My first mini solar setup

ellisxellisx Registered Users Posts: 2
TL;DR: Can I somehow power this 240v/24w pump with this 12v/20w solar panel kit. (I'll probably add a 12v battery in the middle).

Detail: At £40 I couldn't resist getting the solar panel kit with controller and starting to get into solar. I'm very novice at electronics, advanced at computer science, and basically a total geek. Long term I'll want solar to power my EV but I'd rather understand it all and potentially be able to save money by doing some of it myself down the line.

So I'm going to start small with this kit and was wondering what to power, the panels will likely go on my greenhouse or summer house (which both already have power but I'd like to practice by taking them off-grid for now.)

Most stuff suggests I should run the panels to a 12v battery, then power things from that, I'd like something more complex/fun than a few lights so I thought I'd start with my already installed pond pump  if possible and wondered what I'd need in the middle to align the voltage. (I'm already thinking this might be a stupid question, can you increase voltage by 20x ?!)

If not, any recommendations on other things to power instead would be great, just a hobby project at the moment  :)

Thanks in advance!


  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    edited April 2020 #2
    The short answer is almost anything is possible.... The longer answer is it better to stay with what is "generally done" unless you have very special needs and are into experimentation/DIY electronics.

    In this case, yes, you would have a 12 volt battery--This is to store the harvested energy of the solar panels during the day and provide energy when it is dark/cloudy. A basic solar power system works a lot like a car's electrical system. The car battery provide energy anytime it is needed, and the engine (when running) recharges the battery can carries the loads (lights, wipers, engine electrical, car radio, etc.) when driving.

    Solar panels do not output very stable DC voltage (from zero to ~22 volts for your "12 volt" panel). The 12 volt battery "stabilizes" the DC voltage and makes it "useful" for running your loads (DC LED lighting, a DC to AC 230 VAC @ 50 Hz inverter, etc.).

    Since your planned loads exceed the output of your solar panel, so they must be "on" for only a hour or so a day or you will take the battery "dead" and ruin it. Also, AC inverters are pretty power hungry devices--So a system this small (20 Watt panel and a ~10-20 AH @ 12 volt battery) is not really large enough to provide "useful" energy through an AC inverter to a 24 Watt @ 230 VAC pump.

    The next step--You can start with your loads (Watts * hours per day, 12 VDC or 230 VAC through an AC inverter) to define the solar power system. Or you can take what you have (20 Watt panel + 10-20 AH @ 12 volt battery), estimate how much energy it can provide--And power the loads that work with that system (a few LED lights, a small radio, cell phone charger, etc.).

    For a little math... A 12 volts @ ~10  AH battery on a 20 Watt panel. Assuming everything is working, and the panel is mounted in nice/sunny location around Leeds, UK.

    First energy from battery, a nice system would use the battery to store energy for 2 days of sun, and 50% maximum discharge (for longer battery life):
    • 10 AH * 1/2 days storage * 0.50 max discharge = 2.5 AH of daily usage @ 12 volts
    • 2.5 AH * 12 volt battery bus = 30 Watt*Hours
    If you have a couple 1 watt LED lights and a 3,000 mAH phone:
    • 2 * 1 Watt LEDs * 5 hours per night = 10 Watt*Hour of energy per night
    • 3 Amp*Hour cell phone * 3.7 volt battery * 0.50 discharged (50% discharged) = 5.55 WH of energy per day
    • 10+5.55 WH = 15.55 WH
    • 30 WH of stored 12 volt power / 15.55 WH per day = 1.9 days of stored energy from 12 volt @ 10 AH battery bank
    And then there is the amount of energy you can harvest from the sun... Fixed panel, Leeds UK (not a sunny area):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 36° angle from vertical:
    (For best year-round performance)


    Coming into May @ ~4.03 hours of sun per day (long term average):
    • 20 Watt panel * 0.61 DC off grid system efficiency * 4.03 hours of sun per day = 49 WH per day
    Which gives you a bit more than the 30 WH of "overnight/bad weather" usage of a 12 volt @ 10 AH battery... So during sunny weather, you can charge a phone/tablet during the day, and still run LED and read the tablet at night.

    Anyway--A real quick guess at what your present setup can be and would support. Do not just go out and get a car battery (12 volts @ ~80 AH). The battery is for storage and a larger battery does not generate more energy--Your solar panel does that. If you want more power eventually, you need more solar panels and more battery storage together--A "balanced" system design works best.

    Depending on the type of battery you get--If flooded cell, a hydrometer is nice to measure the state of charge. And for detailed maintenance and learning more about DC/AC power systems, an AC+DC Current Clamp DMM is really nice to have:

    For the meter, a 50 Pound or less meter will work for your needs. A 100 Pound or so meter is a nice mid-priced unit (links above are just for you to start your search--I know nothing about what is available in UK).

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ellisxellisx Registered Users Posts: 2
    Wow, that's an absolutely fantastic response, thank you so much! 
    The main aim is to learn so no big deal on the pump, I'll wrap my head around all this and come up with a new proof-of-concept plan  :)
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    edited April 2020 #4
    You are very welcome Elli,

    Some other relatively inexpensive meters that can be used on small DC power systems to measure loads and energy usage: USB Meters DC power meters

    Note, you really need to get a handle on Amps, Amp*Hours, Watts, and Watt*Hours.

    With Amps, you also need to know the working voltage. 1 amp at 12 volts is (p=vi=1a*12= ) 12 watts, vs 1 amp @ 230 VAC (1a*230v= ) is 230 Watts.

    Amps and Watts are power or a rate measurement. Like driving 50 KPH...

    Amp*Hours and Watt*Hours are an amount... Like driving 50 KPH for 2 hours = 100 Kilometers driven.

    Watts are a "complete unit"--It describes the power used.... Amps is an "incomplete" unit, you need to know the working voltage too (5 volts, 12 volts, 230 volts, etc.).

    If you are doing everything on a 12 volt bus (LEDs, DC pump, etc.), you can work in Amps. However, if you are working in different voltages (5 volt USB, 12 volt battery bus, 230 VAC inverter output)--It is easier to convert each item to Watts, do all the math, and convert back to Amps/Volts/etc. when/where needed.

    Solar power is "not cheap". It is always best to find the smallest/most efficient "device" and use that on your solar system. I.e., a small 12 VDC water pump will usually be more efficient than a larger/230 VAC pump (plus there are losses with the AC inverter too).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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