Size Solar Panels & Batteries for Off-Grid Systems

Why Proper Solar Sizing Matters

Under-size a solar system and you'll be dealing with dead batteries and angry users. Over-size it and you've just burned cash on unnecessary panels and batteries — which also means extra weight, a real headache when you're hauling gear to a remote mountaintop repeater site or trying to keep a field sensor deployment lightweight. Getting the math right from the start saves you from both of these failure modes.

The core sizing problem is really just an energy balance. You need to generate at least as much energy per day as you consume, plus some margin for cloudy weather and the inevitable losses in real hardware. Most engineers skip the careful analysis here and just guess — then wonder why their system dies every winter or why they spent twice what they needed to. Let's walk through the engineering properly, then work a real example with actual numbers.

The Energy Balance

Start with the fundamental equation. Your daily energy demand $E_{\text{day}}$ in watt-hours is:

where $P_{\text{load}}$ is the average load power in watts and $t_{\text{on}}$ is how many hours per day the load actually runs. If you've got something running 24/7, then $t_{\text{on}} = 24$. Pretty straightforward.

Now here's where it gets interesting. The solar panel has to produce this energy during whatever sunlight hours are available. The key metric is Peak Sun Hours (PSH) — this is the equivalent number of hours per day at full $1000 \, \text{W/m}^2$ irradiance. Think of it as compressing the day's varying sunlight into an equivalent period at maximum intensity. This number varies wildly depending on where you are and what the weather's like:

• Low (cloudy/northern climates): around 3 hours
• Average (temperate zones): roughly 5 hours
• High (desert/equatorial): up to 7 hours or more

The required panel wattage $P_{\text{panel}}$ becomes:

That $\eta_{\text{sys}}$ term is crucial — it accounts for all the real-world losses you'll encounter. Wiring resistance, charge controller inefficiency, temperature derating of the panels, and gradual panel degradation over time. A typical system efficiency factor runs between $0.75$ and $0.85$. Our calculator uses $0.80$ as a practical middle ground that works for most installations without being overly pessimistic.

Sizing the Battery Bank

Batteries are what keep your system alive when the sun isn't cooperating. The capacity you need depends on how many days of autonomy you want — basically, how many consecutive cloudy days can your system survive without any solar input at all.

The battery capacity equation is:

Here $V_{\text{sys}}$ is your system voltage (typically 12V, 24V, or 48V) and DOD is the maximum depth of discharge you'll allow. This number depends heavily on your battery chemistry. For traditional lead-acid batteries, you typically limit DOD to $0.50$ to avoid killing them prematurely — discharge them deeper than that regularly and you'll be replacing them way sooner than you'd like. LiFePO₄ batteries are more forgiving and you can push to $0.80$ or even higher. Our calculator assumes $0.50$ as the conservative default, which works for any chemistry and gives you room to adjust based on what you're actually using.

Charge Controller Current

The charge controller sits between your panels and battery, regulating the current flow to prevent overcharge. You need to size it properly or you'll either damage batteries or waste panel capacity. The minimum charge controller current rating is:

That $1.25$ safety factor comes straight from NEC 690.8 and it's there for good reason. Solar panels can briefly exceed their rated output on cold, clear days, especially with cloud-edge reflection effects. I've seen panels spike 15-20% above their nameplate rating under the right conditions. Size your controller for the worst case, not the typical case.

Worked Example: Remote Weather Station

Let's size a complete system for a remote weather station that draws 15 W continuously. This is a realistic scenario — you've got sensors, a microcontroller, maybe a small radio transmitter, all running 24/7 in the field.

Given parameters:

• Load power: $15 \, \text{W}$
• Duty cycle: 24 hours/day (continuous operation)
• Location: temperate climate (Average PSH = 5)
• System voltage: $12 \, \text{V}$
• Days of autonomy: 3
• System efficiency: $0.80$
• Maximum DOD: $0.50$

Step 1 — Calculate daily energy consumption:

So you're burning through 360 watt-hours every day. Not huge, but it adds up.

Step 2 — Determine required panel wattage:

A single 100 W panel is the obvious choice here. It gives you about 11% margin over the calculated minimum, which is good practice. That extra headroom accounts for panel degradation over time and worse-than-average weather.

Step 3 — Calculate panel current into the battery:

This is the current that'll flow from your panel into the charge controller during peak sun.

Step 4 — Size the battery bank:

Two 100 Ah lead-acid deep-cycle batteries in parallel would cover this requirement nicely. You could also use a single 200 Ah battery if you can find one at a reasonable price. For a more compact installation, a single 180 Ah LiFePO₄ battery would work beautifully, though it'll cost you more up front.

Step 5 — Select charge controller current rating:

A 10 A PWM or MPPT charge controller handles this comfortably with margin. If you actually go with a 100 W panel (which typically has an $I_{\text{mp}}$ around 5.5 A at its maximum power point when feeding an MPPT controller), a 10 A controller is more than adequate. You could even get away with an 8 A controller in a pinch, but why cut it that close?

Practical Design Tips

System voltage matters more than most people think. Higher voltages mean lower currents for the same power, which translates to thinner wires and dramatically reduced $I^2R$ losses. This becomes critical when you've got cable runs longer than a few meters. A 48V system cuts your current to one-quarter of what you'd see at 12V for the same power level. The wire savings alone can pay for the voltage conversion in larger systems. Don't cheap out on autonomy days. For critical systems — telecom repeaters, medical refrigeration, security cameras, anything where downtime costs you real money or safety — you want 3 to 5 days of autonomy as standard. For non-critical hobby projects or experimental setups, you might get away with 1 to 2 days. But be honest with yourself about what happens if the system goes dark. Account for seasonal variation if you're designing for year-round operation. At temperate latitudes, winter PSH values can drop to 2 or 3 hours, sometimes less. If you size your system based on the annual average of 5 hours, you'll be fine in summer but struggling in December. The calculator's "Low" PSH setting is perfect for this worst-case analysis. Size for winter, enjoy the surplus in summer. Temperature kills panel output. Crystalline silicon panels lose roughly $0.4\%$ per degree Celsius above 25°C. In a hot desert environment where cell temperatures hit 60°C, your 100 W panel might only deliver 85 W. The system efficiency factor partially covers this, but for extreme environments — deserts, tropical installations, anywhere with sustained high temperatures — add explicit derating. I've seen systems in Arizona underperform by 20% because nobody accounted for the temperature coefficient properly. MPPT versus PWM controllers. For small 12V systems, a simple PWM controller is often fine and costs less. But if you're running higher voltages or have significant panel-to-battery voltage mismatch, an MPPT controller extracts 20-30% more energy from the same panels. The extra cost pays for itself in reduced panel requirements.

Try It Yourself

Skip the spreadsheet headache and open the Solar Panel Sizing Calculator to run your own numbers. Plug in your load power, select your peak sun hours and system voltage, set your autonomy requirement, and you'll get panel wattage, battery capacity, and charge controller current instantly. It's the fastest way to sanity-check a design before you start sourcing components or committing to a particular configuration. Tweak the parameters, see what happens to your sizing, and find the sweet spot between cost and reliability for your specific application.