Seasonal energy budgeting

An off-grid solar system sized for the annual average will fail you every January. The sun that delivers 6 peak hours per day in July drops to fewer than 3 in December across most of the northern United States — a production cut of 40–60% that arrives exactly when heating loads climb and days get shorter. Seasonal energy budgeting is the discipline of matching your consumption to what your system can realistically produce month by month, rather than treating the year as a flat average.

This page covers the full annual cycle: quantifying your winter production floor, scheduling generator runtime to bridge deficit months, making productive use of summer surplus, protecting your battery bank through temperature swings, and running the monthly audits that reveal problems before they become emergencies.

The production curve

Before you can budget, you need to understand the shape of your production year. Peak sun hours (PSH) — the number of hours per day equivalent to full-intensity (1,000 W/m²) irradiance — vary dramatically by latitude and month.

Typical PSH ranges by latitude and season for a south-facing array at optimal tilt:

Region	Winter (Dec/Jan)	Spring/Fall	Summer (Jun/Jul)	Ratio (winter:summer)
Pacific Northwest (47–49°N)	1.5–2.5 PSH	3.5–4.5 PSH	5.5–6.5 PSH	1:3
Upper Midwest (44–47°N)	2.5–3.5 PSH	4.0–5.0 PSH	5.5–6.5 PSH	1:2.2
Mid-Atlantic / Great Lakes (40–44°N)	2.5–3.5 PSH	4.5–5.5 PSH	5.5–6.5 PSH	1:2
Upper South / Plains (35–40°N)	3.5–4.5 PSH	5.0–5.5 PSH	6.0–7.0 PSH	1:1.8
Deep South / Southwest (30–35°N)	4.5–5.5 PSH	5.5–6.5 PSH	6.5–7.5 PSH	1:1.5

Your design month is the worst month, not the annual average. If your array produces 3 PSH in December and you sized for the 4.5 PSH annual average, your system is 33% undersized for winter — meaning your battery bank runs short or your generator runs more than you planned.

To find your site's actual PSH values, use the National Renewable Energy Laboratory (NREL) PVWatts calculator or the NASA POWER database, both of which provide monthly irradiance data for any US location. Enter your coordinates, array tilt angle (a tilt equal to your latitude optimizes annual production; increasing tilt by 10–15 degrees optimizes winter production at the cost of summer yield), and read the December and January columns.

Field note

Tracking snow accumulation on your panels matters more than most installers admit. In northern climates, a 2-inch (5 cm) snowfall can cut output to near zero for multiple days. Fixed arrays below 35 degrees of tilt shed snow slowly; steeper angles shed it faster. If winter production is your binding constraint, consider adjustable mount brackets that let you increase tilt from your summer angle (latitude) to your winter angle (latitude + 15°) in October and back in April. A one-time affordable hardware investment pays back quickly in reduced generator fuel.

Winter production shortfall planning

The winter shortfall is the gap between what your system produces and what your household consumes during the coldest, darkest months. Quantifying it before winter arrives tells you exactly how much generator runtime to budget.

Step 1: Calculate winter daily production.

Winter daily production (Wh) = Array wattage × Worst-month PSH × System derate (0.75–0.80)

For a 4,000W array at 2.8 PSH with a 0.77 system derate factor:

4,000W × 2.8 × 0.77 = 8,624 Wh/day (8.6 kWh/day)

Step 2: Calculate winter daily consumption.

Winter consumption is almost always higher than summer. Add loads that run only in cold months: - Furnace blower motor: 300–600W running 6–12 hours/day = 1.8–7.2 kWh/day - Electric water heating supplementation: 1.5–3 kWh/day - More lighting hours due to shorter days: 0.5–1 kWh/day increase - Increased refrigerator cycling due to warm kitchen air (counterintuitively, an uninsulated kitchen can force more compressor cycles)

If your summer daily load was 12 kWh and winter adds 5 kWh, your winter load is 17 kWh/day.

Step 3: Calculate the daily shortfall.

Daily shortfall (Wh) = Winter consumption − Winter production 17,000 − 8,624 = 8,376 Wh/day shortfall

Step 4: Determine generator runtime needed.

Your inverter-charger charges the battery bank while simultaneously powering loads. A typical configuration runs the generator for 2–3 hour sessions, which both tops up the battery and reduces the next day's starting deficit.

For an inverter-charger rated at 100A charging at 48V (4,800W) plus 1,500W of simultaneous loads, the generator must produce 6,300W during a charge session. At 2.5 hours per session:

2.5 h × 4,800W = 12,000 Wh = 12 kWh of battery charging per session

To cover an 8.4 kWh daily shortfall plus typical inverter and charging losses (15–20%):

8,376 Wh ÷ 0.82 efficiency = ~10,200 Wh from generator per day 10,200 Wh ÷ 4,800W charging rate = ~2.1 hours of generator time per day

A 7,000W generator burning 0.87 gallons (3.3 L) of propane per hour at that load consumes about 1.8 gallons (6.8 L) per day in the deepest winter months. Over a four-month deficit season (November–February), that is approximately 216 gallons (817 L) of propane. Plan your fuel storage accordingly — a 500-gallon (1,893 L) propane tank provides comfortable headroom for this scenario. See fuel storage for tank sizing, delivery scheduling, and the cold-weather propane pressure considerations that can cause generator stumble at temperatures below 0°F (-18°C).

