Enter your daily appliance loads to get recommended panel wattage, battery bank Ah, and charge controller size.
An off-grid solar system has four core components working together: panels that convert sunlight to DC electricity, a charge controller that regulates how that electricity flows into your batteries, a battery bank that stores energy for use at night or on cloudy days, and an inverter that converts stored DC power into the AC power your appliances use. Sizing each component correctly requires understanding your daily energy consumption in watt-hours, your location's peak sun hours, and how many days of autonomy — cloudy days the system must cover without solar input — you need to plan for.
The most common mistake in DIY solar design is undersizing the battery bank and oversizing the panels. Panels generate power only when the sun is shining; the battery bank is what actually powers your home the other 16–20 hours of the day. A well-designed system sizes the battery bank to cover 2–3 days of autonomy at 50% depth of discharge for lithium batteries (or 20–30% for lead acid), then sizes the panel array to fully recharge that bank during a single average solar day.
Lithium iron phosphate (LiFePO4) batteries are now the standard recommendation for off-grid solar. They offer 3,000–6,000 full charge cycles (10–15 years of daily use), can be discharged to 80–90% depth without damage, weigh significantly less than lead acid, and require no maintenance. Lead acid (flooded, AGM, or gel) batteries cost less upfront but degrade rapidly if discharged below 50%, require more frequent replacement, and lose capacity in cold temperatures. For any system larger than a small cabin setup, the lifetime cost of LiFePO4 is equal to or lower than lead acid when replacement cycles are accounted for.
The number of panels depends on your daily energy consumption and your location's peak sun hours. Start by adding up the watt-hours per day of every device you use — a typical American home uses 30–40 kWh per day, but an efficient off-grid home can run comfortably on 5–10 kWh per day. Divide your daily watt-hour total by your location's peak sun hours (typically 3.5–6 hours depending on region), then divide by the panel wattage. Add 25% to account for system losses from wiring, charge controller inefficiency, temperature derating, and panel soiling. A moderate off-grid cabin consuming 5 kWh per day in a region with 5 peak sun hours would need roughly 1,200–1,500 watts of panels.
Size your battery bank to cover 2–3 days of energy consumption without solar input, operating at 50% depth of discharge for LiFePO4 (or 50% for lead acid, since you should not discharge below that). For a system consuming 5 kWh per day with 2 days of autonomy and LiFePO4 batteries at 80% usable depth: 5 kWh × 2 days = 10 kWh needed, divided by 0.80 usable depth = 12.5 kWh total battery capacity. At a 48V system voltage that equals 260 amp-hours. Always design for the battery size you can actually afford to maintain — a larger bank that you cannot replace when it fails is worse than a smaller system that stays reliable for 10 years.
Peak sun hours are not total daylight hours — they represent the equivalent number of hours per day when sunlight intensity averages 1,000 watts per square meter, which is the standard test condition for solar panels. The American Southwest (Arizona, Nevada, southern California) receives 6–7 peak sun hours per day on average. The Pacific Northwest and New England average 3.5–4.5 hours. The Midwest and mid-Atlantic states receive 4–5 hours. Northern states and Canada see 3–4 hours. Seasonal variation matters: a Montana system that gets 5.5 peak sun hours in July may only get 2.5 in December. Design your system for the worst-case winter month if year-round reliability is required.
A grid-tied solar system connects to the utility grid and exports excess power for a bill credit (net metering) while drawing from the grid at night or on cloudy days. Grid-tie systems are simpler, less expensive, and require no batteries — but they produce no power during a grid outage unless you add a battery backup. Off-grid systems are completely independent of the utility, relying entirely on panels and a battery bank. They cost more per kilowatt of capacity due to the battery requirement but provide true energy independence. A hybrid system connects to the grid but also includes batteries, allowing backup power during outages while still benefiting from net metering. Off-grid makes practical sense primarily for locations where utility connection costs exceed $15,000–25,000 or where independence is a priority regardless of cost.
Your inverter must be rated to handle the peak simultaneous wattage of everything running at once, including motor startup surges. Add the running watts of all devices you might run simultaneously, then identify the single largest motor load (well pump, air conditioner, refrigerator) and note its startup wattage — typically 2 to 3 times its running wattage. Your inverter's continuous rating must exceed your total running load, and its surge rating (most quality inverters can handle 2x their continuous rating for a few seconds) must cover the peak startup draw. For a household system, a 3,000-watt continuous inverter covers most critical loads; a 5,000–6,000 watt unit handles a broader range including small air conditioners and well pumps.