The Not-So-Dark-Art of Right-Sizing Your Solar Power System

Many people with off-grid solar install their systems and hope for the best. They don’t have the information to improve performance if the solution isn’t generating enough power to meet their needs. 

If they have the resources, they may plunk down another $15k to upgrade the system (ouch, still!) If they don’t, it could mean reading a book by candlelight.

But it doesn’t have to be this way. Do you want to take the guesswork out of dimensioning your solar solution? There’s an app for that. 

You can collect data to see how your system generates power and determine if it’s appropriately sized or where the bottleneck is to boost performance cost-effectively. For example, we helped this client boost his system from 450W to 2,000W at one-third of the cost.

Below is a chart from the companion app of our Victron solar charge controller (SCC). The data is from a few hot, 100-degree summer days when we had two air conditioning units and a bunch of fans working hard all day on top of the usual appliances like the fridge, computers, and Starlink. The bar chart shows how the batteries charge:

Like humans, batteries eat faster when hungry (at the bulk charge state) and sip slower when they’re almost full (at the float charge state).

So, how do you read the bar chart to see if your system is properly dimensioned?

If the batteries constantly push the bulk charge state (i.e., the entire bar is white), your system isn’t generating enough power, and the batteries are gulping down every last drop the solar panels make. The batteries can’t reach full charge, which can lead to imbalances and reduced performance over time for lithium batteries and is very detrimental for lead-acid batteries.

The most likely solution to addressing this bottleneck is to add solar panels to increase production. 

On the other hand, you have an adequately sized system with enough solar panels, SCC capacity, and batteries if you achieve a float charge state almost daily (like the three middle bars in the chart above). 

Meanwhile, you have an oversized system if the bars aren’t showing a predominately bulk charge state (i.e., 60 to 80% of a bar is white)—you’ve probably paid for too much capacity, while the faster charging during the bulk charge state is stressing the batteries unnecessarily, impacting their longevity.

The chart below shows our system during a few cool sunny days when the solar panels produced a lot of power, but the usage was relatively low (i.e., no a/c or heat). It demonstrates how the system was over-dimensioned for our needs during spring and fall.  

Besides generating enough power, right-sizing your system allows the batteries to achieve the optimal state of charge (SoC) level for maximum performance and longevity. Since batteries are the most expensive part of a system, this information can help you save on the total long-term cost of using solar.

This chart doesn’t tell whether you have enough battery capacity for the night. However, a decent battery management system (BMS) for lithium-ion batteries can show you how full the batteries are as you use up power during the evening to understand if you have enough battery capacity.

Your system is properly dimensioned if you can comfortably cover your daily needs day and night and achieve a float charge state almost daily. If you’re reading a book by candlelight, you probably need more battery capacity.

There’ll be some trial and error, and that’s ok! We take this iterative approach to help our clients right-size their systems without paying for more than they need.

When we help clients diagnose their current solar setup, we may install a temporary system with an SCC and panels to measure voltage and current over a week or two to identify bottlenecks and make targeted improvements.

Let's say we see a bulk charge state all the time. We would add more solar panels until we achieve float charge states consistently to ensure the system is adequately sized without spending money on unnecessary capacity. 

Is your existing solar installation properly sized to meet your needs? We can upgrade your system to make it work harder, improve safety, and/or accommodate specific use cases. Learn more about our services and get in touch to see how we can help.