A solar system is a great power generation solution for off-grid living. However, to be totally self-sufficient with your energy generation, you need to be prepared for those weeks where the sun is obscured for several days, providing insufficient light for your solar panels to charge the batteries. This is where a backup generator for your solar system comes in. How do you size a generator for this purpose?
To get the right sized generator, you need to know how much power it takes to run your home. Then, add a 20% safety margin and divide this number by the power factor of 0.8 to get the size generator required. An example of a small home needing 3.5kWh is 3.5kWh / 0.8 = 4.375kVA size generator, round up to 4.5kVA.
When you are choosing a backup generator for your solar system, there are some aspects that should be taken into account to ensure that it has sufficient capacity for your needs. The size of generator will also depend on how you intend to configure it in your solar power system. There are a few configuration choices that will affect the size of generator that you need.
How Are You Going To Configure The Generator Into Your Solar System?
There are two main ways of connecting the backup solar generator into your solar system as a backup power source for the panels.
- Power your entire household and charge the batteries. In this configuration, the generator will provide power to your household and charge the batteries at the same time. Choosing this configuration method will need a larger capacity generator that can carry the load of the power requirements of your home with some additional capacity to charge the batteries simultaneously.
- Only charge the batteries. In this generator hookup configuration, the backup generator will only be used to charge the batteries rather than power your entire home. In this configuration, you can use a smaller capacity generator since its only power demand will be charging the batteries. The power for your home, in this case, will come from the batteries rather than the generator. Once the batteries get to a low voltage, the generator will kick in to charge the batteries again.
Both of these options will work for an off-grid application, but it would be our recommendation to opt for the first option with a larger generator that is capable of powering your entire home on its own should you need it.
The reason for this is that should your solar system fail for some reason, you will have an alternative power source that has enough capacity to power your entire home while you sort out the fault.
With a generator that only has the capacity to charge the batteries, you would be able to hook it up to your home, but it would not have the capacity to power all the devices in your home. You would have to be selective as to what you will be powering with the smaller generator.
For this reason, we will give our attention to the option of using a generator to power the home at the same time as charging the batteries.
Related reading: How many solar panels do I need to run my home off-grid?
How To Size A Solar Generator For Your Home
The first thing you must do is to calculate how much power you will need. Start by adding up the power needed to run each appliance in your home.
There are several ways you can find the power rating of an electrical appliance:
- Look at the power rating label– This is can be found on most appliances and will usually give a value in watts. The label in this picture shows that the power usage of this appliance is 400watts.
If there is a range of values, like on this label, use the higher amount in your calculations. Here you would use 2400watts.
If the label only gives you the values in amps and volts, use this formula to find the power rating:
amps x volts = watts
For example, if the current is 3A and the voltage is 110V, your wattage is 330W:
3 x 110 = 330 watts
- Use a list of appliance wattages – You could get a rough power rating from a list of typical wattages. This is the least accurate way to do this, so it should only be used as a rough estimate.
- Use a watt meter plug – This is a simple watt reader that is attached between your home electricity outlet and the plug of any appliance. Plugging any device into the reader gives you a precise power reading. This is an excellent way to ensure accuracy, and you can usually find a good device for less than $15. Check out the latest price of the one I bought from eBay.
- Research the wattage online – Sometimes the information you’re looking for maybe in the user information manual. These can often be downloaded directly from the manufacturer.
- Contact the manufacturer – If you can’t find the relevant details online, you could contact the manufacturer directly for the information.
How long will you use each item per day?
Now that you know the wattage of each item in your home, estimate how long you will need to run it for each day. It’s always best to think of worst-case scenarios, so if you have a light on for 2 hours during summer, but 6 in winter, record it as six hours.
Putting the information in a table like this will be useful:
|Appliance||Power (Watts)||Length of daily use (hours)||Watt hours (Wh)|
|46inch LED TV||70watts||4hours|
Now you can use this formula to work out the watt hours required each day.
