Solar Batteries - When it makes sense, how to build for offgrid and more!

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Solar Dominates, Batteries are Needed

Batteries, and more specifically, energy storage will be the hot energy topic of conversation and R&D throughout the 20s... not solar. Solar has already proven itself to be the most cost efficient energy generator.

energy cost comparison chart

The biggest problem with solar is not the cost to produce the energy, but what to do with the energy at the time of production.

Solar produces the majority of its energy during the same 4-6 hours every day - we call it peak sun hours. However, us pesky humans tend to have the highest energy demands in the evening, when the sun don't shine.

This is where batteries dominate. We overproduce in the midday and batteries help spread the energy out into the evening hours and early morning, until our panels can begin generating all over again. 

Batteries should be viewed as a solution for 2 scenarios.

  1. Time of Use - Spreading out your energy production throughout the day
  2. Off Grid - Complete energy independence, large battery systems to literally and figuratively weather storms

Batteries for Time of Use

One of the most common requests from Project Solar customers is for us to sell them a battery to go "off grid".

The reality is, going off grid requires much more than a single battery - more on this later in the "off grid" section. 

People generally add 5-20kW worth of batteries, primarily for optimizing time of use.

The majority of homes are connected to utility grids that charge different rates based on the time of day the energy is being used. So early early morning or mid day are usually the cheapest times, and then energy rates can skyrocket in peak hours, often 5-10pm... the time when people are home from work using their TVs, dryers, AC, and other power equipment. 

Most homes size their solar systems to generate close to as many kWh's a month as they use, but that leaves them over producing at the time of production, and under producing at times when they use it most. Batteries smooth this out, and algorithmically can optimize when to use battery power to minimize your pull on the grid during the expensive "peak" times, resulting in significant money savings. 

If your daily consumption is 40kWh and you have an 9kW system, your solar should generate enough energy to offset your bill, if you have a 1 to 1 net metering agreement (9kW * 4.5 sun hours = 40.5kWh). For those with low net metering rates (looking at you AZ), pairing with a battery should make it so YOUR solar generated energy last's the entire day. 

Without a battery, you would sell the excess energy back at wholesale rates, then buy it back at retail rates.

Understanding the timing of energy demand through time use data: Time of  the day dependence of social practices - ScienceDirect

Cost of Batteries

Batteries are great... in theory. Currently, their price is horribly prohibitive. The two most popular battery solutions, Tesla and Enphase, are about $1,000 per kW of storage or about $1/watt. 

A general estimate for how much battery capacity you will need is for every 5kW of solar, you'd need 10kW of storage. Other details like amp load of heavy duty equipment may affect this number, especially on the smaller single 10kW battery systems. 

Keep in mind, adding a battery, especially relative to our low cost solar solution at Project Solar, can equate to 2-3x more total expense. 

After incentive example - a 10kW system with Project Solar fully installed would cost about $14,000. This would require about 20kW of battery storage capacity, which would add about $18,000, bringing your total to $32,000. 

Determining if batteries makes financial sense is the big question. For most, it does not, when strictly reviewing ROI. A general rule of thumb for determining IF a battery makes sense for you financially is to subtract your kWh rate from your net metering credit rate (in UT it's 10¢ - 6¢ leaving a 4¢ net). If the net rate is lower than the 6¢, then we'd advise no battery. Remember, in most states with tiered prices based on consumption, your overage will most likely be billed in the lowest and cheapest tier. Use this lower price as the starting point. 

Sizing for Off Grid

With the recent chaos in Texas and the frequent blackouts in Northern California, demand for "off grid" has never been higher. 

However, we need to define what "off grid" actually means. 

Unless you are literally disconnected from the grid, fully reliant on solar and batteries, you are not considered off grid. 

Having 1-3 batteries (totally 10-30kW of storage) could certainly cover the majority of your energy usage, spreading out what you collect from solar into the evening and early morning when the sun is not shining, but unless you have enough capacity to cover you through a winter storm, you won't have enough to be "off grid" and truly disconnected from city power. You'll still be 90% + (weather permitting) energy independent, but off grid typically requires a much heftier investment. 

The rule of thumb when designing an off grid system is 2-3 days worth of storage. To calculate this, simply look at your monthly kWh usage and divide by 30 to get your daily usage rate. Then multiply by 2-3 (days) and that is your storage needs. 

For example, I use about 2100kWh a month at my house. 2100/30 = 70kWh a day on average. Multiply that by 2.5 (I'm in UT which is relatively sunny), that equates to 175 kWh of needed storage. At $1000/watt, that's $175,000 if I opt for standard lithium ion batteries. 

Most people looking for true off grid solutions do so for mountain homes where grid tie is not even an option if they wanted. The gome size is usually much smaller, AND most will opt for lead acid batteries over lithium ion. A little more high maintenance and not nearly the longevity of lithium ion, but way cheaper. 

off grid solar