How Much Battery Capacity Do I Need For My Solar System?

We don’t know.

Don’t you hate that answer? But that’s the honest truth.

If someone says you need X kWh of batteries without asking about your consumption pattern and guarantees you can keep your appliances running through the night — they’re most likely trying to sell you thousands of dollars worth of battery capacity you don’t need.

Factors affecting how much battery capacity you need

Why do we care about battery capacity?

We aim to help our clients make their investment back in ~2-3 years (e.g., through lowering electricity bills). As such, we must make the best use of their money without compromising their quality of life. Since batteries are typically the most expensive component of a solar system, we ensure we don’t spend unnecessary money on them.

Batteries mostly store power for nighttime use. Occasionally, it provides surge power for starting monster appliances like power tools. Therefore, your lifestyle and habits may affect your solution’s battery capacity.

For example, you may need more battery capacity if:

• You don’t have a gravity-fed water tank and pump water directly from the well into the house. You’ll need battery capacity to power your pump if you water the plants and take long showers at night.

• You open and close the fridge 53 times during the evening to cook dinner and dig for ice cream. You’ll need more battery capacity than someone with the same fridge but doesn’t use it after sunset.

• You like to bake cookies with your electric oven in the evening or blast the air conditioning throughout the night. (Quick tip: Chill your bedroom during the day and turn off the A/C at night — the room will stay cool.)

You get the idea. You should reduce nighttime power consumption, especially for appliances that generate or remove heat, if you want to reduce the battery capacity you need and lower the cost of your solar system. 

How do we determine battery capacity for our clients?

It’s not magic. We ask questions, LISTEN, crunch the numbers, make an educated estimate, collect data, and iterate. Good old-fashioned scientific approach.

First, we talk to our clients to understand their consumption habits, usage patterns, and ideal scenarios. For example, having involuntary candlelight dinners now because you have a poorly designed solar solution doesn’t mean you want to do so after an upgrade.

We collect data about our clients’ appliances and estimate the battery capacity required. Since most people have a backup power source (e.g., the grid or generator), we adopt the minimum-viable approach — test the limit by dimensioning the initial literation as close to the point of running out as possible.

After the client puts the initial system through its paces, we review the data (e.g., when the grid or generator kicks in) to see how much more battery capacity is required. Since our solutions are modular and expandable, adding batteries won’t involve overhauling the system or causing hours-long disruptions.

But battery capacity isn’t the whole story…

We consider more than kWh when selecting a battery solution. Battery chemistry affects the efficiency, power density (i.e., how much space your battery pack takes), discharge curve, and the pack's longevity. Moreover, you may adjust parameters with the battery management system (BMS) to influence battery behaviors.

For example, a client wanted their battery to last as long as possible. We custom build a battery pack with high-endurance lithium iron phosphate (LFP) cells and set the BMS to charge the cells to 80% and discharge to 20% to maximize the pack’s lifespan. We also discuss the tradeoff: His system may switch back to the grid more often at night, and they’re ok with that.

When we build a mobile solar generator solution for a client to run his workshop while at home and power tools while on the go, we mix lithium cobalt cells with a more aggressive discharging curve (i.e., higher output for a shorter period) with those that discharge at a more sustained manner to get the best of both worlds.

A custom battery build in action.

Why aren’t more solar installers doing these custom solutions?

Don’t get us started. But the bottom line is that shoveling what’s already on the shelf to customers is way easier and more profitable than listening to each of them, measuring their power consumption, crunching the numbers, and customizing the solar system design and battery packs.

We’re kinda OCD and get a kick out of optimizing everything. So, we make a point of helping clients maximize their investment in solar power solutions and make their money back in 2-3 years, not 10-15. That means we must be very strategic about how we spend the money.

Plus, the typical solar installer doesn’t know much about battery technology… and we don’t blame them. The Samsungs and LGs of the world invest millions in developing battery technology for good reasons — it’s an expensive, highly technical, niche, and potentially hazardous💥 discipline.

So, how do we do this?

Bert has been the founder/head nerd of a battery technology company for 15 years, and gnawing on wires since he was two (orange cables with a big dose of plasticizers are his favorite). We have the knowledge, sources, AND willingness to work with each client to build custom solar and battery solutions for their requirements.

Simple, but not simple.

Let’s solve your solar challenges: Learn more about our off-grid solar solution and get in touch to see how we can help.