How to size your solar home system?

This is a sample to show your how to size your solar home system,how much do you need for each solar components.

Sample Sizing Calculation

In order for you to size the system correctly, you need to note the power rating of each appliance that will be drawing power from the system.

For this example, we will calculate the power requirements for a campervan with:

  • 2 x 15W 12VDC Fluorescent Lights
  • 1 x 60W 12VDC Water Pump
  • 1 x 48W 12VDC Fridge
  • 1 x 50W 240VAC TV
  • 1 x 600W 240VAC Microwave

(Note that a 600W microwave will consume approximately 900W of power)

1. Calculate Loads

Calculate total DC and AC loads:
DC Loads

  • Lighting - 2 x 15W DC Lights - each used 2 hours per day = 60Wh/day
  • Pump - 1 x 60W DC Pump - used 1/4 hour per day = 15Wh/day
  • Fridge - 1 x 48W Fridge - runs 8 hours per day = 384Wh/day
    Total for DC Loads = 459Wh/day

AC Loads

  • Television - 1 x 50W - used 2 hours per day = 100Wh/day
  • Microwave - 1 x 900W - used 15 min per day = 225Wh/day
    Total for AC Loads = 325Wh/day
    Allowing for inverter efficiency of 85% = 382Wh per day (ie. 325 / 0.85)

Total for AC and DC Loads = 841Wh per day

2. Calculate Required Solar Input

In Central to Northern NSW expect a usable average of around 5 peak sun hours per day.

Required solar panel input = (841Wh / 5h) * 1.4 = 235W

Note: The 1.4 used in this formula is a factor we have found that can be used to simplify the calculations for basic systems.

To ensure that adequate power is produced in the winter months, use a figure of around 4.0 to 4.5 peak sun hours per day instead of 5.

3. Select Solar Panels

Select solar panels to provide a minimum of 235W. Always best to go bigger if possible:

  • 2 x 123W solar panels chosen which, when connected in parallel, will provide 246W or 14.32 Amps.

4. Select Solar Regulators

The rated short circuit current of the 123W solar panels is 8.1 Amps each, giving a total of 16.2 Amps.

Select a solar regulator that is more than capable of handling the total short circuit current: 16.2 x 1.25 = 20.25 Amps

Note that, as described in the notes above, you must allow 25% extra capacity in the regulator rating as solar panels can exceed their rated output in particular cool sunny conditions. A 30A regulator will allow for an additional panel in the future.

5. Select Inverter

Select an inverter that is more than capable of supplying the maximum anticipated combined AC load required. In this example, maximum load would occur if the microwave and TV were running at the same time. Load in this case would be 900W + 50W = 950W.

Note that this calculation assumes that the inverter selected has a suitable surge rating to cope with the start-up surges of the microwave or other loads. A 1000W inverter would appear to be suitable, but a 1200W - 1500W inverter would be recommended.

  • 1200Watt pure sine wave inverter chosen.

Note: A pure sinewave inverter is the preferred choice, but if the budget is tight, a modified sine wave unit could be used.

6. Select Battery

Select a battery, or a matched combination of batteries, that is capable of supplying the total power usage without being discharged more than 70%.

In most cases it is recommended that the batteries are sized such that they have around 3 to 4 days back-up capacity. This allows for days with low sunlight and reduces the daily depth of discharge resulting in longer battery life.

With 3 days storage capacity, the battery sizing would be as follows:

  • Ah Required = (841Wh * 3 / 12V) / 0.7 * 1.1 = 330Ah.

Note: The 1.1 is used in this formula as batteries are generally only about 90% efficient.


The appliance ratings used in the above examples may not be accurate. They have been used for example purposes only. Check the ratings on your appliances before performing any calculations.

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