Watt & Hour is reader-supported. When you buy through links on this page we may earn a commission, at no extra cost to you. How we make money.
Contents
Solar generator sizing has two separate answers, and both start from your daily load. Battery capacity sets how many days you can ride through without sun: divide the daily load by 0.85 and multiply by the days you need. Solar input sets how fast you can put that energy back: divide the battery energy you use per day by 0.75 × your peak sun hours. The station must accept that many solar watts. If either side is short, a bigger version of the other side does not fix it.
This guide is based on manufacturer specifications and our own arithmetic. We have not tested any station or panel. The 0.85 and 0.75 factors are planning assumptions, explained below.
This page is about balancing the battery and the solar input for a daily load. For the panel side (how many panels, series or parallel, and the cold-weather voltage check), see our guide to how many solar panels it takes to charge a power station.
Solar generator sizing in five steps
- Daily load (Wh per day at the outlets): watts × hours for each device, added up. For cycling appliances like fridges, use the average draw.
- Battery energy per day: daily load ÷ 0.85. The 0.85 is our allowance for inverter losses and a small reserve.
- Battery capacity: battery energy per day × the number of days you want to cover with no useful sun.
- Solar input needed: battery energy per day ÷ (0.75 × peak sun hours). The 0.75 is the same delivery factor our panel guide uses: it assumes panels put about 75% of their label rating into the battery once heat, angle, dust, wiring and charging losses are counted.
- Check the station: its maximum solar input must be at least the figure from step 4, and its capacity (with any expansion batteries) at least the figure from step 3.
Multiply or divide by 0.85? Mixing these up is a common sizing error, so here is the difference.
Going from battery to appliance, you lose energy, so you multiply: a 1,024 Wh battery delivers about 1,024 × 0.85 = 870 Wh to your devices.
Going from appliance need to battery size, you need extra, so you divide: to deliver 1,024 Wh to your devices you need about 1,024 ÷ 0.85 = 1,205 Wh in the battery.
The same logic applies to the 0.75 solar factor: to put 1,205 Wh into the battery, the panels’ label rating × sun hours must be 1,205 ÷ 0.75 = about 1,607 Wh.
What a peak sun hour means here
Panels are rated at an irradiance of 1,000 W per square metre and a cell temperature of 25°C, as the IAEI's PV systems math explains. A peak sun hour is one hour's worth of energy at that 1,000 W/m² level. So a location that gets 4 kWh per square metre on an average day in a given month has 4 peak sun hours that month: 4,000 Wh/m² ÷ 1,000 W/m² = 4 hours. Use the figure for the weakest month in which you'll rely on solar. Our panel guide shows where to look it up for your location.
Worked example: a fridge and internet for an outage
Load, for illustration: a Samsung RS27T5202** side-by-side fridge rated at 546 kWh per year by ENERGY STAR, which averages 546,000 ÷ 365 = 1,496 Wh per day, plus a router and modem at an assumed 20 W around the clock, 20 × 24 = 480 Wh. Daily load: 1,496 + 480 = 1,976 Wh.
Battery energy per day: 1,976 ÷ 0.85 = 2,325 Wh.
Battery for one day with no sun: 2,325 Wh. For two days: 4,650 Wh.
Solar input at 5 peak sun hours: 2,325 ÷ (0.75 × 5) = 620 W.
At 3 peak sun hours: 2,325 ÷ (0.75 × 3) = 1,033 W.
At 2 peak sun hours: 2,325 ÷ (0.75 × 2) = 1,550 W.
Matching the load to real stations
Now match that against real stations. The table lists each station's published capacity and maximum solar input, plus two figures we calculated from them.
As an Amazon Associate I earn from qualifying purchases.
| Model | Capacity | Max solar input | Solar watts per kWh of battery | Peak sun hours to refill from empty at max input | Expansion | Amazon options |
|---|---|---|---|---|---|---|
| BLUETTI AC180 | 1,152 Wh | 500 W (12–60 V, 10 A) | 434 W | 1,152 ÷ 375 ≈ 3.1 h | Up to 4,224 Wh total | Check on Amazon.comCheck on Amazon.ca |
| Anker SOLIX C1000 Gen 2 | 1,024 Wh | 600 W (60 V max) | 586 W | 1,024 ÷ 450 ≈ 2.3 h | Not supported | Check on Amazon.ca |
| EcoFlow DELTA 3 Plus | 1,024 Wh | 1,000 W (2 × 500 W, 11–60 V) | 977 W | 1,024 ÷ 750 ≈ 1.4 h | Up to 5 kWh | Check on Amazon.com |
| Anker SOLIX F3800 | 3,840 Wh | 2,400 W (2 × 1,200 W, 11–60 V) | 625 W | 3,840 ÷ 1,800 ≈ 2.1 h | Up to 26,880 Wh with expansion batteries | Check on Amazon.comCheck on Amazon.ca |
Sources: AC180 capacity, solar input and expansion from BLUETTI's AC180 page. C1000 Gen 2 figures and the no-expansion note from Anker's C1000 Gen 2 page. DELTA 3 Plus solar input from the DELTA 3 Plus user manual and expansion from the DELTA 3 series page. F3800 solar input from Anker's F3800 page and expansion from the F3800 user guide.
