What size solar generator do I need? Work it out in two sums
Sizing a solar generator is two sums, not a guess. Multiply each appliance's watts by the hours you will run it to get watt hours, add them up, and that total has to fit inside the unit's usable capacity. Then check one more number: your single biggest load has to sit under the unit's continuous output rating, or it will not start at all.
The calculator below does both sums. It deliberately asks for your watt figures rather than shipping a table of generic ones, because the number printed on your appliance is the only one that describes your appliance.
If your label gives amps rather than watts, convert it first with our watts to amps converter, which also shows what an AC load pulls from a 12 volt battery once inverter losses are counted.
The four rows are pre-filled with the loads we model in our Grid Doctor 3300 review so you have a working example to edit, not a claim about your kit.
The first of them, the refrigerator, is the load most people are really asking about, and we work it through on its own in solar generator for a refrigerator. The furnace blower row has its own guide too: portable power station for a gas furnace.
The calculator
Edit the watts and hours, delete what you do not need, add your own rows. Everything runs in your browser, nothing is sent anywhere and nothing is stored.
| Appliance | Watts | Hours per day | Watt hours | Remove |
|---|---|---|---|---|
| Total per day | 0 Wh | |||
How to read the result
Two different numbers decide whether a unit works for you, and a unit can pass one and fail the other.
- Watt hours decide how long. Your daily total against the unit's usable capacity tells you whether it lasts the night, the day or the weekend.
- Watts decide whether at all. Your single biggest appliance has to sit under the continuous output rating. The Grid Doctor 3300 and the Power Tower share a 3,300W continuous, 6,600W surge inverter, which is why an 1,800W induction burner starts on them and a small unit refuses it.
- Startup spikes are separate. Motors and compressors pull a brief surge well above their running watts. If your list is full of fridges, pumps and power tools, leave room against the surge rating rather than the continuous one.
The runtimes shown are arithmetic, not measurements. They assume the conservative end of the 10 to 15 percent inverter loss we use across this site, they assume nothing else is drawing from the unit, and they ignore temperature. Cold weather and near-empty batteries both cost you more than the maths suggests. Treat the output as a shortlist, then confirm against the spec sheet of the unit you actually buy.
The three capacities on this site
The calculator compares your total against the three Grid Doctor units we cover, using the capacities published on our solar generator hub, where the current prices and the full picks live.
| Unit | Rated capacity | Usable at 85 percent | Realistic role |
|---|---|---|---|
| Grid Doctor 300 | 320Wh | 272Wh | Phones, lights, a router, a CPAP overnight. Not a fridge unit. |
| Grid Doctor 3300 | 2,048Wh | 1,740Wh | Fridge plus essentials through a night or two. |
| 3300 Power Tower | 16,384Wh | 13,926Wh | Essentials for days rather than hours. |
One honest caveat on the smallest unit: we publish a capacity for the Grid Doctor 300 but not a continuous output rating, so the calculator does not run the inverter check against it. If your biggest load is anything with a heating element or a motor, read that unit's own label before you assume it will start.
Where to go from here
If the sums say a battery fits your week, our best solar generators roundup has the picks and the current prices. If the total looked frightening, the honest next question is whether a battery is the right tool at all, and our power station vs generator comparison answers it. If you are still deciding whether the category is worth the money, start with are solar generators worth it.
Common questions
What size solar generator do I need?
Add the watt draw of everything you will run, multiply each by the hours you will run it, and total the watt hours. That total is your daily energy need. The unit you buy needs that much usable capacity plus reserve, and its inverter must be rated above your single biggest load.
Sizing from the labels on your own appliances beats any generic table, because two refrigerators of the same size can draw very different power.
Where do I find the wattage of an appliance?
It is printed on the appliance itself: a nameplate or sticker on the back, the bottom or inside the door, and it is repeated in the manual. Look for a figure in watts, or for volts and amps, which you multiply together to get watts.
A plug-in energy meter measures it directly and is the most accurate option of all, because it captures what the appliance really draws rather than its maximum rating.
Why do I not get the full rated capacity of the battery?
The inverter that turns stored DC into household AC loses some energy as heat. On this site we work with a 10 to 15 percent loss, so a 2,048Wh unit realistically delivers somewhere around 1,750 to 1,850Wh to an AC appliance. This calculator uses the conservative end of that band. Running at very low or very high load, and running in the cold, both cost you more.
What is the difference between watts and watt hours?
Watts measure how hard a device pulls at any moment; watt hours measure how much energy it uses over time. The inverter rating has to cover your watts, the battery capacity has to cover your watt hours. This is why a 1,800W induction burner can defeat a large battery in minutes while a 40W CPAP runs on a small one all night.
Does the surge rating matter more than the continuous rating?
Both matter, for different moments. Anything with a motor or a compressor, so a fridge, a furnace blower, a pump or a power tool, draws a brief spike far above its running watts when it starts. The continuous rating has to carry the running load, the surge rating has to survive that startup spike.
A unit that passes on continuous watts but fails on surge will simply shut off when the compressor kicks in.