Solar Generator Size Calculator

How the Calculator Works (The Math)

A solar generator — really a battery-based portable power station with an inverter, often bundled with folding solar panels — is sized by answering two separate questions: how much energy do you need to store (capacity, in watt-hours) and how much power do you need to deliver at once (output, in watts). This solar generator size calculator works both out from a single appliance list.

Start with your daily energy. For every appliance you tick, the tool multiplies running watts by quantity by the hours per day you run it, then sums across everything:

dailyWh = Σ ( runningWatts × quantity × hoursPerDay )

That daily figure is then stretched across your chosen days of backup and inflated to cover real-world inverter and round-trip losses (a power station never delivers 100 percent of its nameplate energy), giving the required usable capacity:

requiredCapacityWh = (dailyWh × daysBackup) / 0.85

The 0.85 factor is the efficiency allowance — about 15 percent of stored energy is lost to the inverter, conversion, and standby draw. The result is then rounded up to the next common station size (300, 500, 700, 1000, 1500, 2000, 2500, 3000, or 5000 Wh) so the recommendation maps to a real product you can actually buy.

Output is a different calculation entirely. Your continuous output requirement is simply the sum of running watts of everything that could be on at the same time:

continuousOut = Σ ( runningWatts × quantity )

Your surge output requirement adds the worst-case start-up spike. Motors and compressors — fridges, pumps, power tools, air conditioners — briefly draw two to three times their running watts when they kick on. Because two large motors almost never start in the same instant, the tool models surge as the steady continuous load plus the single largest start-up surge delta:

surgeOut = continuousOut + largestSurgeDelta

Finally, the recommended solar recharge wattage is set so the matched station can refill in roughly one sunny day. With an average of about four peak sun hours, that means panel wattage near one quarter of capacity:

recommendedSolarW = capacityWh / 4
runtimeHrs        = usableWh / continuousOut

Runtime is the usable energy of the matched station divided by your continuous draw — the heart of any solar generator runtime calculator. Draw less and it lasts longer; draw more and it empties faster.

Why capacity and output are not the same: a tiny 300 Wh station might happily run a 1,000-watt microwave (output is fine) but only for a few minutes (capacity is tiny). A huge 5,000 Wh station with a weak inverter might store days of energy yet refuse to start a power tool. You must clear both the watt-hour and the watt requirement — which is exactly what this tool checks when it flags a station as a fit.

Worked Example: Full-Size Fridge + Lights for 2 Days

Suppose you want a power station for a 2-day grid outage running a full-size fridge, four LED lights, a wifi router, and charging two phones. Let us size it by hand and confirm the tool agrees.

Step 1 — daily watt-hours. A full-size fridge averages 150 W but cycles, so over 24 hours we count roughly 8 effective hours. Lights run 5 hours, the router runs all day, phones charge a couple of hours each.

Fridge:   150 W × 1 × 8 h   = 1,200 Wh
LED x4:    40 W × 4 × 5 h   =   800 Wh
Router:    15 W × 1 × 24 h  =   360 Wh
Phone x2:  10 W × 2 × 3 h   =    60 Wh
-------------------------------------------
dailyWh                        = 2,420 Wh

Step 2 — required capacity. Across 2 days, with the 0.85 efficiency allowance:

requiredCapacityWh = (2,420 × 2) / 0.85 = 5,694 Wh

Rounded up to the next common station size, that is the 5,000 Wh class and up — realistically an expandable station with an add-on battery, since a single ~2,000 Wh unit will not carry two full days of fridge. (Drop to 1 day of backup and the requirement falls to about 2,847 Wh, squarely in 3,000 Wh territory.)

Step 3 — output. Continuous load with everything on at once is 150 + (40×4) + 15 + (10×2) = 345 W. The fridge compressor surges to roughly 3× its running watts on start-up, a delta of about 300 W, so surge output needed is 345 + 300 = ~645 W. Almost any 1,000 Wh-plus station clears that easily.

Step 4 — solar recharge and runtime. Recommended solar to refill a 5,000 Wh class station in a day is about 5,000 / 4 = 1,250 W of panels. Runtime on a single 2,000 Wh station's usable ~1,700 Wh at the 345 W continuous draw would be roughly 4.9 hours flat-out, which is why the daily fridge cycling matters more than the nameplate.

Result: for two days you want the largest expandable station you can manage plus serious solar; for one day of fridge backup, a 3,000 Wh unit with ~750 W of panels is the practical sweet spot. The calculator above runs this whole chain for you and highlights the stations that fit.

