Correctly sizing an inverter for a solar system is one of the primary tasks to get right. Take the following into account before buying:
1️⃣ How much power is needed for the home, RV, or portable solar system?
2️⃣ How much power the solar panels will produce, measured in watts.
3️⃣ The inverter efficiency.
Sizing solar energy systems, including their respective components, is what I do in running a solar system supply and installation company.
I want to share some practical insights and simplified theory related to the sizing of a solar system inverter that I’ve picked up in the past five years.
🔀 Solar Inverter Sizing (Home, RV, Portable)
An inverter’s primary task is to convert the direct current (DC) generated by solar panels to alternating current (AC).
A solar system inverter is critical to any solar panel system generating AC. This includes solar systems that power homes, RVs, and portable solar panel systems.
Solar inverters are the brains of the operation when it comes to solar systems. The inverter is the central meeting point for the power coming from the solar panels, grid power in and out, battery power in and out, and sometimes a generator port.
The inverter controls the power flow to the various solar system components while also having the critical task of converting DC to AC.
📖 Related article: Understanding the Differences Between AC and DC
Solar inverters are rated according to their maximum output in VA, KVA, or Watts. A 5kw inverter will deliver a maximum of 5000 watts of AC power.
Microinverters coupled with a single solar panel have particular solar panel requirements in terms of DC input to the inverter.
Calculating the size of the inverter required is straightforward. If no measuring equipment is available, the home consumption must be manually calculated by adding up the wattage of each item you want to run off the inverter.
Below is an example.
From the above table, the combined usage of the chosen appliances is 1050 watts (1.05Kw).
To calculate the inverter size in KVA, we need to apply the following calculation:
KVA = KW / Power factor (constant at 0.8 for homes)
= 1.05 / 0.8
Make sure to use the continuous use rating of at least 1.5KVA. So in the above example, a 1.31KVA inverter would be required.
Finding an inverter of the exact size would probably be impossible, so selecting one in the next available standard size, such as 1.5KVA, would be correct.
The same applies to Hybrid inverters designed to work with batteries. They’re generally fitted with an internal MPPT that may have a different input rating than the output of the inverter.
5kw Inverter MPPTs are not the same and can vary between 4000 watts and 6000-watt DC input.
Inverter Sizing To The Home
Some installers size the inverter according to the solar array’s output, while others size the inverter according to the home’s power requirements. The truth is that solar PV panels and inverters need to work in unison.
Assuming you already have solar panels installed, you’d need an inverter large enough to accommodate all the power that the solar panels can potentially generate.
If you have a grid-tied solar system with solar panels supplying power during the day to your home, you need a grid-tied inverter.
The priority is to select an inverter that can comfortably power the selected electrical circuits in the home while running at its continuous power supply rating and have sufficient output power to feed excess energy to the grid in regions where this is allowed.
When determining your home’s power consumption, it’s best to enlist the assistance of a solar installer with the required equipment to do the measurements.
Most use a data logger that will record the home’s power usage over a few days giving an accurate summary of the home’s electricity consumption trends, including the surge power of appliances.
Naturally, you’d need to spec the inverter to deliver the maximum power required, even if it is only for a short time, such as when accommodating surges from a motor or fridge compressor starting.
Should the home draw, for example, 2500 watts at peak consumption, the inverter should have a continuous power output rating of at least 2500 watts.
Inverters also have a surge rating which is often double that of the constant power rating but is only useable for a second or two.
The surge power rating is an internal surge protection limit for the inverter and should not be used as a maximum power output measurement when sizing an inverter.
The type of inverter you choose will be determined by its DC input and AC output ratings.
Secondly, sufficient power must be generated by the solar panels to supply the inverter and batteries, if fitted, with enough ability to run the home appliances and to charge the batteries within a reasonable time.
Inverters For RVs
Determining the right-sized inverter for an RV is the same as sizing an inverter for a home.
Inverters used in RVs tend to be smaller in terms of power output capacity as they need to run fewer appliances. As a result, fewer solar panels and backup batteries are required when the load is less.
RV solar systems can either be off-grid with battery backup or use the grid as backup power when available.
Portable Solar Systems
Portable or mobile solar systems vary widely in power output and battery capacity.
Mobile is generally charged via the grid and contains a built-in inverter, battery, and charger. Power is usually supplied only to selected items such as PCs, laptops, and phones.
Being mobile, the inverters can supply power in various applications where grid power may not be available. Few mobile units offer solar panel compatibility. So they need to be recharged using an alternate power sauce.
☀️ Inverter Sizing to the PV System
Solar panels are rated in watts, amongst other measurements. However, in real-life situations, this may vary considerably depending on the location where the solar panels are used.
For example, a 300-watt solar panel produces a maximum of 300 watts DC under lab conditions at a temperature of 77°F (25°C).
Local weather conditions significantly affect the number of solar panels a solar system requires. For example, more solar panels may be needed in rainy and overcast regions than in mostly sunny places like Florida.
