Understand Your Energy Needs First

Before drawing any solar panels or batteries, sit down with a notepad and list everything you want to power. According to the Virginia Cooperative Extension’s energy estimating guide, the starting point is each appliance’s wattage and how long it runs. For each appliance, locate its nameplate wattage, usually stamped on the bottom or back of the unit. If only the current draw in amperes is listed, multiply the amps by the voltage the appliance uses (most household items in the United States run on 120 volts) to find the wattage.

Roadmap summary: A complete DIY solar plan starts with a load list, moves to site production estimates, then sizes the battery, voltage and wiring before finishing with mounting and protection details.

The extension guide offers a formula every DIY planner should memorize: (Wattage × Hours Used per Day) ÷ 1000 equals daily kilowatt-hour consumption. A kilowatt is 1,000 Watts. For a refrigerator, the guide points out a common mistake: although it is plugged in all day, the compressor cycles on and off. Virginia Cooperative Extension recommends dividing the total time a refrigerator is plugged in by three to approximate actual runtime at its maximum wattage.

Also account for hidden loads. Many appliances, like televisions, computers and kitchen gadgets, continue to draw a small amount of power even when switched “off.” The extension guide calls these phantom loads and notes they can be avoided by unplugging the appliance or using a power strip with a switch. Write down every load, its watts, the estimated hours it runs per day and the resulting kilowatt-hours. This list becomes the foundation for every sizing decision that follows. If the concept of kilowatts and Watthours feels unfamiliar, review the Solar Electricity Basics page before continuing.

Distinguish Watts from Watthours

Confusing power and energy is one of the easiest planning mistakes to make. The U.S. Energy Information Administration (EIA) explains the difference clearly. Electricity is measured in Watts and kilowatts at a specific moment, but electricity use over time is measured in Watthours and kilowatthours. One kilowatt is equal to 1,000 Watts, and one kilowatthour is one kilowatt of power sustained for one hour. For instance, the EIA notes that a 40-Watt light bulb left on for five hours uses 200 Watthours, or 0.2 kilowatthours.

When you complete your load list, express daily totals in Watthours or kilowatthours, not just Watts. A device that draws 500 Watts running for two hours uses 1,000 Watthours. A different device drawing 1,000 Watts for 30 minutes also uses 500 Watthours. The energy number is what matters for planning battery capacity and solar panel production. Keep this distinction front and center throughout your worksheet. If you want to run different system types through this exercise, the System Types guide walks through how each configuration affects daily energy math.

Estimate Solar Production for Your Site

Once you know how much energy you need, estimate how much energy you can realistically generate at your specific location. The PVWatts Calculator, now operated by the National Laboratory of the Rockies, is a free online tool designed exactly for this purpose. According to the tool’s website, it allows homeowners, small building owners, installers and manufacturers to develop estimates of the performance of potential photovoltaic installations.

The inputs described on the tool’s help page include system size in DC kilowatts, module type, array type, system losses, tilt and azimuth. Its results include solar radiation and AC energy figures, which can be downloaded as monthly or hourly results. The website cautions that photovoltaic system performance predictions include many inherent assumptions and uncertainties and do not reflect variations between PV technologies nor site-specific characteristics except as represented by the calculator inputs. The expected range is based on 30 years of actual weather data at the given location and is intended to provide an indication of the variation you might see. Use the results as a planning estimate. To explore the math behind the predictions, see the Energy Production fundamentals page.

Design the Battery Bank

With daily energy needs and estimated panel production in hand, the next step is conceptualizing the battery bank. Victron Energy’s “Wiring Unlimited” guide describes the fundamental principles. According to the guide, batteries are interconnected to increase voltage or capacity, or both. When batteries are connected in series, the voltage increases. When connected in parallel, the capacity increases. A series/parallel arrangement raises both.

For large battery banks, the Victron guide advises caution about running many parallel strings. The maximum recommended is around three or four paralleled strings of 12-volt lead acid batteries. The reason is that with a large bank, small wiring variations and slight differences in internal battery resistance create an imbalance. An imbalanced bank works the bottom battery harder, discharging and charging it more deeply, which leads to premature failure. The guide explains that correct parallel wiring means ensuring the total path of the current in and out of each battery is equal, using methods like busbars, connecting halfway or connecting diagonally with identical cable lengths.

After reading the battery bank concepts, visit the Battery Sizing planning page to step through capacity calculations and the Batteries component guide for lead-acid and lithium details.

Choose System Voltage and Account for Current

One of the most impactful early planning decisions is the system voltage. The power formula, described in the Victron guide, explains why: P = V × I. Power equals voltage multiplied by current. The same power level at a higher voltage requires less current. According to the guide, a 2,400-watt load on a 12-volt system draws 200 amps, while the same load on a 48-volt system draws just 50 amps.

Lower current is not just a number on a page. The Victron guide explains that as current flows through a cable, the cable’s resistance creates a voltage drop. The voltage drop can be calculated with V = I × R. In a 12-volt system with a 2,400-watt inverter and two 1.5-meter 16 mm² cables, the total voltage drop is about 0.64 volts. That means the inverter receives 11.36 volts instead of 12 volts, and the current increases to compensate for the loss, making the system less efficient. The guide recommends keeping voltage drop below 2.5 percent. While larger cables and shorter runs help, raising the system voltage is a powerful lever: at 48 volts the same setup produces a voltage drop of only 0.3 percent. When planning, write down each cable run length so voltage drop can be checked, see Measurement Tools for the meters used to check a finished system, and review Cables and Connectors for conductor selection factors.

