03/08/2022

A 300 watt solar panel with full irradiance will run a constant AC load of 270 watts, taking into account inverter losses of 10%. This includes appliances such as blenders, desktop PCs, vacuum cleaners and treadmills. A 300 watt solar panel will also run a small fridge with 120Ah lithium battery.

Before working out what a 300 watt solar panel will run we need to understand how much energy in watt-hours it can produce and under what conditions.

I’ll be focusing on the energy in watt-hours a 300 watt solar panel can generate, rather than instantaneous power in watts . It’s a much more useful value for matching panel output to load.

Important solar panel electrical specifications are:

- Open circuit voltage (Voc) – measured by multi-meter across the + and – leads
- Short-circuit current (Isc) – measured by multi-meter inline with leads shorted together
- Maximum power voltage (Vmp) – the volts at which maximum power is generated
- Maximum power current (Imp) – the currrent that flows when maximum power is generated

With an average irradiance value of 4 peak-sun-hours a 300 watt solar panel produces 1.2 kilowatt-hours (kWh) of electrical energy per day, or 438kWh per year, The exact amount will vary according to location irradiance. If supplying AC appliances, deduct at least 10% for inverter losses, which depends on inverter size and efficiency.

You’ll come across the term Maximum Power Point again and again as you research solar power generation. This is when the panel volts and current are at optimum values for maximum power.

Power (watts) = volts x amps

This only happens under certain conditions:

- when irradiance is sufficient (how much sun energy is available)
- when the load characteristics match the panel requirements

Solar panel output is heavily dependent on irradiance and varies a lot during the day – maximum in a few hours around mid-day and very low in early morning/evening.

Because of this, solar professionals talk in terms of average hours of maximum irradiance in a day and use this to calculate the average power a panel delivers over time.

In terms of energy, the amount of power can be calculated in this way:

Maximum panel rating x peak-sun-hours/day = watt-hours per day

If we take Houston as an example, then a 300 watt solar panel would generate:

300 watts x 4.254 = 1.276kWh/day = 465.7kWh/year

The maximum amps of a 300 watt solar panel is called Imp (Current at Maximum Power) and is provided on the specification sheet by the manufacturer. An average current is 9.5 amps DC for a 300 watt solar panel with Voc 42 volts. The equivalent AC appliance current (USA) is about 3 amps.

The easiest way to find how many amps a 300 watts solar panel can deliver is to read the specification sheet. An average value is about 9.5 amps DC.

In terms of AC current, we need to take away inverter losses of at least 10 percent and assume the panel is delivering maximum power.

If Vmp is 36 volts, then for an AC load in USA we need to reduce the DC current by the difference in the DC/AC voltage. A simple equation could be:

AC amps = DC amps x 36/120 = 9.5 x o.3 = 2.85 amps AC

Remember that the inverter efficiency varies according to load and I’m guessing at 90 percent when fully loaded. However, this also depends on the size of the inverter used.

Final maximum AC load current = 2.85 – (2.85*10/100) = 2.56 amps AC

At first sight you would think the answer is obviously 300 watts, but it’s always best to over-estimate your needs for a couple of reasons.

Let’s say for example you were running a fridge or freezer. It might run quite comfortably in full sunshine as the average refrigerator has a continuous running watts of 40 to 100 watts.

However, when the compressor cycles, and the motor starts ups, it draws up to 3 times more power due to the surge current.

The 300 watt solar panel output may not be enough to supply this load, particularly if if the panel isn’t operating at maximum output, which is mostly the case except for 4 or 5 hours around mid-day.

In that case a battery is essential and would be coupled to the 300 watt solar panels using a solar charge controller.

The average current produced by a 300 watt solar panel is between 9 and 9.5 amps, so a solar charge controller rated at 10 amps do nicely.

However, it would be prudent to upgrade to a 30 or even a 60A model in case you want to increase solar capacity later.

Most MPPT controllers have an input voltage rating of 60 volts, so a 300 volt solar panel with Voc of between 40 to 44 volts will work.

A 300 watt solar panel can charge a 12 volt battery and the time it takes depends on the state of battery discharge and the irradiance level at their solar panel location.

With an irradiance of 5 peak sun hours per day a 300 watt solar panel will produce 1500 watt-hours per day.

A 100Ah 12 volt battery is equivalent to 1200 watt-hours, so a 300 watt solar panel using an MPPT solar controller will recharge a fully discharged 100Ah 12 volt battery in less than 5 hours.

However, it’s very rare for a battery to be full discharged. A lead-acid deep cycle battery is normally discharged to 50 percent of it’s total capacity, so a 300 watt solar panel kit would recharge it in less than 2.5 hours.