EG4 and Sol-Ark are the two names that come up most often when people shop for a 48V hybrid inverter for a home battery system. Both build split-phase 120/240V inverters that combine a solar charge controller, a battery inverter and a grid connection in one box. Both have 10-year warranties on most current models. And both have model names that hide the number that matters most during a blackout: how many kilowatts the inverter can deliver from the battery alone, at night, with the grid down.
This comparison lines up the EG4 6000XP, 12kPV, 18kPV, FlexBOSS18 and FlexBOSS21 against the Sol-Ark 12K, 15K and 18K using the manufacturers' spec sheets, then works through the sizing math, the price per kW and the installation details that decide which brand fits your house. Every model below is in stock and active in our store on October 24, 2026, shown at the store price before any promo code.
1. EG4 vs Sol-Ark at a glance
If you only read one section, read this one. The short version:
- Price: EG4 inverters cost roughly 25-45% less than a Sol-Ark with a similar battery-only rating. The EG4 FlexBOSS21 and 18kPV deliver 12 kW from the battery for $3,600-$4,900; the Sol-Ark 15K does the same for $5,650.
- Peak power and surge: Sol-Ark wins at the top end. The 18K is the strongest single 48V inverter we stock: 18 kW continuous from the battery and 36 kW of surge for 10 seconds - useful for central AC, well pumps and shop motors.
- Battery ecosystem: EG4 inverters talk closed-loop to EG4 and BigBattery batteries and are listed as UL 9540 systems with them. Sol-Ark works with a long list of approved third-party lithium batteries, which helps if you already own a battery bank.
- Install and support: Sol-Ark ships its residential models as pre-wired all-in-one units with integrated breakers and disconnects and is widely known by installers. EG4 has a very large DIY and installer community and a lower entry price, starting at $1,499 for the 6000XP.
The chart shows why model names are misleading. The FlexBOSS21 is marketed around 16 kW, but 4 kW of that needs solar or the grid running at the same time; on battery alone it is a 12 kW inverter. The Sol-Ark 15K behaves the same way (15 kW with solar, 12 kW from the battery). The Sol-Ark 12K is a 9 kW continuous inverter - the "12K" refers to the solar power it can route to the battery and loads. The EG4 18kPV is named after its 18 kW solar input; its output is 12 kW.
2. Full spec comparison (prices checked October 24, 2026)
| Inverter | Price today | Battery-only output | With solar/grid | Surge / peak | Max solar input | Grid pass-through | Warranty |
|---|---|---|---|---|---|---|---|
| EG4 6000XP | $1,499 | 6 kW | 6 kW (off-grid, no export) | 12 kW | 8 kW, 2 MPPT | 50 A bypass | 5 years |
| EG4 12kPV | $2,999 | 8 kW | 8 kW | 16 kW (0.5 s) | 12 kW, 2 MPPT | 80 A | 10 years |
| EG4 FlexBOSS18 | $3,199.99 | 10 kW | 13 kW | 20 kW (0.5 s) | 18 kW, 2 MPPT | 90 A bypass | 10 years |
| EG4 FlexBOSS21 | $3,599.99 | 12 kW | 16 kW | see spec sheet | 21 kW, 3 MPPT | 90 A bypass | 10 years |
| EG4 18kPV | $4,898 | 12 kW | 12 kW (14.7 kW for 10 min with PV) | 13.5 kW for 10 min without PV | 18 kW, 3 MPPT | 200 A | 10 years |
| Sol-Ark 12K | $4,350 | 9 kW | 9 kW AC (12 kW PV to battery + loads) | 16 kVA (10 s) | 13 kW, 2 MPPT | 63 A | 10 years |
| Sol-Ark 15K | $5,650 | 12 kW | 15 kW | 24 kVA (10 s) | up to 19.5 kW, 3 MPPT | 200 A | 10 years |
| Sol-Ark 18K | $6,415 | 18 kW | 18 kW | 36 kW (10 s) | 3 MPPT (28.8 kW listed) + 19.2 kW AC-coupled | 200 A | 10 years |
All eight are 48V-battery, 120/240V split-phase inverters that can run 240V loads such as well pumps and central AC. Ratings come from EG4 and Sol-Ark spec sheets as reproduced on our product pages; surge figures use different durations by brand (0.5 s, 10 s or 10 min), so compare like with like. The 6000XP carries EG4's 5-year warranty and the other EG4 models 10 years; register EG4 inverters within the first year to keep the warranty valid.