Plan Mounting, Tilt and Orientation

Mounting is about more than just propping up panels. The U.S. Department of Energy’s “Solar Photovoltaic System Design Basics” states that PV arrays must be mounted on a stable, durable structure that can support the array and withstand wind, rain, hail and corrosion over decades. The structures tilt the PV array at a fixed angle. To obtain the highest annual energy output, the Department of Energy advises that modules in the northern hemisphere be pointed due south and inclined at an angle equal to the local latitude.

For ground-mounted arrays, tracking mechanisms are an option. One-axis trackers follow the sun from east to west, while two-axis trackers keep modules pointed directly at the sun all day. The Department of Energy notes that tracking involves more up-front cost and that sophisticated systems require more maintenance, though the cost-benefit picture has improved for ground-mounted systems. Rack mounting remains the most common method for its robustness and simplicity. When you are ready to sketch mounting locations and check roof or ground viability, consult the Solar Panel Sizing guide and the Mounting Equipment reference.

Include Protection Devices in the Plan

A safe solar plan includes fuses, circuit breakers and isolation switches from the start. Victron’s “Wiring Unlimited” guide states that a fuse protects wires in a circuit from excessively high currents, which can cause overheating or fire. The fuse is placed in the supply cable to an electrical device. Fuses protect against overcurrent and short circuits. The guide divides fuse types into one-time wire fuses, thermal circuit breakers and magnetic circuit breakers.

The guide explains that location matters. Each consumer connected to a battery needs an individual fuse in the positive cable, no matter the power rating. A typical DC circuit includes a main battery fuse with branch fuses for each consumer. For AC circuits, circuit breakers are located near the point where the public grid or a generator enters the switchboard. Fuses and breakers must be selected based on four criteria: current rating, voltage rating, interrupt rating and speed. Victron also describes battery isolation switches, which allow disconnecting the battery from the rest of the circuit for maintenance or long idle periods. Removing the main fuse can serve the same purpose. Do not pull fuse ratings or cable sizes from a generic table; check the product manual and follow your local electrical code. Any system that connects to household wiring or the grid requires a qualified electrician. Before touching wires, read Electrical Safety and Battery Safety.

Build Your Planning Worksheet

With all the concepts in hand, transfer your numbers to a written worksheet. Record each step, the value you determined and where the number came from. A thorough worksheet keeps the plan traceable:

Step What to Record Where the Number Comes From
1. Load list Each appliance, nameplate watts (or volts × amps), hours used per day, daily Watthours Nameplate, label or clamp-on ammeter reading per Virginia Cooperative Extension
2. Total daily energy Sum of all load Watthours, converted to kilowatthours Addition of load list items; EIA units reference
3. Site production estimate Monthly and annual AC energy estimate, solar radiation data PVWatts Calculator run for your location and array parameters
4. Battery bank concept Bank voltage, capacity needed, number of series and parallel strings Load list and days of autonomy; Victron parallel string guidance
5. System voltage and current Chosen system voltage, maximum expected current, calculated voltage drop Power formula and voltage drop calculation from Victron theory
6. Mounting, tilt and orientation Array tilt angle, azimuth, mounting type, structural notes Department of Energy mounting and tilt recommendations
7. Protection devices Main fuse and branch fuse locations, isolation switch placement Victron fuse and protection guidance, local electrical code

Print the worksheet and keep it with your project notes. As the design evolves, update it in one place. A filled-out worksheet makes conversations with an inspector or an electrician faster and more precise. For blank templates and extra calculation aids, open the Solar Calculators and Planning Worksheets tool pages. When your worksheet is complete and you are ready to look at specific project types, browse the Portable Solar Setups, Backup Power Projects or Off-Grid Projects sections to adapt the plan to your real-world build.

Frequently Asked Questions

Do I need an electrician even for a small DIY solar system?

Yes, for any system that connects to a home’s wiring or to the grid. Victron’s safety warnings say electrical work should always be carried out by a qualified electrician or technician, that local safety guidelines and requirements must be followed and that undersized wiring or bad connections can cause a fire. A licensed electrician ensures the installation meets your local regulations and can check your planning worksheet for safety gaps.

How accurate are PVWatts production estimates?

PVWatts provides an estimate based on 30 years of historical weather data at the selected location. The National Laboratory of the Rockies website cautions that predictions include many inherent assumptions and do not reflect variations between PV technologies nor all site-specific characteristics. Use the results to set realistic expectations and compare scenarios, not as a precise production figure.

Can I mix old and new batteries in the same bank?

The Victron “Wiring Unlimited” guide highlights how wiring variations and slight differences in internal resistance cause imbalances that overwork some batteries and lead to premature failure. Placing old and new batteries with different internal resistances and capacities in the same parallel string is likely to make that imbalance worse, so check the battery manufacturer’s guidance before mixing batteries in one bank.

For the full sequence of planning steps, return to the DIY Solar Planning hub. It links to detailed guides on energy needs, panel sizing, battery sizing, inverter sizing and system design that walk through each topic covered here in greater depth.

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