Step 1. Off Grid Load Calculation: Assess Your Power Needs

This is the boring step. It's also the one that decides whether your whole off grid solar system works or doesn't.

An off grid load calculation is just a list. Every device you own, how many watts it pulls and how many hours it runs. Add it up and you have an estimate of how much energy your household needs per day.

Skip it and you'll buy the wrong battery bank, the wrong inverter and the wrong number of panels. Money down the drain.

Key Components Of An Off Grid System

One daily figure sets the size and capacity of almost every part:

• The battery bank, which stores tonight's power.
• The inverter, which turns battery power into the kind your appliances use.
• The solar array and charge controller, which refill the battery each day.
• The wiring, which gets thicker as the current grows (Step 4).

So let's get that figure right.

Calculate Your Energy Usage With A Notebook

Walk around and list every appliance in your household that runs on electricity. Lights, fridge, TV, laptop, phone chargers, washing machine, the wifi router blinking in the corner. Every single thing.

For each one, write down two things.

How many watts it uses. This is the input power, usually printed on the bottom or back, on a sticker called the nameplate. If you only find volts and amps, multiply them and you have watts.

How many hours a day it runs. A laptop maybe 5 hours, a TV maybe 4. Include the small stuff too. A fridge is sneaky. It's plugged in all day, but its motor switches on and off to stay cold, so a common rule is to count a third of the time. That's 8 hours out of 24.

The Load Calculation Formula, With A Sample Result

For each item: watts × hours = watt-hours per day. Add every line together and divide by 1000 to get kilowatt-hours (kWh), the unit most batteries are sold in.

Here's a sample list. It's the same one our energy use calculator starts with, so you can open it, enter your own devices and watch the result change.

Is 3.1 kWh a lot? The average American home bought about 899 kWh a month in 2022. That's about 30 kWh per day, almost ten times this list.

That gap is the secret of good off grid solar design. Every kWh you don't use is a piece of battery and solar array you don't have to buy.

Inverter Sizing For Surges

How big a surge does your inverter need to handle? It needs two ratings. A continuous one that covers everything running at once with 25 percent to spare, and a peak one big enough for motors starting up.

Motors are the troublemakers. When a fridge, pump or power tool switches on, the motor can grab 5 to 8 times its running current for a moment. Electricians call that "locked rotor current", which is just a fancy name for the big gulp a motor takes before it gets spinning.

So add up the power of the devices that could run together to determine your peak demand, add 25 percent for the continuous rating, and check the peak rating separately. The inverter size calculator does both checks for you.

Safety Margins For Energy Losses

Nothing has perfect efficiency, so a good off grid system design plans for losses. A good inverter, like Victron's MultiPlus-II, tops out at 93 to 96 percent, so a few percent of every kWh turns into warmth before it reaches your fridge. The wiring loses a little, and the battery loses a bit every time it charges and discharges.

A simple rule of thumb: add about 20 percent to your daily figure. Our 3.1 kWh list becomes about 3.7 kWh to plan with.

Battery Bank Size: How Much Battery Capacity Do I Need To Go Off-Grid?

Short answer: your daily kWh, times how many days you want to last without sun, divided by how deep you can safely drain the battery.

Lithium (LiFePO4) batteries are often planned at 80 percent depth of discharge, which means you only use 80 percent of the box. So 3.1 kWh per day for 2 cloudy days is 6.2 kWh of storage, and divided by 0.8 that's a battery bank with about 7.75 kWh of capacity.

The battery calculator runs this for you and turns it into amp-hours at 12, 24 or 48 volts. For long gray periods, some people add a backup generator instead of a giant battery. Choosing the actual battery is Step 2.

From Your Load List To Off Grid Solar System Sizing

The same daily figure decides how big your solar array has to be, because the panels have to produce enough to charge the battery back up every day. Fewer kWh means fewer panels, a smaller charge controller and a cheaper installation.

Solar production depends on your location too, because a sunny desert makes far more power than a gray northern town. Peak sun hours tell you how much your spot gets, and this guide to your solar energy needs walks through the math. If you'd rather skip it, the solar panel calculator will estimate the panel count for your off grid solar system.

The Final Check: Why It Pays To Be Honest

People almost always underestimate their energy usage. They forget the laundry machine. They forget the coffee maker. They guess low because they're hopeful.

Then they buy too small a system, the lights go out in January, and they have to spend more money fixing it.

Take a full week to track your real electricity use, not a guess, and include the small stuff. The notebook is the cheapest tool in this whole project, and the most reliable one.

→ Calculate your daily energy use

→ Read more about figuring out your wattage

→ Read more about picking a battery

→ Read more about inverters

Sources

• Virginia Cooperative Extension, Estimating Appliance and Home Electronic Energy Use
• US Energy Information Administration, How much electricity does an American home use?
• Michigan State University, Electrical Tech Note 314: Induction Motor Characteristics
• Victron Energy, MultiPlus-II inverter/charger datasheet
• Victron Energy, Lithium iron phosphate batteries Smart datasheet