Ethan BrooksBy Ethan Brooks

Updated November 2026

To calculate an off-grid solar system, you total your daily energy use in watt-hours, add a buffer for cloudy days and inefficiency, then size the battery bank, solar array, inverter and wiring to match. This guide walks through each step with a worked example so you can check your own numbers before you buy anything. By the end, you will have a rough system size you can refine with real equipment specs.

Why Sizing Matters Before You Buy Anything

Most off-grid problems trace back to undersizing. A battery bank that is too small runs dry on the second cloudy day, and a panel array that is too small never fully charges it in winter. Oversizing wastes money, but undersizing leaves you without power when you need it. Careful math up front prevents both.

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

The calculation has a simple logic. Energy flows from panels to batteries to loads, and every conversion loses some energy along the way. You work backward from what you use, adding losses at each stage.

Step 1: Calculate Your Daily Energy Use

Start with a list of every device you plan to run off the system. For each one, you need three numbers: power draw in watts, hours of use per day, and quantity.

Multiply watts by hours to get watt-hours per day for each item. Then add them all together. Use the nameplate or label rating on each device, and be realistic about hours. A refrigerator does not run 24 hours at full draw, but it does cycle on and off, so many people use a rough average of around 8 to 12 hours of compressor run time per day for a standard unit.

Worked Example

Load Watts Hours/Day Watt-Hours/Day
LED lights (6 fixtures) 60 total 5 300
Refrigerator (efficient model) 80 running 10 800
Laptop and phone charging 65 total 4 260
Well pump (short cycles) 750 0.5 375
Washing machine (2 loads/week, averaged) 500 0.3 150
Total 1,885 Wh/day

In this example the household needs about 1,885 watt-hours per day. Your own list will differ, so build it carefully.

Step 2: Add Inverter and Wiring Losses

If any loads run on AC power, the inverter converts DC from the batteries, and that conversion is not perfectly efficient. Many inverters run around 85 to 92 percent efficient under typical load, so divide your AC load total by the efficiency figure to find the DC energy you must supply. Wire losses are usually small, often a few percent when wiring is sized correctly.

Using 88 percent for the inverter: 1,885 ÷ 0.88 ≈ 2,142 Wh per day from the battery bank. Round up to about 2,150 Wh for planning.

Step 3: Size the Battery Bank

Batteries store the energy for nights and cloudy stretches. The formula is:

  • Required usable capacity (Wh) = daily DC energy × days of autonomy
  • Nominal capacity (Wh) = usable capacity ÷ depth of discharge (DoD)
  • Amp-hours at system voltage = nominal Wh ÷ system voltage

Days of autonomy is how many days you want to run without sun. Two days is a common starting point for a backup-minded home. Depth of discharge depends on chemistry: lithium iron phosphate (LiFePO4) batteries are often rated for around 80 to 90 percent usable capacity, while flooded lead-acid is commonly limited to about 50 percent to preserve lifespan.

Example with LiFePO4 at 90 percent DoD and two days of autonomy: 2,150 × 2 = 4,300 Wh usable. 4,300 ÷ 0.90 ≈ 4,778 Wh nominal. On a 24-volt system, that is about 199 Ah. Round to the nearest available battery size, such as a 200 Ah 24V pack or a combination of smaller batteries.

Choosing System Voltage

Higher voltage reduces current for the same power, which means thinner wire and lower losses. Small systems under about 1,000 watts often run at 12V, mid-size systems at 24V, and larger homes at 48V. Match the voltage to your inverter, charge controller and battery options.

Step 4: Size the Solar Array

The array must replace the energy you use each day, plus charging losses, in the worst month of the year. Use peak sun hours, which is the equivalent number of full-sun hours your location receives on an average day in the least favorable season. Data from NREL’s PVWatts tool or local solar resource maps can give you this figure.

  • Array watts (STC) = daily DC energy ÷ (peak sun hours × system efficiency)

System efficiency combines charge controller, battery charging, temperature and dirt losses. A planning value of about 0.75 is common.

Example with 3 peak sun hours (winter month): 2,150 ÷ (3 × 0.75) ≈ 956 W. Round up to about 1,000 W of panels. Several 400 W panels in series-parallel would meet that target.

Step 5: Select the Charge Controller

The charge controller must handle the array’s current. Calculate it as array watts divided by system voltage, then add a margin. For a 1,000 W array on a 24V system, the controller needs to handle about 42 amps, so a 50 A controller is a reasonable choice. Also confirm that the controller’s maximum PV input voltage exceeds the open-circuit voltage of your panel string on the coldest day you expect. Cold temperatures raise voltage significantly.

