Sizing guide
Use this step-by-step sizing flow: calculate daily energy use, size panels for sun hours, choose battery capacity, and confirm inverter and wiring limits.
List every device you want to power, then multiply watts by hours used per day. Add up the results for total daily energy use in watt-hours (Wh).
Example: 100W x 5 hours = 500Wh
For appliances that cycle on and off (like fridges), use the average power or daily energy if available. This step is the foundation for every other calculation.
Peak sun hours is the average daily solar energy for your location. It compresses daily sunlight into an equivalent number of full-sun hours. A typical range might be 3 to 6 depending on climate and season.
Use conservative numbers for winter or cloudy seasons. If you plan for year-round reliability, size for the weakest months.
Write down the values you are using for sun hours, loads, and battery assumptions. This makes it easier to change one variable at a time and see the impact on panel and battery size. It also helps when you compare different system voltages or panel configurations.
If you are unsure about a number, choose a conservative value and revisit it after you gather more data.
Divide your daily Wh by peak sun hours to estimate the panel watts needed, then adjust for losses. A common rule of thumb is to add 20 to 30 percent to account for inefficiencies.
Example: 2,000Wh / 4 sun hours = 500W; add 25% = 625W
Use the panel output calculator for a more detailed estimate.
Losses come from temperature, wiring, inverter efficiency, dust, and real-world conditions that are not captured in panel ratings. If you size too close to the theoretical number, you may fall short during hot or cloudy periods.
A 20 to 30 percent buffer is common for off-grid systems. Grid-tied systems can often use a smaller buffer if you have reliable utility power.
Battery sizing depends on how many hours or days you want to run without sun. Convert daily Wh into battery capacity, then adjust for depth of discharge and system voltage.
For example, a 2,000Wh daily load with one day of autonomy at 12V requires roughly 2,000Wh / 12V = 167Ah, then adjust for usable capacity. If your battery is rated for 50 percent usable depth of discharge, you would need about 334Ah.
Use the battery capacity calculator to work through the numbers.
Lead-acid batteries have lower usable capacity because deeper discharge shortens lifespan. Lithium batteries usually allow deeper discharge and higher usable energy, but cost more upfront. This choice changes the total battery size you need.
Compare options in li-ion vs lead-acid before finalizing capacity.
The inverter must cover your peak load plus any surge loads (motors, compressors). Add up the watts of devices that may run at the same time, then check the surge ratings.
A slightly larger inverter can be helpful, but oversizing too much can reduce efficiency at low loads. The inverter sizing guide walks through this step.
If most of your devices are DC (LED lighting, USB charging, small electronics), you may reduce inverter use and improve efficiency. AC appliances require an inverter, which adds losses. Understanding your load mix can change inverter size and daily energy estimates.
DC loads can simplify wiring on small systems and reduce standby losses.
Higher system voltage reduces current and wire size. Small systems often use 12V, while mid-size systems use 24V or 48V. Your inverter and battery choice set the voltage.
Real systems lose energy in wiring, charge controllers, and inverters. Add a buffer so the system still works on less-than-perfect days. A 20 to 30 percent margin is common for off-grid systems.
Also ensure wiring, fuses, and breakers are sized for the current. The wiring decisions checklist can help you plan safely.
Imagine a small off-grid setup for lights, a laptop, and a mini fridge:
Total daily use = 820Wh. If local sun hours are 4, panel watts = 820 / 4 = 205W. Add 25 percent = about 260W. For one day of battery backup at 12V with 50 percent usable depth, capacity = 820Wh / 12V = 68Ah, then doubled to 136Ah.
This is only an example. Your actual devices and weather will change the outcome.
Loads tend to increase over time. If you expect to add devices later, consider leaving room for extra panels or a larger battery bank. Oversizing wiring and protection early can make future upgrades simpler and safer.
Backup systems are often sized for critical loads only. Off-grid systems must cover all loads and poor weather, which increases panel and battery size. If you are only powering essential devices during outages, you can size much smaller.
Use-case guides like RV sizing and cabin sizing show how use case changes the numbers.
Electrical work can be hazardous. If your system involves service panels, battery banks, or high-voltage wiring, consult a licensed electrician or installer. Always follow manufacturer guidance and local code requirements.
Revisit sizing after a few months of real use. Actual loads often differ from estimates.
It depends on your daily Wh, sun hours, and system losses. Use the panel output calculator to estimate.
Yes, larger batteries reduce cycling depth, but they increase cost and charging requirements.
If you need year-round reliability, use winter sun hours or plan for backup power.
Plan for growth if possible. Leave room for extra panels and battery capacity if your budget allows.