Calculator guide
Solar Kit Size Calculator: How to Size Panels, Batteries, and Inverter
The safest way to buy a solar kit is to size the system around the energy you actually need, then choose equipment that fits the load, climate, and backup goal.
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Estimate daily energy use, divide by peak sun hours, add a loss factor, then match battery storage and inverter size to the loads you want to run. For a full home, start with your electric bill. For cabins, RVs, sheds, and backup systems, start with an appliance list.
Step 1: estimate daily energy use
Solar sizing starts with watt-hours. A watt-hour is simply watts multiplied by time. A 100-watt appliance running for 5 hours uses about 500 watt-hours, or 0.5 kWh. If you have an electric bill, divide your monthly kWh by 30 to estimate daily use. If you only know the dollar amount, divide the bill by your electric rate to estimate monthly kWh.
Monthly kWh ÷ 30 = daily kWh
Example: a $220 bill at $0.16/kWh is about 1,375 kWh per month, or about 45.8 kWh per day. A 100% offset system would need to produce roughly that much energy on an average day before losses.
Step 2: convert daily use into solar panel watts
Peak sun hours are not the same as daylight hours. They are a simplified way to represent usable solar production. Many US locations fall somewhere around 3.5 to 5.5 peak sun hours depending on season, weather, shade, and geography.
The loss factor accounts for inverter losses, heat, wiring, panel angle, dust, imperfect weather, and real-world conditions. A 1.25 multiplier is a reasonable beginner-friendly starting point. Shaded roofs, snow, long wire runs, or less ideal panel angles may need more.
Step 3: size the battery bank
Batteries are not required for every grid-tied home solar system, but they matter for backup, off-grid, cabins, RVs, and hybrid systems. Battery sizing depends on how much energy you want to use when the sun is not producing and how deeply the battery can be discharged.
LiFePO₄ batteries are popular because they usually allow deeper usable discharge and more cycle life than lead-acid options. A lead-acid bank may need to be much larger on paper to provide the same usable energy.
Step 4: choose inverter size
The inverter must handle the loads that may run at the same time. It also needs surge capacity for motors and compressors. Refrigerators, freezers, well pumps, sump pumps, air conditioners, microwaves, and power tools can briefly demand much more power at startup than while running.
For small RV and cabin systems, a 1,000W to 3,000W inverter may be enough. For larger cabins, workshops, and home backup, 3,000W to 6,000W+ is more common. Full-home systems often need professional design and code review.
Step 5: pick 12V, 24V, or 48V
Small portable and RV systems often use 12V. Medium systems often make more sense at 24V. Larger systems usually benefit from 48V because lower current makes wiring and inverter design more manageable. If your estimate is pushing beyond a small weekend setup, do not assume 12V is the best choice just because it is familiar.
What to buy after sizing
- Solar panels or panel kit sized to your watt target
- Charge controller matched to panel voltage/current and battery voltage
- Battery bank with enough usable kWh
- Pure sine wave inverter with adequate continuous and surge output
- Mounting hardware, fuses/breakers, disconnects, wiring, and monitoring
Use the calculator first, then compare product categories. Specific product recommendations should come after the system size is clear.