Power-Station Size Reference Table

Use this as a sanity check against the calculator. "Typical run" assumes a station is full and feeding that single load until empty; real trips mix loads and recharge daily.

Watts vs Watt-Hours (and Surge)

The single most common confusion in power-station shopping is mixing up watts and watt-hours. They answer different questions:

• Watts (W) are power — the rate of energy flow at one instant. The inverter's continuous wattage rating tells you whether a station can run a device at all. A 600 W station cannot run an 1,800 W hair dryer no matter how big its battery.
• Watt-hours (Wh) are capacity — how much energy is stored. They tell you how long it runs. A 1,000 Wh station feeding 100 W lasts about 10 hours minus losses.
• Surge / peak watts are the brief spike a station can deliver for a second or two. Motor and compressor loads need this headroom on start-up. If a fridge running at 150 W surges to 450 W and your station peaks at 400 W, it shuts off the instant the compressor kicks in.

That is why the calculator reports three output numbers — required capacity, continuous output, and surge output — instead of a single figure. For a deeper treatment of pulling those numbers together for a wired system, see our load and appliance calculator.

Sizing Your Solar Recharge

A power station without solar is just a big battery — it runs down and stays down. To make it a true solar generator, pair it with enough panel wattage to refill it during the day. The rule the calculator uses is panel wattage equal to roughly one quarter of capacity, because most locations average about four peak sun hours:

recommendedSolarW = capacityWh / 4

So a 1,000 Wh station wants about 250 W of panels to refill in a sunny day; a 3,000 Wh station wants about 750 W. Treat that as a floor, not a ceiling — clouds, panel angle, heat, and cable losses all eat into real output, so more panels mean a faster, more reliable refill. Also confirm your station's maximum solar input rating; feeding it more panels than its charge controller accepts simply caps at the limit. For a deeper look at array sizing and refill math, see the off-grid system calculator and our system sizing guide.

What to Look For in a Power Station

Once the calculator has told you the capacity and output you need, these are the brand-neutral features that separate a station you will keep for a decade from one you will resent:

• LiFePO4 (LFP) chemistry. Lithium iron phosphate cells last 3,000 to 6,000+ cycles versus roughly 500 for the older NMC lithium in cheap units — often a decade of daily use versus a couple of years. LFP is also far more tolerant of heat and abuse. If a listing does not say LiFePO4, assume it is the shorter-lived chemistry.
• Pure sine wave inverter. Sensitive electronics, CPAP machines, and many motors need clean pure-sine AC. Modified-sine output can buzz, run hot, or refuse to run at all. Every station worth buying is pure sine — verify it.
• Expandable capacity. The smartest hedge against under-sizing is a station that accepts add-on battery packs. Buy for today's load, then bolt on more watt-hours when your needs grow instead of replacing the whole unit.
• Fast recharge and high solar input. Look for high maximum solar watts and quick AC recharge. A station that refills in two to three hours from the wall or a single sun day is dramatically more useful than one that takes all day.
• Adequate continuous and surge output. Match these to the calculator's output numbers. A "1,000 Wh" unit with only a 600 W inverter is a different tool than one with a 1,500 W inverter and a 3,000 W surge.
• Useful ports and an app. Enough AC outlets, USB-C PD for laptops, a 12 V car socket for fridges, and Bluetooth/app monitoring so you can watch state of charge.

Common Sizing Mistakes This Calculator Prevents

• Sizing for capacity but ignoring output. A big battery that cannot start your fridge is useless. The tool checks both watts and watt-hours.
• Forgetting surge. Compressor and motor start-up can be 3× running watts. Size to surge, not just steady draw.
• Ignoring conversion losses. Nameplate watt-hours are not usable watt-hours. The 0.85 factor keeps you honest.
• Buying a station with no real recharge plan. Without enough solar, a "generator" runs flat and stays flat. Match panel wattage to capacity.
• Over-buying. The biggest unit is rarely the right unit. Size to real daily watt-hours and days of backup, then add a modest margin or choose an expandable system.

Frequently Asked Questions

Related calculators & guides

• Off-Grid Solar System Calculator — size a full wired panel, battery, controller, and inverter system.
• Battery Bank Calculator — size a DIY battery bank in amp-hours by chemistry and DoD.
• Wire Size Calculator — get the right AWG, voltage drop, and fuse for any DC run.
• Load / Appliance Calculator — total your daily watt-hours appliance by appliance.
• Best Solar Generators 2026 — tested picks ranked by capacity, output, and recharge.
• Off-Grid Refrigeration Guide — how much power a fridge really draws off grid.