Power x Time = watt hours
|Appliance||Power (Watts)||Length of daily use (hours)||Watt hours (Wh)|
|46inch LED TV||70watts||4hours||280|
Many systems count power usage in Kilowatt-hours. This is a unit containing 1000 watt-hours. To convert watt-hours to kilowatt-hours, they must be divided by 1000.
2860Wh ÷ 1000 = 2.86kWh
Now you will need to add an extra 20% as a safety margin. Generator values are described in kVA as opposed to kWh, which is what this value is in. To find the kW rating of your generator, you would need to multiply its kVA rating by 0.8, which is known as the power factor.
If you already have the kWh amount that your home requires, you would divide this amount by 0.8 to get the size kVA generator that you need to power your home.
Here are some examples of different sized homes to further explain the principle.
Generator Sizing For A Small Home
The average small home will have a power consumption of 2.86kWh, and if we add a 20% margin for safety, we come to a figure of 3.43kWh, which we can round off to 3.5kWh for the purpose of our calculation.
To establish the size generator needed for this type of power consumption, you would divide this number by 0.8 as follows.
3.5kWh / 0.8 = 4.375kVA
This can be rounded up to 4.5kVa, which is the size generator you would need to run your home.
The additional safety factors built in provided enough additional capacity to simultaneously charge the batteries and power your home.
Either kW or KVa can be used to indicate the size of a generator. kW is common in the US and kVa is used throughout the rest of the world. As long as you use the power factor to convert between the two, you should be able to use either to find the correct generator.
Generator Sizing For A Medium Home
The energy consumption for a medium-size home is roughly 6.5kWh, and with the 20% safety margin, this goes up to 7.8kWh, which we will round up to 8kWh. If we apply the same calculation with the 0.8 power factor, we have the following.
8kWh / 0.8 = 10 kVA
A medium-sized home would therefore need a 10kVA generator to run the home and charge the solar batteries.
Generator Sizing for a Large Home
The average large-sized home has an energy consumption of 17.8kWh, and when the safety margin of 20%is added, it comes to 21.35kWh, which we will round up to 22kWh. To calculate the generator size that you would need for a home of this size, we apply the calculation once again.
22kWh / 0.8 = 27.5kVA
The 27.5kVA result can be rounded up to a 30kVA generator that would be required to run your household and charge your solar batteries.
My Generator Recommendation
Westinghouse offers a complete range of reliable, electric-start generators. Some of them can be used with gas or propane. Visit Amazon to check the latest prices.
If you are living completely off-grid and power your home with a solar system, it is a good backup strategy to have a gas generator that can act as a backup power source for your solar system.
If you intend to add a generator to the equation, you need to make sure that your charge controller has the connection for a generator input. The charge controller would be able to monitor the charge going to the batteries and shut this connection off once the batteries have reached capacity.
Depending on the sophistication of your charge controller, it can also automatically switch the generator off and revert to powering your home from the batteries. Once the batteries deplete to a certain level, the charge controller can initiate the startup of the generator to power your home and re-charge the batteries.
If you do not have a sophisticated charge controller, you may have to manage these levels manually and shut off and start the generator manually as required. You may find it helpful to read my articles: How to choose an off-grid charge controller and Guide to off-grid solar inverters.
My Off-Grid Product Recommendations
Useful Book: Off Grid Living 2022-2021 – This incredible step by step guide is a great read and gives you useful information about reaching self-sufficiency in just 30 days. Get the paperback on Amazon or read it free with a Kindle Unlimited subscription or listen to the audio version with Audible Plus membership.
Small Solar Panel Systems: Silicon Solar – This is an excellent company that offers lots of products to get you started on your solar journey. Visit Silicon Solar.
Family Water Filter: Big Berkey – For a fast, affordable water filter with no plumbing required, you can’t beat a Big Berkey gravity-fed filter like this one from Amazon.
Cleaning: Fuller Carpet Sweeper –. This carpet sweeper is an ideal way to keep your home clean without using up your energy stores on vacuuming.
Handy Knife: Gerber Serrated Paraframe – This handy all-purpose knife is lightweight and ideal for all those little jobs around your home and garden.