Planning and Sizing Your Off-Grid System

Designing an off-grid system starts with understanding how much energy you need and then figuring out how to supply that reliably. This process can be broken down into a few steps. Essentially, you’ll perform an energy audit, calculate your daily energy consumption, and then size your battery bank and solar array to meet that demand (including a buffer for bad weather). Let’s go through it:

Step 1: Calculate Your Daily Energy Use (Energy Audit)

Begin by listing all the electrical devices and appliances you plan to power, along with their power ratings (watts) and an estimate of how many hours per day (or per week) you will use them. This is called an energy audit. It’s a critical first step – you can’t design the system properly until you know the total Watt-hours you need per day. Include everything: lights, refrigerator, water pump, phone chargers, laptop, TV, tools, well pump, etc. Don’t forget intermittent or seasonal loads (for example, a fan in summer, or a satellite internet modem that runs 24/7).

For each appliance, multiply the power (W) by the daily hours of use to get Watt-hours (Wh) per day. Sum all these up to get your total daily energy requirement in Wh. (1000 Wh = 1 kWh, which is the unit electric bills use.) You may find it useful to use an online load calculator tool for this step – many off-grid websites provide calculators where you can input common appliances. For example, Unbound Solar’s Off-Grid Calculator has a table of common household appliances so you can fill in your usage and it will tally the watt-hours. The key is to be thorough and honest about your usage. It’s better to slightly overestimate than underestimate, to ensure your system can handle real-life usage.

Sample Energy Audit Table (for a small cabin):

In the above example, the cabin needs about 2 kWh per day. Your situation might be very different – a larger home with a well pump, full-size fridge, freezer, and power tools could be 5–10+ kWh/day. An ultra-efficient tiny house might be under 1 kWh/day. The energy audit gives us a target to design for.

Tip: Consider ways to reduce your energy needs as you do this audit. Energy efficiency is effectively the cheapest “source” of power – every watt you save is one you don’t have to generate. Switching to LED lighting, choosing efficient appliances (look for EnergyStar ratings), and eliminating phantom loads can significantly cut your daily watt-hours. It’s often cheaper to invest in efficiency (or change usage habits) than to buy more panels and batteries to supply wasteful loads.

Step 2: Size the Battery Bank (Storage Capacity)

Once you know your daily usage, you can size your battery bank to supply that amount of energy (plus some reserve). Battery capacity is often specified in ampere-hours (Ah) at a certain voltage, or in watt-hours (Wh). It’s easiest to work in Wh or kWh for system sizing.

First, decide how many “days of autonomy” you want – this means how many days can the batteries supply your needs without any solar input. If you’re in a very sunny region and have a generator backup, you might size for only 1-2 days of autonomy. If you want to be able to ride through a week of cloudy weather, you might design for 3-5 days of autonomy. Let’s say we want 2 days autonomy for our example cabin: 2 kWh/day × 2 days = 4 kWh of usable energy needed in the batteries.

Next, account for the fact that you shouldn’t discharge batteries 100%. As mentioned, lead-acid batteries are usually drawn down no more than ~50% to greatly extend their life. Lithium batteries can be drawn down 80% or more, but it’s often recommended to use, say, 80% to be conservative. This limit is called the Depth of Discharge (DOD) – the fraction of the battery’s total capacity that can be used. So, if using lead-acid with 50% max DOD, and we need 4 kWh usable, we’d actually need 8 kWh of total battery capacity (because only half is usable). If using lithium with 80% DOD, 4 kWh usable would require about 5 kWh total capacity.

Also consider battery efficiency – there are some losses when charging and discharging. Lead-acid might waste ~15% of energy in heat, etc., while lithium is more efficient (~95% efficient). For a rough design calculation, you might increase required capacity by ~10% to account for these losses (or factor it into solar generation instead). Additionally, batteries last longer if they aren’t routinely pushed to their DOD limit, so building in extra capacity is beneficial.

Finally, ensure the battery bank voltage and configuration suits your needs. Off-grid battery banks are typically 12V, 24V, or 48V systems. Higher voltages (24 or 48V) are used in larger systems to keep currents lower (which improves efficiency and allows thinner cables). Many modern off-grid inverters are designed for 48V battery input for this reason. Small cabin or RV systems might be 12V or 24V if running smaller inverters or DC loads. You’ll choose battery units (e.g. 6V or 12V batteries) and connect them in series/parallel to achieve the desired bank voltage and capacity.

For our 2 kWh/day cabin example: if using lithium batteries, a single 24V 200Ah LiFePO₄ battery (~24V × 200Ah = 4.8 kWh) could be sufficient (providing ~4 kWh usable at ~80% DOD). If using lead-acid, we might need four 6V 225Ah golf-cart batteries in series for 24V (which gives ~24V × 225Ah = 5.4 kWh, but only ~2.7 kWh usable at 50% DOD, so actually we’d need 8 of those batteries for ~10.8 kWh total to get ~5.4 kWh usable). As you can see, lithium’s greater usable fraction can reduce how many batteries you need, though each battery is more costly.

Step 3: Size the Solar Array (Generation Capacity)

Now determine how many solar panels (and what wattage) you need to replenish that battery energy and power your loads, especially in the worst-case scenario (typically winter). You’ll consider two main factors here: daily energy generation and power output at any given time.

