Power Station Size Calculator
Add the devices you want to run. The calculator works out your daily energy use, the battery capacity you need, and the inverter rating that can handle startup surges.
How this calculator works
Sizing a battery comes down to two separate questions, and you need a "yes" to both:
- Does it hold enough energy? That is the capacity in watt-hours (Wh). It decides how long you can run things.
- Can it deliver enough power at once? That is the inverter's continuous and surge rating in watts (W). It decides what you can run at the same time.
A 2,000 Wh power station with a 300 W inverter has plenty of stored energy but will shut off the moment you plug in a coffee maker. A 3,000 W inverter attached to a 500 Wh battery can start the coffee maker but drains in about 20 minutes. If you are new to the terms, our guide to watts vs. watt-hours explains the difference.
The formulas
Capacity = Daily Wh × days ÷ usable share × (1 + safety margin)
Continuous W = Σ (watts × quantity) × 1.25
Surge W = Σ running watts + (largest motor's watts × (startup multiplier − 1))
Why "usable share" matters
You never get the full rated capacity out of a battery. When a power station turns its battery's DC power into 120 V AC for a wall-style outlet, the inverter loses some energy as heat, typically 10–15%. Lead-acid batteries also shouldn't be drained much past half if you want them to last, which is why the AGM option uses only 45%. LiFePO4 cells tolerate deep discharge well, so most of their capacity is usable.
Why add a safety margin?
Batteries lose some capacity as they age and in cold weather, and real-world use is rarely as tidy as an estimate. The default 20% margin keeps you from buying a unit that is exactly enough on paper and slightly short in practice. If budget is tight, you can lower it, but avoid going to zero.
Worked example: a refrigerator during a power outage
Say you want to keep a fridge, four LED bulbs, two phones and the Wi-Fi router running for one day during an outage. With the default values above:
| Device | Watts | Hours/day | Qty | Wh/day |
|---|---|---|---|---|
| Full-size refrigerator | 150 | 8 (compressor run time) | 1 | 1,200 |
| LED bulbs | 9 | 5 | 4 | 180 |
| Phone charging | 10 | 2 | 2 | 40 |
| Wi-Fi router | 12 | 24 | 1 | 288 |
| Total | 1,708 Wh | |||
1,708 Wh ÷ 0.85 × 1.2 ≈ 2,410 Wh. In practice, that points to a power station in the 2,000–3,000 Wh class, or a smaller unit paired with solar panels to top it up during the day. The fridge dominates the total, so it is worth measuring yours. See how long a power station runs a refrigerator for how to find its real daily use.
Frequently asked questions
Why does the fridge say 8 hours instead of 24?
A refrigerator's compressor doesn't run constantly. It switches on and off to hold the temperature. Eight hours of running at about 150 W is a reasonable stand-in for a typical modern fridge using roughly 1.2 kWh per day. Your fridge's yellow EnergyGuide label (in the US) lists its estimated yearly kWh. Divide that by 365 for a daily figure.
What if I won't run everything at the same time?
The continuous-watts result assumes everything is on at once, which is the safe assumption. If you know you'll never run the microwave and the space heater together, you can size the inverter for the largest realistic combination instead. The capacity result isn't affected, because it's based on total daily use.
Does the calculator work for DIY battery banks?
Yes. Choose the LiFePO4 or lead-acid option. The result is the rated capacity of the battery bank in Wh. To convert to amp-hours, divide by your system voltage (for example, 2,400 Wh ÷ 12.8 V ≈ 188 Ah).
Can I run a space heater from a power station?
Technically, a large unit can, but resistive heaters use so much energy (1,500 W for one hour is 1.5 kWh) that they drain even big batteries quickly. For staying warm on battery power, an electric blanket at around 50–100 W is usually far more practical.