How to properly size a battery system

By John Connell, VP of Crown Battery’s SLI Products Group

Oversize your solar panels, inverters and batteries and you’ll waste money. Undersize your system and you’ll compromise battery life or run out of power — particularly on cloudy days. But if you find the “Goldilocks zone” of ample battery capacity, your solar-plus-storage project will work seamlessly.

By sizing your system using the ROI method — Reduce electrical demand; Observe power draw and calculate amp-hour requirements; and Incorporate a safety buffer (reserve capacity) — you can ensure the best combination of solar and storage will be used to fulfill your energy needs.

Step 1: Reduce electrical demand

Sizing a system without reducing demand is like heating a home in the winter while leaving the front door open. It may be possible, but it isn’t efficient or affordable.

In other words: By lowering your power needs, you can install smaller batteries and fewer solar panels.

First, tackle low-hanging fruit. Swap out incandescent lighting and power-guzzling appliances and electronics for ENERGY STAR models. Eliminate vampire loads with switchable or smart power strips. Set programmable and smart thermostats lower in winter and higher in summer.

Next, perform an energy audit to identify areas to improve your building’s efficiency. An important task is to seal and insulate. In North America, nearly half of household energy is used to heat and cool spaces. Reducing air infiltration and insulating attics, basements and walls minimizes thermal losses — and slashes energy consumption. Replacing low-insulation doors and windows can also make a big difference.

Finally, consider upgrading your HVAC system. High-efficiency air conditioners and furnaces can cut electricity loads significantly.

After reducing energy usage, you’re ready to size your system.

Step 2: Observe power draw and calculate amp-hour requirements

Battery sizing is a balancing act. You’ll want ample electrical reserves for nighttime power, power outages (if grid-tied) and days with low solar production.

To calculate your battery needs, you’ll need to determine how many kilowatt-hours (kWh) you need to store. For grid-tied systems, refer to the last 12 months of utility bills to estimate power requirements. Off-grid users can monitor appliance energy consumption with inexpensive plug-in power meters. For full-home monitoring, consider a submeter, advanced inverter or sensor-based system (examples: Efergy and Sense).

Battery manufacturers and distributors often have online calculators to help solar installers and customers simplify the calculation process, but in general the standard calculation for power requirements is: Watts = Amps x Volts

For example, if you require 1,000 watt-hours (1 kWh) a day and select a 12-volt battery bank, you’ll need 84 amp-hours of storage. In this example, the battery would be discharged 100% during every cycle, because there would be 0% reserve power. Turn on just one more LED bulb, and you’d run out of power every day. That’s why you also need a safety buffer.

Step 3: Incorporate a safety buffer (reserve capacity)

You wouldn’t drive your car for hours with the gas light on because running on fumes can leave you stranded.

It’s the same with solar power systems. The sun doesn’t always shine. HVAC loads can skyrocket during temperature extremes. And sometimes, a building’s occupants use more electricity than anticipated. Without reserves, a system will run out of electricity.

With a solar-plus-storage system, it is critical to design for proper surge capacity and depth of discharge.

Surge capacity is a measure of how well a battery handles high-draw loads. It’s important because some devices — air conditioning units, refrigerators and microwaves — draw two- to seven-times more electricity during startup. They will not work unless there’s ample surge capacity.

The many different battery chemistries vary in surge capacity. Lead-acid batteries have the highest surge capacity while lithium-ion systems are much lower, often below the threshold for air-conditioning startup. More lithium-ion batteries may be needed to meet electricity needs when compared with lead-acid.

Depth of discharge (DOD) refers to how deeply a battery can discharge, or drain, without compromising longevity. A battery labeled as 80% DOD battery means only 20% of capacity will remain. Some manufacturers size batteries for 100% DOD — the battery equivalent of running on fumes.

But be wary — a high DOD can leave you stranded. A high DOD means there is nearly zero reserve electricity for those startup loads, large appliances or days with decreased solar generation. High DOD can shorten battery life.

Renewable energy installers recommend sizing your system for twice the true amp hours determined in step 2 above. The formula is simple: 2 * [amp-hour requirements from step 2]

In our example system from step 2, the battery bank should be 12 V and 168 Ah (2 * 84 Ah). This will double our estimated amp-hour needs and ensure we stay below 50% DOD.

When shopping for batteries or planning a system, always specify no more than 50% DOD. Be sure to compare battery prices using that number. And beware of manufacturers that offer “lower” prices by selling undersized batteries that reduce or eliminate your safety buffer.

By following the ROI method, you can select the right-sized batteries for your solar system — and ensure the optimal performance and operation for many years.