way more capacity than they need or they under estimate and run out of power when they need it the most.


    “kill-a-watt” catchy product name. I’ll have to check that out.


    our local library has a kill a watt to checkout. don’t have to buy. most inverters even the small alligator clip to a batter twelve volt have in and out power reading. you can run one item at a time record the in and out. paying attention to you start-up draw and regular run loads and do the math.

    another huge variable in calculation system requirements is capacitors. in well switches and if your using a transformer to flip 120 to 240. usually a capacitor will hold its charge for 2-3 days so to be accurate flip the breaker servicing that load for a couple of days to get a more accurate start up that includes the capacitor’s load too.

    peace g


    The 30th Anniversary Solar Living Source Book has a system sizing worksheet on page 579-80. Followed on page 581 by a power consumption table of a large variety of electronic devices in the home. Then on page 582 it has the US solar insolation chart, followed on the next pages by magnetic declination, wire sizing, battery bank wiring, and friction loss charts for water pumping. A truly good book to have, or something like it for your country.

    There’s another one here;

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