How Many Solar Panels Do I Need? A California Sizing Guide

Most solar companies will size your system based on your annual electricity usage. If you use 10,000 kWh per year, they sell you a 7 to 8 kW system. This works for grid-tie installs where you export excess power and draw it back later.

Off-grid solar sizing is different. You cannot export to the grid, so you size for daily production and storage. You also prioritize your highest-value loads first -- heating, cooling, refrigeration -- instead of trying to cover everything at once. Here is how to calculate how many panels you actually need.

Step 1: Calculate Your Daily Energy Usage

Pull up your last 12 months of utility bills. Find your total annual kWh usage and divide by 365 to get daily average. A typical California home uses 20 to 30 kWh per day. Larger homes or homes with electric heating, cooling, or vehicle charging use 40 to 60 kWh per day.

If you are going off-grid or hybrid, focus on your essential loads instead of whole-house usage. Add up the wattage and daily runtime for your refrigerator, lights, HVAC, electronics, and other must-have loads. Most VoltSol customers find that heating and cooling dominate -- a mini-split heat pump running 8 hours per day uses 8 to 16 kWh, depending on efficiency and outdoor temperature.

For this example, assume you want to cover 15 kWh per day: 10 kWh for a mini-split heat pump, 3 kWh for refrigerator and lights, and 2 kWh for electronics and small loads.

Step 2: Account for Solar Production in Your Area

Solar production varies by location, season, and weather. In Northern California, a fixed south-facing panel at optimal tilt produces roughly 4 to 5 peak sun hours per day on average. Coastal areas with marine layer get 4 hours. Inland valleys like Fresno and Sacramento get 5 hours. Mountain areas vary by elevation and shading.

A 1 kW solar array producing 5 peak sun hours per day generates 5 kWh. To cover 15 kWh per day, you need 3 kW of panels. Add 20 percent margin for cloudy days, panel degradation, and inefficiency, and you are at 3.6 kW.

Panel wattage has increased over the years. Modern residential panels range from 350 to 450 watts. A 400-watt panel is common. To get 3.6 kW, you need 9 panels (3,600 watts / 400 watts per panel). Each panel is roughly 3.5 feet by 6.5 feet, so 9 panels occupy about 200 square feet of roof space.

Step 3: Size Your Battery Storage

Battery size depends on how much energy you use at night and during cloudy weather. If you use 15 kWh per day and the sun shines 12 hours, you produce and consume energy during the day and draw from the battery at night. Nighttime usage is typically 40 to 60 percent of daily total, or 6 to 9 kWh.

A 10 kWh battery covers one night with margin. A 15 kWh battery gives you nearly two days of autonomy if weather turns cloudy. VoltSol systems typically include 10 to 15 kWh of EG4 LiFePO4 battery storage, which balances cost and resilience for Northern California conditions.

Battery depth of discharge also matters. LiFePO4 batteries can discharge to 80 to 90 percent depth safely, but cycling to only 70 percent depth extends lifespan. A 15 kWh battery cycled to 70 percent delivers 10.5 kWh of usable energy per day, enough to cover nighttime loads with headroom.

Adjusting for Seasonal Variation

Solar production drops in winter. Northern California sees 30 to 50 percent less production in December and January compared to June and July. If your summer production is 5 kWh per day per kW of panels, winter production might be 2.5 to 3.5 kWh per day per kW.

You have two options: oversize your panel array to cover winter, or reduce winter usage. Many VoltSol customers do both. They add an extra 1 to 2 kW of panels beyond summer needs, and they use efficient mini-split heat pumps instead of resistance heaters. Heat pumps deliver 2 to 3 times more heat energy than the electricity they consume, which cuts winter HVAC load dramatically.

Another strategy: add a small backup generator for rare multi-day winter storms. Size your solar for 90 percent coverage year-round, and fill the gap with a $1,000 portable generator that runs a few hours per year. This keeps upfront costs lower than oversizing solar and battery for the worst-case week.

Roof Space and Orientation

South-facing roofs produce the most energy in California. East- and west-facing roofs produce 15 to 25 percent less. North-facing roofs are usually not viable. If your roof does not face south, you can compensate by adding more panels or installing a ground-mount array.

Roof tilt matters too. Optimal tilt for Northern California is 30 to 35 degrees, roughly equal to your latitude. Steeper or shallower roofs still work but produce slightly less. Flat roofs need tilt racks to optimize production and shedding rainwater.

Shading kills production. Trees, chimneys, vent pipes, and neighboring buildings cast shadows that reduce output. Even partial shading on one panel can drag down the whole string if you use a traditional string inverter. Microinverters or power optimizers solve this by isolating each panel, but they add cost. VoltSol assesses shading during site evaluation and designs around it.

Grid-Tie vs Off-Grid Sizing Differences

Grid-tie systems size for annual production, not daily balance. If you use 10,000 kWh per year, you install a 7 to 8 kW system that produces 10,000 kWh over 12 months. You export excess summer production and draw it back in winter. Battery storage is optional and mainly used to shift time-of-use loads or provide backup during outages.

Off-grid systems must balance daily. You cannot export or import, so your panels must produce enough energy each day to cover loads and charge the battery. This usually means smaller systems focused on high-value loads like HVAC, not whole-house coverage. VoltSol off-grid systems are typically 3 to 5 kW, which covers heating, cooling, and essentials for under $10,000.

Hybrid systems blend both approaches. You size for daily off-grid operation on your critical loads, but keep a grid connection for backup or heavy appliances. This gives you energy independence without the cost of oversizing for rare peak usage.

Real Example: VoltSol 4 kW System

A typical VoltSol install includes 10 panels at 400 watts each for 4 kW total. In Fresno or Sacramento, this produces 20 kWh per day in summer and 12 to 15 kWh per day in winter. Paired with a 15 kWh EG4 battery and an EG4 hybrid mini-split heat pump, the system covers year-round heating and cooling plus essentials.

Total cost is under $10,000 installed. After the 30 percent federal tax credit, net cost is around $7,000. At current PG&E rates of 40+ cents per kWh, the system saves $200 to $300 per month and pays for itself in under 3 years. The panels last 25+ years, the battery lasts 15 to 20 years, and the mini-split lasts 15 to 20 years.