Solar Battery Sizing Simplified

What size of solar battery customers need is a frequent question we hear. While there are a few things to know regarding solar batteries, at SunWatts, our goal is to make solar simple and we can also make solar batteries simple. Here are a few things to know to determine what size of solar battery you need for your solar array if the grid goes down. 

First, determine your daily kilo-watt hour (kWh) power consumption. The simplest way to do this is to look at a utility power bill because it should have the average daily kWh used for the recent billing cycle. Some statements will show the daily average for the most recent 12 months.

If the statement does not show the daily average, you can use 12 power bills to calculate the annual kWh consumption. Then divide that by 365 to calculate the daily average.

Next, you should allow for the depth-of-discharge (DoD) recommended by the manufacturer for your battery. Various lead-acid batteries are rated for 50% to 80% DoD, while lithium batteries are typically rated for 80%. Keeping some of the battery in reserve ensures there is always a charge and extends the battery life.

For example, if you need 10 kWh per day and the battery has 80% DoD, then you should look for a 12 kWh battery.

In addition, you may have to over-size the battery to allow for cold temperatures. In cold winter climates, this can range for 10% to 40%. Using the 10 kWh daily example, try this approach: 10 kWh * 1.2 (80% DoD) = 12 kWh * 1.4 (40% cold temp adjustment) = 16.8 kWh. Now, if you want multiple days of battery power storage, then simply multiply the kWh by the desired number of days.

To make it easier to compare, the batteries at SunWatts.com are rated by kWh. However, some batteries may be rated by their Amp hours (Ah) and Vols (V). Just multiply the Amp * Volts to calculate the Watts. For example, a 200 Amp hour battery operating at 12 Volts equals 2,400 watts, or 2.4 kWh. 

If you are unsure or need help, a Solar Design is included with our solar kits to make sure you get the right battery size for your home. The Solar Design helps you make an informed, unbiased decision to find the best solar array with battery storage at the lowest cost.

Solar Battery Size Reference Table