DIY Solar Sizing Calculations: Panels, Battery Banks and Inverters

Planning a small off-grid, backup or portable solar system starts with a few simple formulas. You do not need an engineering degree, just a clear picture of your daily energy use, the battery storage you want, the panel array that can recharge it, and an inverter that can handle your loads. This guide walks through each calculation step by step, using publicly available data and straightforward arithmetic.

How to estimate daily energy use

The foundation of any sizing exercise is knowing how much energy your appliances consume. The Virginia Cooperative Extension provides a simple formula: (Wattage × Hours Used per Day) ÷ 1000 = Daily kilowatt-hours (kWh). The U.S. Energy Information Administration (EIA) explains that a watt-hour is the energy of one watt used for one hour, so multiplying watts by hours gives watt-hours (Wh). For example, a 40-watt light bulb used for 5 hours consumes 200 Wh, or 0.2 kWh.

To find the wattage, check the nameplate on the appliance. If only amperes are listed, multiply amps by the voltage (usually 120 volts in the United States) to get watts. The Virginia Cooperative Extension also notes that many appliances draw a small amount of power even when switched off; these phantom loads can be avoided by unplugging the device or using a power strip. For a more detailed walk-through of listing your loads, see our Energy Needs planning page.

Example: A laptop drawing 50 watts for 4 hours a day uses 50 × 4 = 200 Wh. A small refrigerator with a nameplate rating of 725 watts that cycles on about one‑third of the time (a typical duty cycle) might use roughly 725 W × 8 h = 5,800 Wh per day. Add up all such loads to get your total daily energy requirement.

Sizing the battery bank

Once you know the daily watt-hours, you can calculate the battery capacity needed to ride through cloudy periods. The Hong Kong Electrical and Mechanical Services Department (EMSD) highlights the concept of “days of autonomy”, the number of days the battery system can support the loads without being recharged.

Battery capacity in watt-hours is: Daily Wh × Days of autonomy ÷ Usable fraction. The usable fraction accounts for the fact that you should not discharge a battery completely. The usable fraction depends on the battery chemistry and on what the battery manufacturer allows, so take it from the battery’s documentation; the example below simply assumes 0.5. The EMSD also explains that battery capacity is often given in ampere‑hours (Ah). To convert watt‑hours to amp‑hours, divide by the battery voltage: Ah = Wh ÷ V. For more on battery types and depth of discharge, visit our Batteries component guide.

Example: Assume a daily load of 1,000 Wh, 2 days of autonomy, and an assumed usable fraction of 0.5. Battery bank Wh = 1,000 × 2 ÷ 0.5 = 4,000 Wh. At 12 volts, that is 4,000 ÷ 12 ≈ 333 Ah. A bank of two 12 V, 200 Ah batteries wired in parallel would provide 400 Ah, which covers the need.

Sizing the solar panel array

The panel array must produce enough energy each day to replenish the battery and supply the loads. The formula is: Panel array wattage (W) = Daily Wh ÷ (Equivalent sun hours × Loss factor).

Equivalent sun hours describe how much usable sunlight a location gets in a day. The National Laboratory of the Rockies’ PVWatts calculator (formerly NREL) estimates production for a location you choose and lets you look at monthly results, which you can use as the solar input for this formula; the lab cautions that its estimates include many assumptions and uncertainties.

The loss factor accounts for inefficiencies in the real world. The EMSD states that inverter conversion efficiencies are generally in the range of 85% to 95%. Further losses come from wiring, soiling, temperature and battery charging, so the loss factor is an assumption you choose and should refine with your component data. Our Solar Panel Sizing page covers these losses in more detail.

Example: With a 1,000 Wh daily load, an assumed 4 equivalent sun hours and an assumed loss factor of 0.7, the required array size is 1,000 ÷ (4 × 0.7) = 357 W. Round up to 400 W of solar panels, such as four 100‑watt panels.

Sizing the inverter

The inverter converts DC power from the battery to AC power for your appliances. To size it, add up the wattages of all the appliances that could run at the same time. That sum gives the minimum continuous power rating the inverter must supply.

Motors, such as those in refrigerators, pumps, or power tools, draw a much higher current for a fraction of a second when they start. The Virginia Cooperative Extension notes that a motor can draw about three times more current at startup than when running smoothly. Therefore, the inverter must also be able to handle this surge. Check the inverter’s surge rating (often listed for a few seconds) and make sure it exceeds the starting wattage of the largest motor load. For a deeper look at inverter selection, see our Inverter Sizing guide.

Victron’s Wiring Unlimited book illustrates the current calculation with the formula I = P ÷ V. For example, a 2,400‑watt load on a 12‑volt battery draws 200 amps. While you do not need to size wires here, this relationship helps you understand why higher‑voltage battery banks reduce current and make wiring easier.

Example: Suppose you plan to run a laptop (50 W), a few LED lights (30 W total), and a small refrigerator (150 W running, but starting surge around 450 W). The continuous load is 50 + 30 + 150 = 230 W. The surge requirement is at least 450 W. An inverter rated for 300 W continuous and 600 W surge would be a comfortable fit.

Putting the formulas together

The table below summarizes the four key calculations, the inputs you need, and the assumptions behind them.

For a complete planning workflow that ties these steps together, visit our System Design page. You can also explore the Solar Calculators category for additional tools, and the Solar Sizing Formulas article for a deeper dive into the math.

Frequently asked questions

What if I cannot find the wattage on my appliance?

If the nameplate only lists amperes, multiply the amps by the voltage (typically 120 V for household appliances in the U.S.) to get watts. The Virginia Cooperative Extension suggests using a clamp‑on ammeter to measure the actual current while the device is running, then multiplying by the voltage. Always follow the meter manufacturer’s safety instructions.

How do I find the equivalent sun hours for my location?

The National Laboratory of the Rockies’ PVWatts calculator provides monthly solar radiation data for any location you enter. The value in kWh/m²/day is the same as equivalent sun hours. For a system that must work year-round, plan around the month with the least sun in your estimate.

Can I use these formulas for a grid‑tied system without batteries?

The energy and production estimates still help, but grid-connected systems are designed, permitted and connected by licensed professionals with utility approval, so treat these formulas as background for talking with an installer.

For more planning worksheets and measurement tools, head over to our Solar Calculators page.

Sources

• Virginia Cooperative Extension, Estimating Appliance and Home Electronic Energy Use
• U.S. Energy Information Administration, Measuring Electricity
• Hong Kong Electrical and Mechanical Services Department, Balance of System
• PVWatts Calculator (National Laboratory of the Rockies)
• Victron Energy, Wiring Unlimited