To produce the required daily energy, you need to know the solar resource in your location, often expressed in “peak sun hours per day.” A peak sun hour is essentially one hour of full sun equivalent (1 kW/m² of solar irradiance). Even if the sun is up for 10 hours, the intensity varies, so it might equate to, say, 5 peak sun hours of strong sun. This figure depends on your latitude, climate, season, and panel orientation. You can find peak sun hour data for your area from solar maps or by using tools like NREL’s PVWatts Calculator, which estimates solar energy production based on location and system specifics. For example, let’s say your location averages about 4 peak sun hours per day (this might be typical in a temperate climate).

With 4 sun hours/day, a 1 kW (1000 W) solar array would generate roughly 4 kWh per day (minus some losses). If our cabin needs ~2 kWh/day, in theory ~500 W of solar could suffice (0.5 kW × 4 h = 2 kWh). However, you’ll want to size larger for several reasons: charging inefficiencies, cloudy days, winter shorter days, and to cover running loads while charging batteries. A common rule of thumb is to size solar about 1.3 to 1.5 times the average load needs for a reliable off-grid system (more if you have long cloudy seasons).

So, we might choose about 800 W of solar for the 2 kWh/day need. In practice, that could be e.g. four 200 W panels, or six 135 W panels, etc. This array, in good sun, could produce ~3.2 kWh/day at 4 sun hours, giving some cushion. In summer it will produce more; in winter, maybe a bit less. If we wanted 100% coverage even in winter, we’d have to use the winter sun hours (maybe only 2–3 peak hours in some places) which could double the required wattage. Many people add extra panels or an adjustable tilt to maximize winter production if the goal is year-round self-sufficiency without a generator.

Another consideration: panel placement and orientation. Panels produce their rated output under ideal conditions (full sun at the proper angle, cool temperatures). Make sure you have adequate space oriented south (in N. hemisphere) or north (S. hemisphere) for the array. Avoid shading from trees, etc., as even a little shade can greatly reduce output. If space is limited, opt for higher efficiency panels. Also decide if panels will mount on a roof or ground rack – ground mounting can allow easier cleaning and better cooling (panels are a bit more efficient when cooler) and seasonal angle adjustment, but require yard space.

For a rough check: Take your battery capacity in kWh and ensure your solar can recharge a good chunk of it on a typical day plus supply that day’s loads. If you have 5 kWh usable battery, you’d want to be able to put at least that much back in on a decent sunny day after running your loads. If not, your batteries may never fully recharge in winter, which can shorten their life.

Step 4: Size the Inverter (Power Electronics)

Lastly, ensure your inverter (and other electronics) are sized to handle your peak power needs. Inverter size is measured in watts (W) of AC output. Add up the maximum watts of all appliances you might run simultaneously. Also account for surge – many devices like pumps, fridge compressors, and power tools have a higher startup surge demand (often 2-3 times their running wattage) for a second or two. The inverter must be able to supply that surge.

For example, if you could have a 700 W microwave, a 100 W TV, and some lights (50 W) on together, that’s ~850 W continuous. But if the refrigerator (200 W running, 600 W start surge) kicks on at the same time as the well pump (300 W running, maybe 900 W start), you could momentarily need 1.5–2 kW. In that case, you’d choose an inverter of at least 2000 W (2 kW) continuous, with surge rating say 3000+ W. It’s generally wise to overspec the inverter a bit so it’s not running at absolute max all the time. Common off-grid inverter sizes for small cabins are 2000–4000 W. Larger homes with multiple AC units, electric cooking, etc., may use 6000–10,000 W (often achieved by stacking multiple inverter units).

Also ensure the inverter’s DC input matches your battery bank voltage (12, 24, or 48 V) and that it produces the AC form you need (120 V single phase for most US homes, or split-phase 120/240 V if you have 240 V well pumps or appliances, etc.). Many off-grid inverters now provide 120/240V split-phase output from one unit or a pair of units.

Finally, make sure your charge controller can handle the solar array’s voltage and current. MPPT charge controllers come in various amp ratings (e.g. 40A, 60A, 80A) – choose one that exceeds the max current your panels will produce (for example, an 80A unit for a 800 W / 24V system, since 800W/24V ≈ 33A). Also ensure the controller’s PV input voltage window is compatible with your panel configuration (panels in series have higher voltage). Most MPPT controllers accept 100-150V input, which is fine for strings of 2-3 panels.

This sizing process can be complex, but thankfully there are online tools and calculators to help. As mentioned, NREL’s PVWatts is great for estimating monthly/yearly solar energy output for a given system size in your location. There are also comprehensive off-grid system design calculators (for example, Off-Grid Wizard or solar vendor sites) that let you input your loads, desired autonomy days, sun hours, etc., and they will suggest battery AH and solar wattage needed. These tools consider factors like inverter efficiency and recommended depth of discharge. It’s wise to consult such calculators or formulas when planning, and if possible, get a professional opinion on your design, especially for larger systems.

At the end of the day, proper sizing is crucial. Under-size your system, and you’ll be running out of power or stressing components. Over-size it, and you might spend more than necessary – but between the two, it’s safer (though costlier) to err on the larger side for critical components. Many off-grid beginners start small and then expand as they learn their actual usage patterns. For instance, you might begin with 2 kW of panels and add 2 kW more later when you realize you want extra margin for winter.