a man using his solar generator to connect his computer equipment

Home Solar Generator Sizing Guide

Aniket

Written by qualified solar engineer Aniket. Last updated:

Home solar generators are becoming increasingly more popular among homeowners. More instances of power outages from natural calamities are a growing concern highlighting the importance of energy security and, thus, power generators. Although gasoline or diesel generators have always been in the market, their high operational costs and detrimental effect on the environment are pushing people toward cleaner and more cost-effective options like solar power. The life span of solar generators can also be significantly longer.

A home solar generator sizing guide, such as this, is essential if you want to understand which kind of generator will best suit your home. The sizing of any home solar system mainly depends on the amount of energy utilized and the number of appliances used and their power consumption.

Understanding Power/Energy Consumption

When you size solar power systems, it should begin with understanding the quantity and pattern of energy consumption. Fortunately, calculating your specific energy consumption is relatively simple. However, people prefer sizing solar generators to satisfy only the ‘essential’ power requirements and not their entire home. In the event of a power outage, most of us can do without using several appliances.

To begin with, make a list of your ‘essential’ appliances. Most people opt for indoor lights, refrigerators, a microwave oven, a laptop or two, and a few other basic appliances, such as dishwashers and washing machines.

Power

Once you have made a list of the necessary appliances you want your generator to power, the next step is to add the power consumption to have a total power requirement value. For this, you need the power rating of every appliance measured in watts.

There are two ways to know this. The first one is checking the power rating label on the appliance. Every appliance will have a manufacturer’s label on it with this information.

image showing a label on appliance with power consumption information

The other way is to use a ready-made list of appliances and their wattage requirement. Some appliances may not have their power consumption in watts given on the label, so multiplying the voltage and current (amps) will provide you with the consumed watts per hour.

You must keep in mind that some appliances use more power while starting, such as water pumps. When it comes to backup solar generators for home, avoid these appliances from the calculation altogether.

An average American house needs about 6kW of solar panels. While sizing a solar generator for your home, considering only essential appliances. This value should ideally drop down to less than half, but it can vary based on what you term ‘essential’.

Once you have listed all the appliance power ratings, add them up. The sum is the total power you will require from your home solar generator. For example, if your total adds up to 1000 watts, your solar panels’ capacity must be at least 1000 W (1 kW). Here’s an example list of household appliances and their wattages:

image showing a list of appliances and their respective wattages

Energy

Once you calculate the total power requirement, the next step in sizing your generator system is to understand your energy requirement. Power is consumed or generated in an hour, and energy is when that power gets consumed or generated over several hours.

Let us understand the importance of understanding energy consumption with an example. Suppose that based on your power calculation, you decide to use 1 kW of solar panels. You use 1000 watts per hour for about 8 hours a day, but your panels get enough sunlight for only 5 hours of power generation. This means although your power consumption and power generation match, you are still left with 3 hours without power.

This is exactly where energy comes into play. Understanding energy consumption throughout the day helps size the system to have the right size of solar panels. Going back to our example, let us calculate how much energy you require in a day:

1 kW × 8 h = 8 kWh

Panel Sizing

Our goal is to have enough panels to produce energy sufficient for a full 24-hour usage. Now, considering the 5 hours of sunlight in your area, let us find out how many kW of panels are needed:

8 kWh ÷ 5 h = 1.6 kW

This means that to compensate for the missing three hours, our generator’s power output must be 1.6 kW instead of the 1 kW we initially calculated.

However, a total power output of 1.6 kW can power your home for the same 5 hours as 1 kW panels, so having only sufficient sized panels is not enough. In such a case, we would have excess power during the afternoons. We store this extra energy and use it later during our extra 3 hours, which is typically late in the evening. Hence come into the picture – batteries!

Battery Sizing

We already discussed why large-sized panels are not the complete solution for a generator to power your appliances that run longer throughout the day. You could install hundreds of panels and still not be able to run a microwave oven for dinner. We need batteries to store that excess power. Batteries also ensure continuous power supply during sudden changes in sunlight quantities.

Batteries are chemical devices. They use the electricity to undergo a chemical reaction, which gives a certain potential energy to the batteries. When the battery terminals are connected to an electrical load, a reverse chemical reaction occurs and produces electricity, which we call battery discharging.

Going back to our example, we have already figured out that we need 1.6 kW solar panels, which will generate 8 kWh energy. We now must decide how much energy needs to be stored in the batteries. The entire 8 kWh does not need to be stored in the batteries since some power will be used during the afternoons as soon as it is generated.

Suppose your energy usage during the day is 5 kWh and during late evenings is 3 kWh. This means we must size batteries that will store 3 kWh of solar energy.

There are two options from here. The first is to find out the right size of batteries required, arrange for batteries and connect with the panels. You will also need a solar charge controller to make sure the batteries are charged safely and efficiently from the panels, and an inverter to convert power from DC to AC to make it suitable for use in the house since all the household appliances are designed to operate on alternating current (AC).

The second and much easier option is to go for one of the solar backup generators readily available. These solar generators come as an integrated unit of the charge controller, battery, and in-built power inverter. Modern solar generators often come with several other useful features, such as an LCD display and various power outlets or ports ready to use. A 1000-2000-watt heavy duty portable generator should be sufficient in this case. It may not power an entire home, but that is what gives it its portability.

The ready-to-use option might be slightly costly, although more reliable and durable. If you choose to go with the first option, let us find out how to calculate battery sizing for the 1kW house generator for your home:

Batteries usually come with Ah (amp-hours) and voltage as their specifications. Thus, we will need to convert our required kWh to Ah to find the correct number of batteries for our use. From our example, we know that we need to store 3kWh, or 3000Wh. Let us see how to convert this into Ah and V. Power is a product of current and voltage, thus,

W = A × V

Hence, Wh = Ah × V

We need to store 3000Wh, and batteries are usually rated at 12V, hence,

Ah = Wh ÷ V = 3000 ÷ 12 = 250

In the ideal case, 250Ah of battery storage should be sufficient, but two other factors reduce the actual usability of a battery’s capacity. One is efficiency, where batteries lose some energy in conversion; hence a 250Ah battery would store less energy in it.

The second is the depth of discharge or the allowable discharge level of any battery. Extracting 100% of a battery’s stored energy damages the battery. Battery efficiency is usually around 80%, and the depth of discharge (for solar specific batteries) is around 80% as well. Our actual Ah required then becomes:

250 Ah ÷ (0.8 × 0.8) = 390 Ah

We would thus require around 400 Ah of 12V batteries for our home solar generator’s backup energy. This can be in the form of four connected 100Ah batteries. These batteries can be connected all in series, in parallel, or both, depending on your inverter and charge controller’s voltage rating. You can go with any battery type as long as it suits your needs and budget.

image showing batteries connected in series and parallel

For the charge controller, simply find out the current rating by dividing the solar panel watts by the battery voltage. In this case:

1000 W ÷ 12 V = 83.3 A

Considering a safety margin, a 100 A solar charge controller should be sufficient. The inverter selection is also based on your panels’ size. In this case, a 1kW inverter should be suitable. Many manufacturers also provide off-grid inverters with built-in charge controllers.

Overall, solar generators are powerful machines that can serve your home for decades, especially to power during blackouts. The working principles of a solar generator are simple to understand. The process of sizing home solar generators is not complicated and takes a few easy steps. However, we recommend going for one of the well-built, ready-to-use solar portable generators that will take away the hassle of component selection, wiring, and possible maintenance.

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