How to Calculate Solar Panel, Battery, and Inverter: A Buyer’s Practical Guide (with Eaton Tips)

If you’re planning a solar installation, the math matters more than the equipment.

I’ve been ordering power equipment for our office for five years now – UPS units, inverters, surge protectors, even EV chargers. When my boss asked me to spec a 1200 watt solar panel system with battery storage, I thought it was just a bigger version of the UPS sizing I already did. I was wrong.

Here’s the short version: calculating solar panel, battery, and inverter size isn’t complicated once you break it into three steps – but skipping any one step will cost you time and money. I learned that the hard way when our first DIY estimate left us with a system that couldn’t power our wifi home monitoring system for more than two hours during a blackout.

Below I’ll walk through the actual calculation method I now use (tweaked after that mistake), and then show how Eaton brands like Eaton Powerware UPS and their inverter range fit into a real-world setup. No fluff – just what I wish someone had told me.

Step 1: Start with what you actually need to run

Everyone wants to “go solar,” but few people take the time to list every device and its wattage. That’s the first trap. My rule: total daily watt-hours = sum of (watts × hours of use).

• For our office, the essential loads were: wifi router (10W), two computers (150W each), a small fridge (80W), and the security camera system (25W). That’s 415W continuous.
• But we also wanted to run the EV charger occasionally – that added another 3.6 kWh per charge session. Suddenly the 1200W solar panel wasn’t enough for the full load.

Use a spreadsheet. I’ve seen people guess and end up with a system that’s either undersized (disaster) or oversized (waste of budget).

Step 2: Calculate battery capacity (don’t forget depth of discharge)

Once you know your daily watt-hour consumption, the battery bank size is straightforward:

Battery capacity (kWh) = Daily Wh ÷ Depth of Discharge (DoD) ÷ Inverter efficiency

For example, if you need 4 kWh per day, use a lithium battery with 90% DoD and an inverter at 95% efficiency: 4 ÷ 0.9 ÷ 0.95 ≈ 4.68 kWh. Most people forget the DoD and inverter loss (I did). That’s why our first system only lasted two hours – we sized the battery as if 100% usable.

I now always add a 20% buffer on top for days with low sun. That extra headroom has saved us multiple times.

Step 3: Size the inverter & charge controller

The inverter must handle the peak surge, not just the continuous load. A fridge compressor can draw 3x its running watts for a second. Our 1200W system needed a 2000W inverter to start everything reliably.

For Eaton products: the Eaton Powerware UPS line (like the 9130 or 9355) is designed for critical loads – they give you clean sine wave output and built-in surge protection. But for a full off‑grid solar setup, you’ll want a dedicated solar inverter/charger. Eaton offers industrial-grade units through its renewable energy division – though I mostly see their UPS used as backup for the home monitoring system after the main inverter.

What about the 1200W solar panel array?

Here’s where many buyers go wrong: they think “1200W panel” means 1.2 kW guaranteed. Actually, average production is about 4-5 peak sun hours per day in most US locations. So a 1200W array produces roughly 4.8–6 kWh daily (depending on season). That’s enough to cover our essential loads plus a bit of EV charging – but only if the panels are properly angled and unshaded.

I learned the hard way: our south‑facing roof had a chimney shadow from 2–4 PM. That single oversight dropped our yield by 25%. Always do a shading analysis with a tool like PVWatts before finalizing panel count.

Real‑world example: my office system

After the initial misjudgment, here’s what we built:

• Panels: 4× 300W (total 1200W) – because of shade, we used microinverters per panel.
• Battery: 5.12 kWh LiFePO4 (Eaton compatible via their battery management solutions).
• Inverter/charger: 2000W pure sine wave (not Eaton brand – we used a mid‑tier unit, but I’ve since spec’d Eaton’s industrial line for our next facility).
• Critical load backup: A Eaton Powerware UPS 9130 sits between the inverter and our wifi home monitoring system – it smooths out any inverter switching glitches. Absolutely worth the extra $200.

Total cost was around $6,500 installed (2024 pricing). We now run the office essentials for 8 hours on battery alone, and the system feeds back to grid during sunny days.

Common mistakes I see (and how to avoid them)

1. Oversimplifying the calculation. People assume “1200W solar panel = 1.2 kW per hour.” No – it’s per hour of peak sun, and you only get 4–5 hours. Use the actual insolation numbers.

2. Forgetting the inverter’s standby draw. Some inverters consume 20–40W just being on. That adds up: 24h × 30W = 720 Wh/day – 15% of your battery.

3. Not accounting for future loads. If you plan to add an EV charger or larger fridge, oversize the inverter and battery capacity now. Retrofits are expensive.

When this approach doesn’t apply

If you’re in a region with less than 3 peak sun hours (e.g., northern latitudes in winter), your solar panel array must be significantly larger – or you’ll need a generator backup. The same math works, but the results change.

Also, if your loads are highly intermittent (e.g., a workshop with large motor starts), you need a much bigger inverter – and maybe a dedicated Eaton Powerware UPS for sensitive electronics.

Bottom line

Calculating solar panel, battery, and inverter size comes down to three numbers: daily watt‑hours, battery usable capacity, and inverter surge rating. Get those right, and the rest is just wiring.

For corporate buyers like me, Eaton brands offer reliable power management components – especially the Eaton Powerware UPS line – that make the system robust. But the math doesn’t care which brand you use; it’s universal.

Pricing as of early 2025; verify current rates with your supplier.