Your local solar expert should be able to provide a recommendation on the over-spec required if any.
Solar inverters use a charge controller through which the power generated by the solar panels runs. The charge controller or MPPT controls the energy that flows into the inverter.
The charger or MPPT carries a rating specified in Watts and Amps, which will determine the maximum size PV array it can accommodate.
When multiple solar panels are combined, this is called a string. The inverters’ DC specifications will determine the length of the string.
📖 Related Article: Micro vs String Inverters
Let’s look at an example:
Most Hybrid Inverters would contain an MPPT rating of 60V to 140 Volts and 80 Amps.
60V is the minimum voltage required for the MPPT to start charging batteries, 140 volts is the maximum voltage, and 80 Amps is the maximum amperage the MPPT can safely accommodate.
A 300-watt monocrystalline solar panel delivers an open circuit voltage (VOC) of 41V and 10 Amps.
So if three solar panels are connected in series, the total potential output would be 123V and 10 Amps, well within the range of the MPPT.
The 900 watts available from the two solar panels may not be sufficient to run a house but would most likely be adequate to power an RV and portable system.
If more power is required for the home, two strings of three solar panels (six total) can be connected parallel to the inverters MPPT.
When combining strings of solar panels, the voltage remains the same, but the Amps increase with every added string.
In our example, two strings would generate 123V and 20 Amps. The maximum number of solar panels you could add to this inverter would thus be eight strings of three solar panels totaling twenty-four and delivering 123V and 80A.
Should you live in a very sunny region, adding more solar panels would not be advisable as the MPPT will limit the solar production, preventing additional power from being used and essentially wasting energy generated by the solar panels.
When the solar panels are in a less-than-ideal position or are located in a region known for lower solar production, you may be advised to exceed the recommended maximum number of solar panels to compensate for the poor solar production conditions. Consult a specialist before doing so.
Inverter Considerations For Use With Battery Backup
Battery storage is essential to your solar system when running your home or RV off the grid. However, grid-tied solar systems have traditionally been used without batteries.
In contrast, hybrid solar systems are used with battery backup to store power for use when solar or grid power is unavailable.
Batteries are charged by solar panels or a combination of grid and solar power, depending on the inverter type.
Most inverters used to power homes are 48-volt inverters, meaning the power supplied by the batteries is 48V DC power. The type of inverter should thus have battery charging capabilities.
Most lithium-ion, gel batteries, and traditional lead-acid batteries are connected in various series or parallel connections depending on the system’s required power output.
When connecting multiple batteries to an inverter, remember that the more batteries you must keep charged, the more solar panels you’ll need while keeping within the inverters charging capabilities.
When more batteries are required for evening use, a larger inverter may accommodate the increased number of solar panels needed to run the home.
Smaller solar systems, such as those used in RVs and batteries in portable units, are generally 12V or 24V.
Solar Inverter Efficiency
Solar inverters are electrically powered devices that require electricity to run. They’re cleverly made so solar panels, batteries, and even gas, natural gas, or diesel engine generators can be powered from the grid.
The amount of electricity the inverter consumes while running is the efficiency rating expressed as a percentage.
This percentage is crucial as you don’t want to sacrifice too much solar panels’ power to run the inverter.
When running a grid-tied or hybrid solar system, the inverter efficiency is less critical, but every bit of available power is essential when running off the grid.
Most modern inverters are very efficient, running at above 95% efficiency, but it pays to get the most efficient inverter you can afford.
Can An Inverter Be Too Big?
The inverter uses power to generate power. Using an inverter that is too big for your solar array will result in the inverter losing efficiency. Larger inverters also cost more than smaller ones, so if the inverter is not used optimally, you may find that keeping the system running uses more grid power than needed.
How Do I Calculate What Size Inverter I Need?
Determining the power requirement is the key to correctly sizing the inverter to your needs. The power required determines the size of the inverter needed to supply the home or RV. Additional factors such as the size of the PV array you need to accommodate, prevailing weather conditions, and location must be factored into the calculation.
Can you Undersize A Solar Inverter?
Undersizing an inverter (compared to the maximum output the solar panels can generate) is primarily done to save money on the initial installation cost. A solar inverter can be undersized in two ways, buying a smaller inverter or increasing the number of existing solar panels.
Undersizing the inverter results in more power clipping, meaning that the inverter discards excessive power generated by the solar panels.
🔑 Key Takeaways
Determining the size of the inverter you need is determined by a few critical factors:
1️⃣ Most importantly, you know how much power the home, RV, or portable inverter unit needs.
2️⃣ Once you know the expected power consumption, you’ll learn how much ability the inverter must be able to deliver.
3️⃣ Most inverters are solar panel and battery compatible, so these need to be factored into the equation.
Once the constant power requirement is known, an inverter meeting the DC input requirements from the solar panels and the battery can be sourced.
💡 Determining your power consumption is done by enlisting a solar system installer who will use monitoring equipment to determine the peak power required. Alternately add up the wattage, including the surge power of compressors and motors, and calculate your need from the information.