Two EMP-hardened Sol-Ark versions are also in stock for buyers who want electromagnetic-pulse protection: the Sol-Ark 15K EMP ($7,740, 15 kW / 12 kW from batteries) and the Sol-Ark 12K EMP ($6,649, 9 kW continuous).
3. Price per kW: where the money goes
Dividing the store price by the battery-only rating gives a fair cost comparison across sizes. EG4's FlexBOSS models come in at about $300-$320 per kW and the 6000XP at about $250. The Sol-Ark 15K and 12K cost about $470-$480 per kW. The interesting exception is the Sol-Ark 18K: at about $356 per kW it is cheaper per kW than an EG4 18kPV, because it packs 18 kW into one box. If you need more than 12 kW from the battery, one Sol-Ark 18K can cost less than two smaller inverters in parallel and saves wall space and wiring.
What the chart does not show: the 18kPV's extra price over the FlexBOSS21 buys a 200 A grid pass-through (many homes can skip a separate critical-loads subpanel), while the FlexBOSS line is built to pair with EG4's GridBOSS hub for whole-home load management. Sol-Ark's price includes integrated PV and battery disconnects, arc-fault and rapid-shutdown controls in a pre-wired box.
4. How to size a hybrid inverter for your home
Inverter size and battery size are two separate questions. kWh (battery) decides how long you can run; kW (inverter) decides how much you can run at the same time. This section is about kW.
Step 1: add the running watts you want at the same time
List what must run together during an outage, at night, with no solar. A typical rural backup list:
- Refrigerator and freezer: about 300-400 W running combined
- Lights, Wi-Fi, TV, phone chargers: about 400-600 W
- Gas furnace blower: about 500-800 W
- 1 HP well pump: about 1,500-2,000 W running
- 3-ton central AC or heat pump: about 3,000-3,500 W running
- Microwave or kettle (occasional): about 1,200-1,500 W
Example: 400 + 500 + 700 + 2,000 + 3,500 + 1,500 = 8,600 W if everything overlaps. Add a 20-25% margin so the inverter does not sit at its limit: 8,600 x 1.25 = about 10.75 kW of battery-only output. That rules out the 6000XP, 12kPV and Sol-Ark 12K for this house on a single unit, and points to a 12 kW battery-only inverter (18kPV, FlexBOSS21 or Sol-Ark 15K).
Step 2: check the biggest motor start
Compressors and pumps draw two to five times their running watts for a fraction of a second. A 3-ton AC can start at 10-15 kW without a soft starter. If that start can overlap with other loads, check the surge column in the table above - this is where the Sol-Ark 18K (36 kW for 10 s) and Sol-Ark 15K (24 kVA) have the most headroom. A soft starter on the AC compressor often costs a few hundred dollars and can cut the start current by more than half.
Step 3: make sure the battery bank can feed the inverter
Battery current = inverter kW / battery voltage / efficiency. A 12 kW inverter on a 51.2V LiFePO4 bank draws about 12,000 / 51.2 / 0.95 = about 247 A at full load. One EG4 WallMount 16 kWh battery is rated for 200 A (10.2 kW) continuous, so a 12 kW inverter needs at least two batteries in parallel to deliver its full rating, and an 18 kW Sol-Ark 18K about 370 A - three or more batteries of that class. Undersize the bank and the inverter's headline number is never available.
Step 4: match solar to the MPPT inputs
Each inverter has a maximum usable solar power and voltage window. An 18kPV takes up to 18 kW across three MPPT trackers; the 6000XP takes 8 kW across two. Our Solar Kit Calculator and the sizing guide on running vs starting watts help with both steps.
