Graham FisherBy Graham Fisher

Updated November 2026

A solar off grid system design calculation tells you how much solar array, battery storage, and charge control you need to power your loads without a utility connection. You will work out your daily energy use, size the battery bank for autonomy, then match panels and a charge controller to recharge it. The math is simple arithmetic, and the worked example below shows each step.

Step 1: Calculate Your Daily Energy Use

Start by listing every appliance you plan to run, its power draw in watts, and how many hours per day it operates. Multiply watts by hours to get watt-hours (Wh) per day for each item, then add them together. Use the nameplate rating or a plug-in watt meter for accuracy.

Shopping for one? See our guide: 6 Best DIY Solar Power System Kit Options for Off-Grid Homes (2026).

For example, a 60 W LED lighting circuit running 5 hours a day uses 300 Wh. A 50 W laptop running 4 hours uses 200 Wh. Your total so far is 500 Wh per day. Add a 20 percent margin for inverter losses and wiring, which brings the target to 600 Wh per day.

Step 2: Size the Battery Bank

Battery capacity depends on daily energy, the number of days of autonomy you want (days without sun), and the usable depth of discharge. Lithium iron phosphate (LiFePO4) batteries are often rated for about 80 to 90 percent usable depth, while flooded lead-acid is typically limited to about 50 percent.

The formula is: Battery Wh = Daily Wh × Autonomy days ÷ Depth of discharge. With 600 Wh per day, 2 days of autonomy, and 80 percent DoD, you get 600 × 2 ÷ 0.8 = 1,500 Wh. Divide by battery voltage to get amp-hours. For a 12 V system, 1,500 Wh ÷ 12 V = 125 Ah.

Step 3: Size the Solar Array

Next, work out how many panel watts you need to replace the daily energy. Divide daily Wh by your location’s peak sun hours (the equivalent hours of full-strength sunlight per day, which you can look up for your region). Then add a derate factor of about 0.75 to cover temperature, dust, and wiring losses.

Using 600 Wh per day and 4 peak sun hours: 600 ÷ 4 = 150 W, then 150 ÷ 0.75 = 200 W. Round up to the next practical panel size. Choosing a little extra capacity helps during cloudy stretches and keeps the battery from sitting low.

Step 4: Choose the Charge Controller and Inverter

The charge controller must handle the array’s current. Divide array watts by battery voltage, then add a 25 percent safety margin. For 200 W on a 12 V bank: 200 ÷ 12 = 16.7 A, and 16.7 × 1.25 ≈ 21 A. Choose a controller rated at 25 A or higher. MPPT controllers usually outperform PWM at this scale.

For the inverter, add up the simultaneous load you expect, then choose a continuous rating above it. If your peak load is 300 W, a 500 W inverter gives comfortable headroom for motor startup surges.

Worked Checklist

  1. List loads, watts, and hours; total daily Wh.
  2. Add 20 percent for losses.
  3. Set autonomy days and DoD; calculate battery Wh and Ah.
  4. Divide daily Wh by peak sun hours; divide by 0.75 for array watts.
  5. Size the charge controller at 125 percent of array current.
  6. Size the inverter above your peak simultaneous load.
Parameter Example Value
Daily energy with margin 600 Wh
Battery bank (12 V, 2 days, 80% DoD) 125 Ah
Solar array (4 peak sun hours) 200 W
Charge controller 25 A or higher

For a full walk-through with more variables, see Off-Grid Solar System Calculation: Step-by-Step Sizing for Beginners and Planning an Off-Grid Solar System: 8 Steps to Size It Right. Once the numbers are set, Wiring Diagram for Off-Grid Solar System: A Step-by-Step Guide shows how the parts connect.

Common Mistakes

  • Using nameplate watts instead of measured run time, which inflates or understates daily energy.
  • Ignoring battery depth of discharge and sizing the bank as if you can use all of it.
  • Forgetting that panel ratings assume lab conditions, so the derate factor matters.
  • Sizing the controller to panel watts without checking the battery voltage.
  • Skipping winter sun hours, which are often much lower than summer values.
  • Victron SmartSolar MPPT controller: a widely used MPPT controller that makes the current calculation in Step 4 straightforward to apply. View on Amazon
  • Kill A Watt P3 Meter: plugs between the wall and an appliance to measure real watts and run time for Step 1. View on Amazon

FAQ

How many peak sun hours should I use?

Use the average peak sun hours for your location’s worst month, typically winter, so the system performs year-round. Government solar resource maps list this value for most regions.

Why add a derate factor to the solar array?

Panels rarely produce their rated output because of heat, dust, wiring losses, and charge controller inefficiency. A derate factor of about 0.75 is a common planning value.

Should I consult a professional for system design?

For larger systems or any work involving household wiring and main breaker panels, consult a licensed electrician or qualified solar installer before building.

Conclusion

A solar off grid system design calculation is a sequence of simple steps: measure daily energy, size the battery for your autonomy, then match the array and controller. Double-check your peak sun hours and depth of discharge, since those two numbers most often throw off a design. Work through your own loads on paper before buying any hardware.

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Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our DIY solar power systems guides, checking product details against manufacturer specifications and owner feedback.