Choosing an inverter for a DIY solar system starts with one question: how much power do you need at the same time? Inverter sizing is about matching the inverter’s power rating to the total wattage of the appliances you plan to run simultaneously, while also accounting for start-up surges, power factor, and efficiency losses. This guide walks you through the key numbers you need to gather and the calculations that turn them into a safe, workable inverter choice. For a broader look at planning a complete system, see our DIY Solar Planning section and the pillar article How to Plan a DIY Solar System on Paper.
List the loads that will run at the same time
According to the Virginia Cooperative Extension’s guide on estimating appliance energy use, the first step is to find the wattage of each appliance. The wattage is often stamped on the bottom or back of the appliance, or on its nameplate. If only the current draw in amperes is listed, multiply that by the voltage (typically 120 volts for most household appliances in the United States) to get watts. The guide also notes that a clamp-on ammeter can measure the actual current while the device is running.
Write down every appliance you might use at the same time and add up their running wattages. For example, if you plan to run a laptop (50 W), a window fan (200 W), and a few LED lights (30 W) at the same time, the total running wattage is 280 W. Keep in mind that many appliances draw a small amount of power even when switched off; the Virginia guide calls these “phantom loads.” Unplugging devices or using a power strip with a switch can eliminate that draw. For help building a complete load list, visit our Energy Needs planning page.
Account for motor starting surge
Motors and compressors draw much more current for a brief moment when they start. The Virginia Cooperative Extension states that when measuring the current drawn by a motor, “the meter will show about three times more current in starting the motor than when it is running smoothly.” As a rough example, a motor appliance drawing 200 W while running could need around three times that at the moment it starts. Victron’s Energy Unlimited book (2013, written mainly for boats and vehicles with lead-acid batteries) discusses inrush current of AC electric motors and how to handle it in autonomous power systems. The inverter you choose must be able to deliver this short burst of power without shutting down.
Understand continuous and surge ratings
Inverter specifications separate continuous power from short-term surge capability. As one example, the manual for a 750 VA inverter lists a continuous output power of 750 W, a maximum output power of 800 W for up to 3 minutes, and a surge rating of 1400 W. These numbers show that an inverter can handle loads above its continuous rating for a limited time. When sizing, compare your total running wattage to the inverter’s continuous rating, and compare the highest starting surge of any motor to the inverter’s surge rating. For more on inverter types and features, see our Inverters component guide.
VA, watts and power factor
AC loads do not always draw power in a simple way. Victron’s Wiring Unlimited book explains that in a purely resistive circuit (heaters, incandescent lights) the voltage and current waves are in phase, and watts equal volt-amps (VA). Inductive loads (motors, compressors) and capacitive loads cause a phase shift, so the apparent power (VA) is higher than the real power (W). The book notes that the average residential AC circuit has a power factor of about 0.8. Victron says it is okay to use 0.8 for general calculations, so you can estimate apparent power with VA = W ÷ power factor. For a mixed load of 800 W with a power factor of 0.8, the inverter needs to supply at least 1000 VA.
Efficiency and idle draw affect battery sizing
No inverter converts DC to AC without some loss. The Hong Kong Electrical and Mechanical Services Department states that conversion efficiencies of inverters generally fall in the range of 85% to 95%. In addition, an inverter consumes power even when no load is connected. The 750 VA inverter manual lists a no-load current draw of 1.25 A for a 12 V model, 0.64 A for 24 V, and 0.31 A for 48 V. That idle draw must be included when you calculate how much battery capacity you need, because it runs continuously whenever the inverter is on. Our Battery Sizing guide covers those calculations in detail.
Higher battery voltage reduces DC current
Wiring Unlimited explains that power (watts) is independent of voltage, so the current on the DC side can be found with the formula I = P ÷ V. For the same AC load, a higher battery voltage means a lower DC current. As an example, a 2400 W load on a 12 V battery draws 200 A, on a 24 V battery it draws 100 A, and on a 48 V battery it draws 50 A. Lower current allows the use of smaller, less expensive cables and reduces voltage drop. This is one reason larger DIY systems often use 24 V or 48 V battery banks.
Cable length and fusing
The inverter manual advises keeping DC cables as short as possible and using the wire lengths and sizes recommended in the manual. It also warns against installing the inverter in the same compartment as batteries, because battery gases are corrosive. Electrical codes and inverter manuals require a properly sized fuse on the positive battery cable; always follow the manufacturer’s instructions and consult a qualified electrician. Never attempt to connect an inverter to a home’s main electrical panel without a licensed electrician, the required permits, and approval from the utility.
Quick reference: key numbers for inverter sizing
| Question | Why it matters | Where to find the number |
|---|---|---|
| What is the total running wattage of loads that may be on at the same time? | Determines the minimum continuous inverter rating. | Appliance nameplates, clamp meter measurement, or the Virginia Cooperative Extension typical wattage list. |
| What is the starting surge of any motor loads? | Determines the required surge rating of the inverter. | Measure with a clamp meter (about three times running current per Virginia guide) or check the appliance manual. |
| What is the power factor of the loads? | Converts watts to VA for correct inverter sizing. | Estimate 0.8 for mixed loads, or use appliance specifications. |
| What is the inverter’s continuous and surge rating? | Must exceed total running watts and highest surge. | Inverter datasheet (e.g., a 750 VA model lists 750 W continuous, 1400 W surge). |
| What is the inverter’s efficiency and no-load draw? | Affects battery capacity needed. | Inverter datasheet (e.g., no-load current values) and EMSD efficiency range. |
| What battery voltage will you use? | Higher voltage reduces DC current and cable size. | System design choice; see Wiring Unlimited theory. |
Frequently asked questions
Can I just buy an inverter that matches the total wattage of all my appliances?
No, because you also need to account for starting surges and power factor. The inverter’s continuous rating must be above the total running watts, and its surge rating must handle the highest motor start. Using only the running wattage can lead to an inverter that trips or is damaged when a motor kicks on.
What happens if my inverter is too small?
It may shut down or be damaged when overloaded, especially during motor startup. The 750 VA inverter manual lists overload and short circuit protection, but repeated overloads can shorten the inverter’s life.
Do I need an electrician to install an inverter?
For small, portable setups that plug directly into the inverter’s outlets, you can often do the work yourself if you follow the manual and all safety precautions. However, any connection to a building’s wiring, a main panel, or the grid requires a licensed electrician, permits, and utility approval. Working with high-current DC wiring also demands careful attention to torque, fusing, and cable sizing.
For more detailed planning guidance, visit our inverter sizing category page.
