System Sizing: Off Grid

There are are a few basic factors to consider when designing any kind of system.

1. Energy requirements
   2. Peak Sun Hours
   3. Site Location Evaluation
   4. Battery bank
   5. No. of Solar Panels

   Off-grid systems are designed to work alone and are sized so that they work during the worst conditions of the year, which usually means getting through the month December, when there is bare minimal sunlight. It is known as the month with the worst solar irradiation, critical design month.

1. Energy Requirements:

This is measured in watt-hours or kilowatt-hours per day. Let’s assume the equipment consumes 15 watts of power and operates 20 hours a day:

15 Watts x 20 hours = 300-watt hours per day or.3 kWh per day.

If an inverter is being used to produce AC power for a load, it is important to account for the inverter’s self-consumption and efficiency losses. Inverters consume a small amount of power while they are operating. Inverter self-consumption typically ranges from under 1 watt, to around 30 watts depending on its type and make.

Efficiency losses can be from 5% to 15% depending on the inverter and how much load it’s bearing. This will be important when understanding the size of batteries to install. It’s important to invest in a quality, high-efficiency inverter.

2. Peak Sun Hours

PSH = Peak Sun Hours = Insolation is the amount of sunshine that you get in a particular location on the average day throughout the year at a particular tilt angle. PSH is usually between 4 and 6.

3. Site location Evaluation

Use any online solar mapping tool to estimate available PV resources. The system should be sized based on the month with the highest power consumption and/or lowest solar resource, typically December or January.

A good resource centre is NREL’s PV Watt Calculator.

This helps estimate the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It also allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations.

It is important to make sure there is a bare minimum shade available.

4.Battery Bank size:

We will be working with an example of 240Wh/day based on lead-acid batteries:

First, we need to account for the inefficiency of the inverter (if the inverter is being used). Depending on the equipment, 5-15% is usually reasonable. Check the spec sheet for the inverter to determine the efficiency. We will be using 10% inefficiency for this example:

240 Wh x 1.1 efficiency compensation = 264-watt hours

This is the amount of energy drawn from the battery to run the load through the inverter.

Next, we need to account for the effect of temperature on a battery’s capacity. 

 Since Lead-acid batteries lose capacity as temps go down, we will be using a multiplier to account for the battery temperature at 20*F in winter.

240 Wh x 1.1 x 1.59 = 419.76 watt-hours

Next, we will be accounting efficiency loss that occurs while charging and discharging batteries. Typically used figures are 20% inefficiency for lead-acid batteries, and 5% for Lithium-ion.

240 Wh x 1.1 x 1.59 x 1.2 = 503.71 watt-hours minimum energy storage requirement.

The above calculation is for  single day of autonomy( number of days that the battery can supply the site’s loads without any support from generation sources) so we need to then multiply it by the number of days of required autonomy.

For 5 days of autonomy, it would be:

504 wh x 5 days = 2,520-watt hours or 2.5kWh of energy storage.

The last most important parameter while assessing battery installation is the discharge depth, or how much capacity is discharged from the battery. Sizing a lead-acid battery for a maximum 50% depth of discharge will extend the battery’s life. Lithium batteries are not as affected by deep discharges, and can typically handle deeper discharges without substantially affecting battery life.

5. No. of Solar Panels required:

The solar panels need to produce enough energy to fully replace the energy drawn out of the battery while accounting for all the efficiency losses.

In our example, based on 2.5 peak sun hours and 240 Wh per day energy requirement,

240 Wh / 2.5 hours = 96 Watts PV array size.

Moreover, accounting for other real-world losses caused by inefficiencies, module soiling, ageing, and voltage drop, we take a general estimate to be around 15%:

96 array watts / .85 = 112.94 W minimum size for the PV array.

Example Question:

To have a better understanding of the overall system design we will be working with another example. 

An 8kWdc PV system makes 6 kW ac when 1000W per square meter are shining on it. If this is in a location that will get an average of 5 peak sun hours (PSH) per day and there are 15% losses from shading, then about how much energy will this system produce in a month?

Ans- 6 x 5 (PSH) x (1-0.15) = 25.5 kWh

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