5. Key differences that matter after the purchase
Battery compatibility and closed-loop communication
Closed-loop communication lets the battery tell the inverter its exact state of charge, voltage and current limits, which gives accurate charging and protects the cells. EG4 inverters are designed around EG4 and BigBattery lithium batteries: the EG4 WallMount 16 kWh, for example, is listed as a UL 9540 AC energy storage system when paired with the 18kPV, 12kPV, FlexBOSS18 or FlexBOSS21. Sol-Ark keeps an approved-battery list covering many lithium brands, and its inverters can also run lead-acid banks. If you already own batteries, check the inverter maker's current compatibility list before choosing the brand.
UL listings and inspections
All models here carry UL 1741 grid-interconnection listings for the U.S. (Sol-Ark lists UL 1741 SB and IEEE 1547 on its spec sheets; the EG4 12kPV, FlexBOSS18 and FlexBOSS21 list UL 9540 as well). Inspectors and utilities increasingly ask for a UL 9540 system listing, which covers a specific inverter + battery combination. Buying a pre-matched system such as an EG4 inverter with EG4 or BigBattery ETHOS batteries is the simplest way to show that listing.
6. Our picks
EG4 18kPV - $4,898
12 kW continuous from the battery, 18 kW of solar input across three MPPT trackers and a 200 A grid pass-through that lets many homes connect the inverter between the meter and the main panel without a separate critical-loads subpanel. It runs a typical rural house with a well pump and central AC on battery alone (with a soft starter on the compressor), and it is the inverter in BigBattery's UL 9540 ETHOS systems - for example the ETHOS 20.48 kWh + EG4 18kPV system at $9,433.99.
EG4 FlexBOSS21 - $3,599.99
The lowest price per kW of battery-only output among grid-tie hybrids we stock: 12 kW from the battery, 16 kW when solar is producing, and 21 kW of PV input across three MPPT trackers and five strings - a good fit for large or multi-angle roofs. It pairs with EG4's GridBOSS hub for whole-home load management, and with EG4 WallMount batteries it is listed as a UL 9540 system. A complete FlexBOSS21 + WallMount 16 kWh indoor kit is $7,092; add a second battery to unlock the full 12 kW from storage.
Sol-Ark 18K - $6,415
18 kW continuous straight from the battery and 36 kW of surge for 10 seconds - the strongest single 48V inverter we sell, and cheaper per kW than any other Sol-Ark. It handles big homes, shops and properties with several motors starting at once, takes three MPPT inputs plus over 19 kW of AC-coupled solar, passes through 200 A of grid power and is rated for -40 C to +60 C. Plan the battery bank for about 350-370 A of continuous current.
Sol-Ark 15K - $5,650
12 kW from batteries and 15 kW with solar, up to 19.5 kW of PV, 200 A grid pass-through and a 24 kVA 10-second peak. It is the model installers know best, with integrated PV disconnect, battery breakers, arc-fault detection and rapid-shutdown control in the box, and a long approved-battery list. Choose it over the 18kPV if you value the higher surge and broad battery compatibility more than the roughly $750 price gap.
EG4 6000XP - $1,499
6 kW continuous and 12 kW surge at 120/240V with 8 kW of solar input - enough for a cabin, a small off-grid home or a critical-loads subpanel with a fridge, well pump, furnace and lights. It is the most-searched EG4 model and the lowest cost per kW we stock. Remember it is off-grid only (no export), and it parallels up to 16 units if your needs grow. The 6000XP + WallMount all-weather 16 kWh system is $5,414.97.
7. EG4 or Sol-Ark: which should you choose?
- Choose EG4 if price per kW matters most, you are buying batteries at the same time (EG4 or BigBattery for closed-loop and UL 9540 listing), or you want an entry-level off-grid system. Browse all EG4 inverters or complete EG4 solar power kits.
- Choose Sol-Ark if you need more than 12 kW from the battery in one unit, have big motor loads, want a pre-wired box your installer already knows, or plan to use batteries from another brand on Sol-Ark's approved list. See all Sol-Ark inverters.
- Not sure yet? Compare every hybrid solar inverter we carry, and read our whole-home battery backup cost guide to price the battery side of the system.
