Watts, Voltage and Amps

Ohm's law calculations can be useful for calculating your battery storage limits and the power delivered to it from the solar panel(s) against the power needed to run the lights and the electrical equipment you require.


P (watts) = Amps x Volts

I (Amps) = Watts ÷ Volts

V (Volts) = Watts ÷ Amps


Example 1 - using the calculation for Current I=P/E

Lets assume you have an 100 amp hour leisure battery with one 30 watt solar panel charging it, to find how many amps the solar panel will deliver in a full sunshine hour we need to do our calculations. We need to convert the solar panel output watts to amp hours, so to find the amps we need to divide watts by volts (I=P/E, I (amps) = P (watts) ÷ E (volts)):-

i.e. 30 watts (solar panel) divided by the panels nominal voltage17.5 Volts (approx) = 1.7 Amp's per hour. This would be under exceptionally good circumstances (brilliant sunny day, not to hot or cold) and the most likely average in usual weather would be closer to 1.25 Amp's per hour.

If your total energy requirement is say a total of 200 watts, if we convert this to amps (Watts ÷ Volts) = 16.67 amp's per hour we can now work out how many hours we will get from our storage battery. We can see clearly that if using the 100 amp hour leisure battery mentioned above that we can maintain our power requirement for between 5 and 6 hours. This of course would not be recommended as it would bring the battery to a very low point and may mean permanent damage has been done.


Screen shot from 'the12volt.com website

the12volt.com Is a good website for working out or verifying your Current, Power, Resistance or Voltage calculations

Example 2

Often domestic electrical equipment is stated in amps (or milliamps) especially when the consumption is low. Ohms Law calculations can be useful to convert all the milliamps into watts to match the other electrical equipment you are trying to calculate your usage for.


Good additional reading:

energy.gov

bbc.co.uk

thegreencarwebsite.co.uk



Ohms Law - (defined)

Ohm's Law defines the relationships between P ((P) power), E ((E) voltage), ((I) current), and R ((R) resistance). One ohm is the resistance figure through which a single volt will maintain a current of a single ampere.


Current (I) is like a garden hose and is what flows on a wire or conductor. Current is measured in Amps or Amperes.

Voltage (E) is the measurement of electrical potential between two points in a circuit. It is the strength or pressure behind the current flow.

Resistance (R) is the measurement of current through a circuit or component. Resistors are used to control voltage at current levels. High resistance reduces the amount of current flowing and is measured in Ohms.

Power (P) is the amount of current multiplied by the voltage level in a circuit and is measured in watts.