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When the lights go out at dinner time and the fridge sits silent in the dark, the size of inverter for solar power is the question that decides whether the lights, the fridge and the phone chargers can all run from the battery bank. An inverter that is too small shuts off the moment a motor starts, while an inverter far larger than the loads wastes battery power on standby draw. The steps below work from the loads you actually plan to run, then match the inverter to the battery bank and the panels.
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The quick answer
Add up the running watts of every load you plan to use at the same time, then pick an inverter whose continuous watts cover that total with room to spare. Check the surge watts as well, because motors and compressors draw a short spike when they start, and the surge figure must cover the largest spike. Then match the inverter input voltage to the battery bank, choose pure sine wave output for motors and electronics, and check that the panels and charge controller can keep the bank charged.
Add up the loads you plan to run at once
Start with a list of the loads, not the inverter. Walk through the cabin, the van or the house and write down every appliance you expect to run from the inverter, then read the watts on the label of each one. A plug-in watt meter between the appliance and the outlet measures real use over time, which gives a better figure than the label when the load cycles on and off, as a fridge does.
Only the loads that run at the same moment count toward the total. A fridge and a few lights at night are one total; a fridge plus a microwave plus a kettle at the same moment is a much larger one. Add the running watts of the loads in the largest group you expect to run together, and use that sum as the floor for the continuous watts of the inverter.
| Load | What to read on the label | Notes |
|---|---|---|
| Fridge or freezer | Running watts and startup draw | Compressor cycles, so a meter reading over a day helps |
| Lights and phone chargers | Watts per lamp or charger | Small alone, they still add to the total |
| Microwave or kettle | Running watts | High draw for a short time |
| Fan or small pump | Running watts and startup draw | Motors pull a spike at startup |
| Laptop or television | Watts on the power brick or back panel | Prefers pure sine wave output |
These are planning notes only. Labels differ between models, and a meter reading from your own appliance is the figure to trust.
Allow for surge when motors start
Capacity sets how long the loads run. Output sets whether they run at all. A motor or compressor pulls a short spike of power when it starts, well above its running watts, and the inverter must cover that spike or it shuts off each time the motor kicks in. See the difference between continuous watts and surge watts before you compare listings.
- Continuous watts: the steady output the inverter can hold while the loads run. It must exceed the running watts of everything on at the same time.
- Surge watts: the short peak the inverter can hold for a moment at startup. It must exceed the largest startup spike in the group.
- Starting loads together: when a fridge and a freezer start at the same moment, their spikes add up.
- Stagger the starts: turning loads on one at a time keeps the spike down.
If the surge figure is too low, the inverter trips each time a compressor starts, even when the continuous watts look ample.
Match the inverter to the battery bank
The inverter input voltage must match the bank voltage, and the bank must hold enough watt-hours to cover the loads for the hours you need. A bank that is too small drains fast under a large inverter, and an inverter that is too large for the bank spends its time near the low-voltage cut-off. See the best deep cycle batteries for an inverter when you size the bank.
Follow the inverter manual for cable size and fusing between the bank and the inverter, and keep the cable run short. A licensed electrician should install any inverter that connects to home wiring, and this guide gives no wiring steps.
Match the panels to the inverter and controller
For a system that runs longer than a day, the panels have to put back what the loads used. The charge controller sits between the panels and the bank, and its input limits set how much panel wattage the system can take, so check the controller input against the panel array and the panel voltage against the controller input range. When the array is larger than the controller can take, the extra panel watts go unused, so match the array to the controller rather than to the inverter alone.
Pure sine wave or modified sine wave
Most loads run on either waveform, but motors, compressors and sensitive electronics prefer pure sine wave output, which matches the shape of household AC. Modified sine wave output can run simple resistive loads such as an old-style kettle or a basic heater, yet it may make motors run hot and noisy, and some electronic controls refuse to start on it at all. See the pure sine wave vs modified sine wave inverter comparison.
For a solar power system that runs a fridge, a freezer or a well pump, pure sine wave is the safer choice. A modified sine wave inverter suits a light-duty setup with resistive loads only, and the listing should state the waveform plainly.
How to choose the size of inverter for solar power
The checklist below puts the steps together.
- List the loads: write down every appliance you plan to run, with the running watts from its label or a meter reading.
- Find the largest group: add the running watts of the loads that run at the same time, and take that sum as the floor for continuous watts.
- Cover the surge: check the startup spike of the motors in the group and make sure the surge watts cover it.
- Match the bank: match the inverter input voltage to the battery bank and size the bank for the hours you need.
- Match the array: check the panels and the charge controller against each other, so the bank stays charged.
- Pick the waveform: choose pure sine wave for motors, compressors and electronics.
See the best pure sine wave inverters for a solar system and the power inverter vs power station comparison when you weigh an inverter against a portable station with a built-in inverter.
FAQ: power away from the grid
What size inverter do I need for solar power?
Add up the running watts of the loads you plan to run at the same time, allow for the startup spike of any motors, and pick an inverter whose continuous watts cover the total and whose surge watts cover the spike. Then match the inverter to the battery bank voltage.
Can a small inverter run a fridge?
Only when its continuous watts cover the fridge running watts and its surge watts cover the compressor startup spike. Check the label on the fridge or measure it with a watt meter, and see the best inverters for a refrigerator for units made for that load.
What is the difference between continuous watts and surge watts?
Continuous watts are the steady output the inverter can hold while the loads run. Surge watts are the short peak it can hold for a moment when a motor starts. A fridge that runs on modest watts can still need a much larger surge figure, so check the surge watts before you buy.
Do I need a pure sine wave inverter for solar power?
For motors, compressors and sensitive electronics, yes. Pure sine wave output matches household AC and runs those loads without the heat and noise that a modified sine wave can cause.
How do I match an inverter to my battery bank?
Match the inverter input voltage to the bank voltage, size the bank in watt-hours for the hours you need, and follow the inverter manual for cable size and fusing. A licensed electrician should install any inverter that connects to home wiring.
Related solar guides
See every Controllers and Inverters guide and all solar inverters with a charger. Also see the solar generator calculator and the best portable power stations for home backup.
