Size the inverter to the continuous load first, then to the battery current, then to the solar panel set. For a battery-backed hybrid system, that order prevents more purchase-order mistakes than any other rule I use. I started ordering solar equipment for a commercial installer in 2021 and manage about 70 equipment orders a year. This article is the checklist I wish I had before my first hybrid inverter purchase order.
I say that as the person who writes the purchase orders, not as an engineer. I don't design the system, but I have to make sure the inverter, battery, and modules arrive on the same site at the same time. The Deye SUN-6K-SG03LP1-EU inverter specifications are a useful example: the model number says 6 kW, but the decisions are in the PV input and battery port sections, not in the 6 at the front.
When someone asks me how to size a solar inverter, I try to move them away from the AC nameplate. The nameplate is an output rating. It is not the reason a system works.
The three checks that matter
- Continuous AC load. In a hybrid system, the inverter passes power from solar and battery to the loads. If the critical loads during a power cut are 5 kW, a 6 kW hybrid can usually carry them; a smaller unit cannot, no matter how many panels or kilowatt-hours are behind it.
- Battery current. Buyers remember kWh. The inverter cares about amps. When a quote request says magazyn energii deye bos-g, it usually describes a BOS-G stack by number of modules or total capacity. I still verify the continuous charge and discharge current limits on the battery side against the battery port on the inverter before approving anything.
- PV string voltage and current. The inverter doesn't see a solar panel set as one total kWp number. It sees the voltage and current of each string. The max DC voltage is a hard limit, and cold weather raises module voltage. A string that seems safe in summer can exceed the inverter input limit on a cold clear morning.
The check that usually gets skipped is battery current. I still kick myself for a 2023 order where I matched the battery capacity to the inverter output but not the discharge current. The battery had enough stored energy; it just couldn't deliver it at the rate the inverter wanted for the backup load. The system had to be re-configured for a smaller load until the correct battery module arrived.
Why the solar panel set can be larger than the inverter
This is the part that makes buyers nervous. A 7.2 kWp solar panel set on a 6 kW hybrid inverter is not a sizing mistake. In grid-tied systems, a DC/AC ratio of about 1.1 to 1.3 is normal. Panels rarely make nameplate power under real operating conditions, so some extra DC keeps the inverter closer to its rated AC output for more hours of the day.
Yes, there is clipping. Clipping means the inverter limits its output during the few hours when the DC power is higher than the AC rating. In most residential and small commercial designs, that lost production is a fair trade for the extra energy captured in the morning, afternoon, and cloudy periods. With a hybrid inverter, the battery also changes the picture: if the battery isn't full, the surplus DC can be stored before it reaches the AC output.
What I check instead of the total ratio is the inverter's maximum DC input voltage, minimum MPPT voltage, and maximum DC input current. Those three values decide whether a specific string design will start, run, and survive cold weather. The ratio alone won't tell you that.
Does a car power inverter drain the battery?
This question appears in our inbox often enough that I will answer it. Does a car power inverter drain the battery? Yes, if the engine or another charging source is not running. The inverter draws its power from the 12 V battery, and conversion is not free. A 150 W load through a car inverter with 90% efficiency draws roughly 14 A from the 12 V system. With the engine off, that current comes out of the battery.
The reason I mention it: the word inverter makes people think of a simple DC-to-AC box. A solar hybrid inverter also manages PV voltage, battery state of charge, the grid connection, and backup load switching. The sizing rules follow from those jobs, not from the conversion function alone.
How to size a solar inverter when I write the order
The sequence I use on battery-backed projects is:
- Define the largest continuous load the inverter has to serve during an outage or in off-grid mode.
- Check the battery voltage range and the maximum continuous charge and discharge current on the battery BMS and the inverter datasheet.
- Calculate the string voltage for the coldest local temperature and compare it with the inverter MPPT voltage range and the maximum DC input voltage.
- Confirm the inverter model has the certificates and grid code approvals needed for the country. If the local grid operator limits export, that limit may force a smaller AC rating than the array suggests.
This last step matters more than it used to. In 2024, I had to expedite a replacement because the inverter we approved was not listed on the DSO approved model list. The rush freight was €400; the idle crew was more than that before lunch. I don't treat the certificate check as paperwork anymore.
When I don't follow this exact order
If the system has no battery and no backup mode, I start from the array and the export rule, not the load. A pure grid-tied string inverter is sized to the PV array and the site's grid connection. Load is irrelevant in that design because the inverter is not serving local consumption during an outage.
I'm also careful with three-phase sites. The Deye SUN-6K-SG03LP1-EU is a single-phase hybrid inverter. If the site has a three-phase well pump or workshop load that must run during an outage, one 6 kW unit is only part of the solution.
Take the local grid detail with a grain of salt because export rules and DSO lists change. As of February 2025, the versions I use are the current ones, but I still verify them for each project.
So, if you are preparing a PO for a battery-ready project this week: start with the load table, check the battery current, compare the cold-weather string voltage, and let the nameplate be the final confirmation instead of the first guess.