How to Size a Solar Power System: Load, Battery, Panels, and Inverter

Sizing a solar power system means answering four questions in order: how much energy do you use, how much of it do you want to store, how many panels does it take to refill that storage, and what size inverter moves it all safely. Skip a step, or size one part off a guess, and the rest of the system inherits that mistake.

This guide walks through the same four steps a real installer works through, using plain math you can do yourself. The last section covers something just as important: the point where ballpark planning has to stop and your specific equipment's real numbers take over.

Step 1: Start With Your Daily Load

Everything else is sized from this number. List every device you want the system to run, how many watts it uses, and how many hours a day it actually runs. Multiply watts by hours for each device, then add them all up.

• A fridge running 24 hours a day at 100W uses 2,400 Wh per day.
• Three 60W lights running 5 hours a day use 900 Wh per day.
• A well pump running 1 hour a day at 800W uses 800 Wh per day.

Add every device the same way, then convert the total to kWh per day by dividing by 1,000. This is the same Device, Qty, Watts, Hours approach our power station sizing calculator already uses, it works the same way whether you're sizing a portable power station or a full home system.

Step 2: Size Your Battery Bank

Your battery bank needs to hold enough usable energy to cover your daily load for as many days of backup as you want, even on a cloudy day with no solar input.

Usable battery capacity needed = daily kWh × days of backup ÷ depth of discharge

Depth of discharge (DoD) is how much of the battery's rated capacity you can safely use. Most lithium (LiFePO4) batteries allow 80 to 100 percent DoD. Lead-acid batteries are usually limited to around 50 percent to avoid shortening their life. That difference matters: a lead-acid bank often needs roughly double the rated capacity of a lithium bank to deliver the same usable energy.

Battery banks also step up in nominal voltage as systems get bigger, commonly 12V for small systems, 24V for mid-size systems, and 48V for whole-home systems. Higher voltage means lower current for the same power, which generally means thinner, cheaper wiring for a large system.

Step 3: Estimate Your Panel Count and Wattage

Panels need to replace what the battery bank gives up each day, plus cover system losses along the way.

Array wattage needed = daily kWh ÷ average peak sun hours for your area ÷ system loss factor

Peak sun hours are not the same as daylight hours, they measure the equivalent hours of full-strength sunlight your location gets on average, and they vary by region and season. System loss factor accounts for wiring resistance, panel temperature, dust, and inverter conversion loss, commonly estimated between 0.75 and 0.8. Once you have your total array wattage, divide by your chosen panel's wattage rating for a rough panel count.

Step 4: Estimate Your Inverter Size

Size the inverter to your highest expected continuous load, not your daily average. If you might run several things at once, add those wattages together for your peak continuous figure.

Then check surge capacity separately. Motors, compressors, and pumps briefly draw far more than their running wattage the instant they start, often three to five times more for a second or two. A fridge that runs at 100W might momentarily pull 400W or more on startup. An inverter sized only for continuous load can brown out or shut off at exactly that moment, even though the average math looked fine.

Inverter selection also depends on where in the world the system is installed. Most of North America runs 120V/240V at 60Hz, while most of the rest of the world runs 220 to 240V at 50Hz, with a few regions using other combinations entirely. An inverter has to be built or configured for the local voltage and frequency, this is not a setting you can change after the fact on most equipment.

Where a Generator Fits In

Many off-grid and hybrid systems pair batteries with a backup generator instead of sizing for worst-case weather alone. The typical setup: batteries handle everyday use, and the generator starts automatically when the battery drops to a set charge level, recharges the bank, then shuts off. This can meaningfully shrink the battery and panel sizing needed, at the cost of fuel use and generator wear during long stretches of poor sun.

Getting a generator to start and stop automatically is its own separate problem from getting it to charge the batteries cleanly. Some generators have a genuine remote-start interface built in; many do not, despite looking similar on the outside. And even a generator that starts on command still needs its output voltage, frequency, and waveform checked against what the inverter will actually accept. Treat generator integration as something to verify with real equipment, not something to assume from a spec sheet.

Why Your Exact Wiring Still Depends on Real Numbers

Everything above gets you a solid planning estimate, how much battery, how many panels, roughly what size inverter. What it deliberately does not give you is your exact wiring configuration, and that's on purpose.

Two identical-looking solar setups can need completely different wiring. A panel's voltage output changes with temperature, colder panels actually produce higher voltage, and every inverter has its own safe voltage window it can accept. Whether your panels should be wired in series, in parallel, or some combination of both depends on your specific panel's datasheet numbers checked against your specific inverter's limits. Get that wrong and the risk isn't a slightly undersized system, it's equipment damage or a real safety hazard.

The only way to know your exact series-and-parallel configuration is to check your specific panel against your specific inverter, or have someone qualified do that check for you. That's not a step to skip past, it's the actual engineering work, and it's worth a second set of eyes before anything gets wired or energized.

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