How each station handles the example
Applying the example load of 2,325 Wh of battery energy per day:
- AC180: solar-limited and battery-limited. Even at 5 sun hours it needs 620 W, above its 500 W input. Its 1,152 Wh holds about half a day (1,152 ÷ 2,325 = 0.50).
- C1000 Gen 2: just short on solar at 5 sun hours (600 W vs 620 W) and cannot be expanded. Not a fit for this load.
- DELTA 3 Plus: enough solar input at 5 sun hours (1,000 W vs 620 W) but not at 3 (1,033 W). On its own, 1,024 Wh is battery-limited; with two 1 kWh extra batteries it reaches about 3 kWh, which covers one day without sun.
- F3800: balanced for this load. 2,400 W covers the 1,550 W needed at 2 sun hours, and 3,840 Wh covers about 1.65 days without sun (3,840 ÷ 2,325).
Solar-limited or battery-limited? How to tell
Once you have the two numbers, the shortfall tells you what to change:
- Solar-limited: the station's maximum solar input is below the watts from step 4. More battery does not help, because each day you can only put back what the input allows. You need a station with a higher solar input, a lighter load, or another charging source.
- Battery-limited: the input is enough, but the capacity is below the figure from step 3. You get through sunny days but run out during cloudy spells or overnight. Add expansion batteries, if the model supports them, or choose a larger station.
- Balanced: both pass for your weakest month, with some margin.
A quick screen is the "solar watts per kWh" column. At 0.75 delivery and 4 peak sun hours, a station needs about 1,000 ÷ (0.75 × 4) = 333 W of solar input per kWh of battery to refill a fully used battery in one day. All four stations in the table clear that ratio, so for these models the solar input is unlikely to be the bottleneck for refilling the battery itself. It becomes the bottleneck when the daily load is larger than the battery, as in the example.
Recovering after cloudy days
Battery capacity buys you days without sun. Afterwards, the solar input has to cover that day's load and refill the battery. With the example load on the F3800, after one cloudy day: 2,325 + 2,325 = 4,650 Wh to put back. At 5 sun hours that needs 4,650 ÷ (0.75 × 5) = 1,240 W, inside its 2,400 W input. At 2 sun hours it would need 3,100 W, so recovery would take more than one day.
If winter sun is your weakest case, also check the station's charging temperature limits. Our Canadian winter outage guide lists them by model.
The input limits behind the watts
A station only reaches its maximum solar watts if the panels deliver enough power within its voltage and current window. The AC180 accepts 12–60 V at up to 10 A; the DELTA 3 Plus 11–60 V at up to 15 A per port; the C1000 Gen 2 has two bands (11–28 V at 8.2 A and 29–60 V at 14.5 A); and the F3800's current limit depends on its firmware version. Our panel guide works through these limits and the cold-weather voltage check with example panels. Never connect an array whose cold-weather open-circuit voltage exceeds the station's stated maximum.
Which station for which balance
- Small daily loads, steady sun: BLUETTI AC180. Best for loads under about 1,275 Wh per day at the outlets: 500 W × 0.75 × 4 sun hours × 0.85. It has the most capacity of the 1 kWh-class models here and can expand to 4,224 Wh.
- Most solar input for its size: EcoFlow DELTA 3 Plus. 1,000 W of input on a 1,024 Wh battery, expandable to 5 kWh. A good fit when you have the panels and want to add battery over time.
- Larger loads and winter sun: Anker SOLIX F3800. 2,400 W of input and 3,840 Wh, with expansion. It is heavy (Anker lists 132.3 lb), so it suits a fixed setup.
The C1000 Gen 2 is a compact 1 kWh station with 600 W of input and no expansion, so it suits smaller loads where the battery alone is enough. We link it only on amazon.ca; the amazon.com listing on file is a two-pack.
Frequently asked questions
Should I buy a bigger battery or more solar?
Work out both numbers first. If the station's solar input is below the watts your daily load needs, more battery only delays running out. If the input is enough but you run out overnight or on cloudy days, add battery.
Can I connect more panel watts than the station's maximum solar input?
The station will not take more than its rated input. Whether a larger array is allowed, and how much it helps on dull days, depends on the manufacturer. Check the manual, and never exceed the voltage limit.
How long does it take to recharge from solar?
Peak sun hours to full = capacity ÷ (array watts × 0.75), as long as the array is within the station's input limit. A 1,024 Wh station on a 600 W array: 1,024 ÷ 450 ≈ 2.3 peak sun hours, with nothing else running.
Why 0.75 for solar and 0.85 for the battery?
They cover different losses. The 0.75 covers panel heat, angle, dust, wiring and charging; the 0.85 covers the inverter and a reserve on the way out. Both are planning assumptions, not measured values; use lower numbers if your setup is less than ideal.
How we research: we compare manufacturer specifications, check published sources and run the numbers. We do not claim hands-on testing. Our methodology.
