The generator is usually the thing people already own. It sits in the shed on a pallet, gets pull-started twice a year, and comes out the morning after a storm smelling like last season's gas. Then a battery goes up on the wall and somebody asks which one they should have bought.
Both, sometimes. Neither, sometimes. It comes down to a question almost nobody gets asked, which is whether your problem is a short interruption or an outage that runs for days.
They answer different questions
A battery decides whether you notice the outage at all. On an EG4 18kPV the switching time on the backup output is 10ms, a hundredth of a second. Nobody walks outside. Nobody pulls a cord. Whether a particular appliance rides that handoff through is a question about the appliance, but the handoff itself is not something you sit and wait out.
A generator decides what day four looks like. And here is the limit of a battery that people miss. A battery does not make energy, it holds it. Whatever your panels put in during the day is the whole budget. Get a stretch of thick gray weather after a storm, the kind the Gulf Coast specializes in, and the array underproduces while the bank drains a little further every night. The battery is not broken. It is empty, and nothing else on your property can refill it.
A generator refills it. That is the real job, and it is a smaller job than most people think.
The part almost nobody tells you
Left alone, a generator is a manual appliance. Somebody has to be awake, walk out in the rain, start it, then run it all day because nobody wants to go back out at midnight.
Wired to a hybrid inverter, it stops being manual. The 18kPV has a set of GEN terminals plus a two-wire start contact on the communication board. Give the inverter that dry contact and it starts and stops the generator on its own. The conditions live in the settings. EG4 starts the generator when the utility is down and any one of three things is true: the bank has hit its cut-off, the bank has asked for a force charge, or the battery has dropped past the charge-start point you picked. That last one is the setting most people actually tune. Once the battery climbs back above the charge-end point, the inverter shuts the generator off.
Think about what that changes. The generator stops being the machine that carries your house all day. It runs in bursts, puts a charge into the bank, and shuts off. The battery carries the house between the bursts, and the generator turns into a refill pump that answers to charge settings you configure once. How long a burst lasts depends on your loads, your bank, and your charge settings, so nobody can hand you an hours figure from a blog post. The shape of it is the point: less run time, less fuel, and neighbors who stop hating you.
Size it to the inverter, not only the house
This is the step that gets skipped, and it is the one that bites.
For the 18kPV, EG4 asks for a 120/240VAC 60Hz generator between 6kW and 21.6kW, with total harmonic distortion under 3 percent. That distortion number sits on the generator's own spec sheet. Read it before you buy. EG4 publishes those three requirements for a reason, and a generator that misses them is a purchase you get to make twice.
The pass-through relay on the GEN terminal is rated 90A. Everything moving through it counts, your loads plus whatever current is heading into the battery, and the total has to stay under that ceiling. EG4's own guidance is to size the backed-up subpanel to the generator, with a 60 amp minimum per inverter.
One behavior worth knowing, because it changes what you buy: if the generator's power is not enough for everything running at that moment, the inverter does not choke. The manual says it pulls from the battery bank as supplemental power. The generator does not have to be sized to carry your worst moment by itself.
When one is enough
You have a working grid and outages that come a couple of times a year and end the same day. A battery sized against your list of must-run loads is usually the whole answer, and you never think about fuel again. Put the money into more battery or more panel instead of a second machine.
You are genuinely off grid, no utility at the road. The answer flips. EG4 tells purely off-grid owners to strongly consider a two-wire start backup generator for extended cloudy periods, and recommends an 8kW to 12kW diesel per 18kPV when they do. How often it runs comes down to the loads, the bank, and the charge settings you picked. Nothing gets you to never.
The house in the middle, a grid connection you do not trust and outages that run into days, is where both earn their keep. The battery handles the short interruptions and the quiet part of a long one. The generator is there for the stretch where the sun does not come back and the bank stops recovering.
What to do with this
Work out which of those three houses you live in before you shop for either box. Count the outages you actually had last year and how long each one ran. If they ended the same day, the money goes into the battery. If they ran into days, plan for the refill too. If there is no power line at your road, EG4 says to strongly consider the generator for the long cloudy stretches.
None of that gives you a size. The size comes from the load list, the bank you buy, and how much sun your roof catches in a bad week, and anyone who quotes you a runtime without those three has not done the work.
Whichever way it lands, the generator interconnect is licensed electrical work. A state-licensed local contractor does that wiring and the transfer arrangement. Ape Solar designs the system and runs the build.
Sort the load list first, because the list decides the size of everything else. Our piece on what stays on in an outage walks through that part. Then run your numbers in the system builder, or call the shop and we will size the bank and tell you straight whether you need the generator at all.