Going Solar Chapter 7 : Know Your Load

Before you considering how big you need to scale your solar PV system, it is important to know your load. The load will include anything you attached to the output of a system, including lighting, motors, digital equipment etc.

Load calculation is very crucial in a off-grid solar PV system as there is no utility power grid support like a grid-tied system do. Failure to estimate your load will cause overloading to your critical devices like charge controller and battery or run out of battery during critical operation. Frequently near fully discharge a battery will shorten its lifespan.

Watt, KiloWatt and KiloWatt hour

Watt (W) is a unit of power in the International System of Units (SI). It is defined as 1 joule per second (or 1 Newton meter per second). In electrical terms, one watt is the rate at which work is done when one ampere (A) of current flows through a electrical potential difference of one Volt (V). Hence Watt = Volt x Amps. This quantify the instantaneous power. The higher the wattage, the more power, or equivalently the more electrical energy is used per unit time (second).

One kilowatt (kW) is equals to one thousand watts. In terms of solar irradiance, on a clear day at mid day close to the equator, one square meter on Earth will receives typically about one kilowatt of sunlight from the sun (1kW/m2). To simplify things up, most of the units in solar is using kilowatt (kW).

Kilowatt hour (kWh) on the other hand is unit of energy equavalent to 3.6 megajoules (1 joule x 3600 seconds), which means the energy being sustained transmitted or used over a period of time. The total energy in kilowatt hours is the power in kilowatts multiplied by the time in hours. It is commonly used by electric utilities to quantify the energy used in order for billing.


How is this important ? With same amount of 1kWh it will run a 1000 watt device for 1 hour but also can run 100 watt device for 10 hours. Hence to scale the solar PV system, we need to know how much energy will all the devices generated, stored and used.

Estimating your daily load, usage and beyond

Illustration on how to estimate the size of your solar PV system. It is all back to your load, hence load calculation is an important part of a off-grid solar PV system.

First list down all the electrical appliances (light, fan etc) that you will be connected to your solar PV system.

Identify the schedule and time table that how long each appliance will be using. This will give you a rough idea of how much energy (kWh) each appliance will using. Since there will be different energy usage when the appliance is start work from cold, warm start, baseline work etc, hence a power meter will be helpful. For example, a Kill-a-Watt power meter will give you a clear picture on how much energy your device is used.


Add all the kWh used in a day, and it will be the baseline for your power demand. Also considering around 30% energy lost in the system due to electrical resistance, heat loss during energy transfer and battery storage efficiency. Hence the total kWh times 1.3 (corresponding to 130%) will come to a estimate daily need (demand).

With knowing the estimate daily need, you can estimate the storage capacity you need to store the energy. Remember that you have only a limited hours of the day with sunlight, hence the minimum storage capacity will be equivalent to your daily need. It corresponding to one full day of power supply. A 12 volt 100 Amp hour battery will estimate store 1200 watt hours (1.2kWh) of energy.

However if you have only the minimum storage capacity for your system, you will end up fully discharge the battery each day and risk spoiling the battery. To prolong the battery life, it is advisable to partially discharge a battery. More information will be on the battery section.

For example if your daily need is 1kWh, then you need a minimum recommended 2kWh (~170Ah 12V) lead acid battery (lead acid battery have Depth of Discharge – DoD recommendation of 50% and below) to act a backup storage for one day. Obviously a 3kWh (~250Ah 12V) lead acid battery will be a better choice as it will be only 33% DoD.

Furthermore depends on weather and seasons conditions, there are days that the sun light is less (e.g. cloudy days, rainy days, snow days) , hence you need to scale up your storage depending on how many days of backup energy you intended to store. In places with poor weather condition, you might need a backup of 4-7 days.

Finally with that much of storage in place, you can estimate the solar PV array size. Your aim is to charge up all the storage as fast as possible when the sun is up. Back to the understanding of sun hours, a 100watt rated solar panel will generate estimate 0.5kWh (500Wh) of energy daily if it is subjected to 5 sun hours. (total energy = solar PV rating x sun hours). Noted most of the day the solar PV would not generate the rated 100Watt consistently as the watt rating is based on a standard condition of 25 celcius and subject to solar irradiance of 1 kW/m2 of maximum unobstructed mid day sun at the equator.

The minimum total estimated solar PV panel energy output per day must equivalent or more than total estimated energy usage per day to prevent a negative balance system that will end up totally discharged the storage after a few day of usage.

Hence, load calculation is not as easy as strapping a 100Watt device onto a 100Watt solar PV panel. After all the calculation you might end up needing a 1000Watt solar PV panel to run a 100Watt load for 24hrs non stop.

An example on load calculation

The example based on above situation:

  • 100 Watt device running for 24hrs non stop will need 2400Watt hour per day. (100 x 24 = 2400)
  • Considering a estimate 30% energy lost from the system, it will need 3120Watt hour per day. (2400 x 1.3 = 3120)
  • 3120 Watt hour per day will need a minimum 624 Watt rated solar PV panel in area of 5 sun hours per day. (3120/5 = 624)
  • 3120 Watt hour per day will need a minimum 780 Watt rated solar PV panel in area of 4 sun hours per day. (3120/4 = 780)
  • In winter or area of frequent cloudy days with 2 sun hours per day will require a minimum of 1560 Watt rated solar PV panel to ensure it will generate 3120 Watt hour per day. (3120/2 = 1560)

NEXT CHAPTER >> Know Your Solar PV Cells and Panels