Step 6: Size the Inverter

The inverter must handle your largest combined simultaneous load plus surge. Add the running watts of everything you might run at once, then add surge capacity for motors. A well pump or refrigerator can draw two to three times its running watts at startup. In the example, the pump and refrigerator could start together, so a 2,000 to 3,000 W continuous inverter with adequate surge rating would be appropriate.

Step 7: Size the Wiring and Breakers

Use wire gauge charts and voltage drop limits. A common target is 2 percent or less on DC runs from the array and batteries, and 3 percent on AC branch circuits. Higher currents need thicker wire. Fuse or breaker each conductor at the battery and at the panel string, following the manufacturer’s and local electrical code requirements.

Tools and Checklist

  1. Appliance list with watts (from labels or a plug-in power meter)
  2. Spreadsheet for daily watt-hour totals
  3. Peak sun hour data for your site
  4. Battery specs: nominal voltage, usable capacity, DoD, temperature limits
  5. Panel specs: Pmax, Voc, Isc, temperature coefficient
  6. Wire gauge and voltage drop chart
  7. Calculator, tape measure and a multimeter for verification

Common Mistakes

  • Using nameplate watts without checking real run time, which can inflate or understate loads.
  • Forgetting inverter losses and battery DoD, which leaves the bank short.
  • Sizing the array for summer sun instead of the worst winter month.
  • Ignoring cold-temperature voltage rise when choosing the controller.
  • Skipping surge capacity for well pumps, compressors and tools.
  • Undersizing wire, which causes voltage drop and wasted energy.

Tips for a Safer, More Reliable Build

Build in a 20 to 30 percent margin on the array if your budget and roof space allow, because real-world output often falls short of ideal ratings. Keep your load list updated, since adding one space heater can change the whole calculation. Read the manufacturer manuals for every component, and confirm local code requirements before installation.

Once you have a rough size, compare it against prepackaged designs and online tools. The internal guides below show how other builders approach the same numbers:

A plug-in power meter helps you measure real appliance draw instead of relying on labels, which often overstate or understate use. A clamp meter lets you check actual current on DC and AC circuits during commissioning. A multimeter is essential for confirming open-circuit voltage and battery state.

  • Kill A Watt P3 Electricity Usage Monitor: measures watts, watt-hours and run time for plug-in loads. View on Amazon
  • Klein Tools CL800 Digital Clamp Meter: checks current on AC circuits and helps verify loads during testing. View on Amazon
  • Fluke 117 Electricians Multimeter: confirms voltage, continuity and battery readings during setup. View on Amazon

FAQ

How many solar panels do I need for off-grid living?

It depends on daily energy use and peak sun hours. Divide your daily watt-hours by peak sun hours and an efficiency factor near 0.75 to get array watts, then divide by panel wattage to find the panel count.

What is depth of discharge and why does it matter?

Depth of discharge is the percentage of a battery’s capacity you use before recharging. Using a lower DoD extends battery life, so the battery bank must be larger than your usable energy needs.

Should I size for winter or summer?

Size the array for the month with the least sun, usually winter in most northern sites. Summer will then produce surplus energy, which is normal.

How many days of battery backup should I plan for?

Many off-grid homes plan for one to three days of autonomy. More days cost more in batteries but protect you during extended cloudy periods.

Do I need to include the inverter in the calculation?

Yes. Inverter efficiency reduces usable energy, and the inverter’s continuous and surge ratings must cover your loads.

Can I use a rule of thumb instead of detailed math?

Rules of thumb give only a starting point and often miss important details like surge loads or seasonal sun. Use them for rough planning, then complete the full calculation.

Should I hire a professional for the design?

For homes, larger systems, or any work involving your main electrical panel, consult a licensed electrician or qualified solar installer. Local codes and safety rules vary, and errors with high-voltage DC and battery systems can be dangerous.

Conclusion

Calculating an off-grid solar system comes down to a repeatable sequence: total your daily watt-hours, add inverter losses, size the battery bank for your chosen autonomy, then size the array against your worst-month sun. Charge controller, inverter and wiring follow from those numbers. Check each figure against real equipment specs, and leave room for growth.

Ready to choose one?

Ethan Brooks

About Ethan Brooks

Ethan Brooks 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.