How to Calculate Solar Panel Battery and Inverter

solar panels on roof

Being ready to go off-grid first requires knowing what items are essential to power, how much electricity they use, and then balancing the needs of the solar system to power them and charge the batteries for night use. Knowing the necessary appliances and their wattage is the first step. Next is calculating what those numbers mean regarding the number of solar panels and batteries and the inverter size.

We will go through how to calculate your electrical needs and then explain what the numbers mean when paired with solar panels, batteries, and the inverter. Additionally, we will touch on peak sun hours and incorporate that into your calculations. Have your pen and paper handy, and let’s get started.

Figuring Out What to Power

Solar and battery power are balancing acts when calculating how much is enough. It’s possible to power your entire home or just power critical items when no power is coming from any other source, and the difference comes down to cost. Powering the whole home will require much more solar capacity, significantly larger or more batteries, and an inverter that can handle the size of the system needed. That means a much higher cost.

If you just want to be prepared for that emergency moment and not shell out a large amount of money for a whole-home solar and battery storage solution, start by figuring out what items are essential to power. For most, these items will include:

  • Well water pumps
  • Refrigeration
  • Some lighting
  • Heating (if electrical is an option)
  • Outlet access to charge power tools or power communications systems

Calculating Solar Needs

First, it is essential to know your sun peak hours (when the sun is high in the sky). Peak hours are the amount of time your panels are exposed to full sunlight. A system for off-grid use won’t be doing net metering with a utility company, which is when the utility gives you credits for solar system over-production. Therefore, it should not be calculated on the average peak hours per year. For any off-grid solution, base the system size on the amount of sunlight on short winter days.

Plan on seasonal variation in power usage. Winter means more power is used for lights, heating, etc. A system sized for winter extremes  will need to be bigger than an on-grid system. To size it properly, we will figure out what your area can produce and how much power you need and then calculate how many panels and batteries are required.

You’ve figured out what critical systems you want to power, now it’s time to figure out how much power they use. A simple way to estimate your needs is to look at the wattage specified on the device and multiply that by how long you expect to use the appliance each day. A refrigerator might specify it is a 200 watt unit, which means it uses approximately 200 watts an hour or 4800 watt-hours (WH) a day when used 24 hours a day. An 8 watt LED bulb that is on 4 hours a day, only uses 24WH a day.

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Under Creative Commons License Photo by Solar Energy World, LLC on Flickr

Solar Panel Amount

Add up all of the total WHs per day you calculated, and that is the number we need to match for our solar and battery system.

To figure out the number of solar panels needed, take the daily peak hours in winter and multiply that number by the wattage of the panel. Let’s say you have 3 hours of peak sun in winter, and the average solar panel produces 330 watts per hour, that is 990 watts per day. Multiply the 990 by .75 to account for system inefficiency, and you have a total of 743 watts per day. Now, take the total WHs you calculated and divide it by that daily panel output.

Daily WHs needed to power appliances divided by daily panel output = The number of panels needed.

If you calculate your daily needs as 7500WHs and divide the total power produced by a single panel of 743WHs, you’ll get 10.09. Round up, and there you have it; eleven 330W solar panels. That is a 3.63kW solar system (330 x 11 / 1000).

Under Creative Commons License Photo by gw68 on Flickr

Battery Count

Batteries are calculated in amp-hours (AH) which is a measurement of the battery’s capacity. To get the amp hours, we will take that total daily need of 7500WHs and divide it by the battery voltage (let’s assume 12-volt batteries) 7500/12 = 625AH. If we found some deep cycle batteries rated at 200AH each, it’d be best to get four of those. In cases of being off-grid, it is best to have 2 days’ worth of battery power available, so doubling four batteries to eight would be smart.

Where it gets tricky is that even if a battery is rated for 200AH, that doesn’t necessarily mean it will meet that rating. The discharge rate of the battery can be affected by things like temperature and battery age, thus overestimating  batteries is a good thing. In our scenario, purchasing 800AH for a need of 625AH is smart.

Under Creative Commons License Photo by So Gecko Ltd on Flickr

Inverter Size

A solar inverter is what converts DC electricity into AC. Simply match the inverter to the maximum hourly AC need or maximum load wattage. If every single appliance on your list was running at once, what would that wattage be? If you come up with 1000 watts, then an inverter rated for 1500 watts would be perfect.

This hypothetical system needs eleven 330 watt panels, eight 200AH batteries, and one 1500 watt inverter.

The numbers are estimates and will be rough, but we built in enough slack that your system should be more than enough to run the critical functions you have identified. Best of all, it will continue to do so for many years if necessary.

What do you think? Has this been helpful in figuring out the size of the system you’ll need? Let us know in the comments.

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