8. What our customers ask
Questions about hybrid inverters come in by phone, text and email in a few repeating patterns (shared here without any personal details):
- "Is the 12K really 12 kW?" The most common confusion. We point buyers to the battery-only rating - 9 kW for the Sol-Ark 12K - because that is what runs the house at night during an outage.
- Complete 12 kW kits. Many requests are for a whole system - a 12 kW inverter, 20-30 kWh of storage and panels - often for rural homes with well pumps. We size these from an appliance list or a utility bill using the Solar Kit Calculator.
- Mixing brands. Buyers who already own batteries ask whether a new inverter will talk to them. The answer depends on the inverter maker's current compatibility list, so we check model by model.
- Delivery and handling. Inverters weigh 50-140 lb and batteries far more, so customers ask about tracking, delivery dates and whether they need a truck or trailer. Plan for two people and a dolly.
9. Current codes
As of October 24, 2026 these codes are active at checkout (one code per order): SAVE250 ($250 off orders of $4,000+) and SAVE500 ($500 off $8,000+), which cover the Sol-Ark 15K and 18K, the EG4 18kPV and most complete systems, plus ECO100 ($100 off $1,500+) for the FlexBOSS models and 12kPV, and ECO50 ($50 off, no minimum). ECOBLACK2026 ($150 off $2,000+ on selected products) is scheduled for November 20 - December 1; our Black Friday solar deals hub tracks every verified price and code.
10. FAQ
Is EG4 or Sol-Ark better?
Neither is better for every home. EG4 gives more battery-only kW per dollar - about $300-$410 per kW for the FlexBOSS21 and 18kPV - and pairs closed-loop with EG4 and BigBattery batteries. Sol-Ark costs more per kW on most models but offers higher surge, the 18 kW single-unit 18K and a broad approved-battery list.
What is the difference between the EG4 18kPV and the Sol-Ark 15K?
Both deliver 12 kW continuously from the battery and pass through 200 A of grid power. The Sol-Ark 15K reaches 15 kW when solar is producing and has a higher 10-second peak (24 kVA); the EG4 18kPV takes 18 kW of solar input and costs about $750 less ($4,898 vs $5,650 on October 24, 2026).
How much power does the Sol-Ark 12K actually deliver?
The Sol-Ark 12K is rated for 9 kW of continuous AC output, including from batteries alone, with a 16 kVA 10-second peak. The "12K" refers to up to 12 kW of solar power it can deliver to the battery and AC outputs. For 12 kW from the battery, look at the Sol-Ark 15K, EG4 18kPV or EG4 FlexBOSS21.
Can the EG4 6000XP sell power back to the grid?
No. The 6000XP is an off-grid inverter. It can charge batteries from the grid or a generator and pass grid power through to your loads, but it does not export. For grid-tie with export, use the EG4 12kPV, FlexBOSS18, FlexBOSS21, 18kPV or a Sol-Ark hybrid.
What size inverter do I need to run central air on batteries?
Add the running watts of everything that can be on at the same time, including the AC (about 3,000-3,500 W for a 3-ton unit), multiply by 1.25 and compare with the inverter's battery-only rating. Most homes with central AC need 10-12 kW of battery-only output, plus enough surge or a soft starter for the compressor start.
Can I use EG4 batteries with a Sol-Ark inverter?
Possibly - Sol-Ark approves a number of third-party lithium batteries, but the list changes with firmware and battery generation. Check Sol-Ark's current approved-battery list for your exact EG4 battery model and set up closed-loop communication as the inverter manual describes. Matching EG4 inverters with EG4 batteries is the simplest route to a UL 9540 system listing.
Want a second opinion on your system? Send us your appliance list or utility bill and we will size the inverter, battery and solar array for you - or compare all 48V inverters and EG4 batteries in stock today.
Serhii Dovzhenko, Solar & Energy Storage Specialist at Eco Solar Kits · LinkedIn
Serhii is based in Vancouver, BC, and works on product data, system sizing and buyer guides for Eco Solar Kits. He attends solar and energy storage industry conferences and is involved in the community of electricians in Vancouver, BC. Credit for the data verification in this guide goes to Adsec.ca.