Generator runtime is not infinitely flexible

A generator run at 30% load burns nearly as much fuel as one run at 70% load, while producing far less useful power. Never run a generator just to "top off" a battery that's at 80% state of charge — wait until the bank reaches your configured low-SoC threshold (typically 30–40% for LiFePO4, 50% for lead-acid) before starting a full charge session. Short, frequent generator runs waste fuel, accumulate more engine hours, and leave wet stack deposits from incomplete combustion.

Summer surplus utilization

Summer is the mirror image of winter: production often exceeds consumption, and batteries reach full charge by mid-morning. A system sized correctly for winter will produce 2–3 times more than it needs on a clear July day. Wasted production is harmless but represents free energy you could put to work.

Absorb first, then redirect. The priority sequence for surplus production:

Keep batteries full — Allow the daily cycle to complete, reaching 100% state of charge (SoC) consistently. Full charge allows the battery management system (BMS) to balance individual cell voltages and is the normal operating state.
Divert to water heating — A resistive diversion load controller (sometimes called an immersion controller or dump load relay) monitors battery SoC and automatically routes surplus power to an electric water heater element when the bank is full. A standard 4,500W or 5,500W water heater element consumes large amounts of surplus without any manual intervention. Hot water stored in a 40-gallon (151 L) or 80-gallon (303 L) insulated tank functions as thermal storage — effectively a free battery for heat energy.
Run high-draw discretionary loads — Schedule electric clothes dryers, power tools, water pumping to storage tanks, dehydrator runs, and chest freezer defrost cycles during the peak production window (typically 10 a.m.–3 p.m. solar time). These loads run on solar energy that would otherwise be clipped or wasted.
Equalization for flooded lead-acid — Summer is the best time to run equalization cycles on flooded lead-acid batteries. Equalization requires 15.5–16V (at 12V nominal) for several hours — a controlled overcharge that breaks up sulfation and brings all cells to equal voltage. The plentiful solar production of summer means the generator does not need to run for equalization; the panels supply the elevated voltage while the MPPT (maximum power point tracking) charge controller is set to equalization mode. Schedule this quarterly during summer months. LiFePO4 does not require equalization but benefits from full-charge balancing sessions, which summer production facilitates automatically.
Extend runtime on non-time-critical (NTC) loads — Air conditioning is a realistic summer load for systems with adequate surplus. A window unit drawing 800–1,200W running 6 hours during peak production adds 4.8–7.2 kWh of consumption — often well within the summer surplus budget.

Field note

A diversion load water heater typically pays back the controller cost (inexpensive to affordable) within a single summer season in propane or electricity savings. The Victron Energy BMV battery monitors can trigger a relay at a configurable SoC threshold; combine this with an immersion heater element and a standard tank water heater, and you have an automatic surplus absorption system that requires no daily management.

Load-shedding priority list

When production cannot meet demand — in deep winter, during extended cloudy periods, or after several days of high load — you need a predetermined list of what gets cut and in what order. Making this decision in advance, under no pressure, produces better choices than making it in the dark.

Tier 1 — Critical loads (never shed): - Medical devices (CPAP, oxygen concentrator, insulin refrigeration) - Refrigerator and freezer - Well pump and pressure system - LED lighting (minimum safe illumination) - Communications (phone charging, radio, router) - Heating system controls and blower motor

Tier 2 — Essential loads (shed under sustained deficit): - Water heating (switch to heating water on wood stove or solar thermal) - Laptop and office equipment - Additional lighting beyond minimum illumination - Network-attached storage, servers, or always-on devices

Tier 3 — Discretionary loads (first to shed): - Clothes dryer - Electric range or oven (switch to propane, wood stove, or outdoor cook) - Air conditioning - Power tools - Entertainment systems and gaming equipment - Supplemental heating via electric space heaters (most energy-intensive load category)

Print this list and post it near your inverter or battery monitor. Add the estimated daily watt-hours for each Tier 2 and 3 item so you can immediately see how much headroom shedding a given load creates. Cross-reference your battery state-of-charge display against this list daily during deficit months — when SoC drops to 60%, begin Tier 3 shedding; when it drops to 40%, evaluate Tier 2. Do not let the bank reach 20% SoC waiting to see if the weather improves.

Battery bank health in cold weather

Cold weather attacks your battery bank from two directions simultaneously: it reduces available capacity, and it makes charging more complex.

Capacity reduction in cold is chemistry-specific: - LiFePO4 at 32°F (0°C): 10–15% capacity reduction - LiFePO4 at 14°F (-10°C): 20–25% capacity reduction - LiFePO4 at -4°F (-20°C): up to 40% capacity reduction - Lead-acid at 32°F (0°C): 20–30% capacity reduction - Lead-acid at 0°F (-18°C): up to 50% capacity reduction

These are not permanent losses — capacity returns as the battery warms — but they mean your autonomy calculation is wrong in winter if you sized it for room temperature performance. A 20 kWh LiFePO4 bank stored in an uninsulated outbuilding at 14°F (-10°C) delivers about 15–16 kWh of usable energy: your four-day autonomy becomes three days.

The charging prohibition for LiFePO4 is absolute: do not charge below 32°F (0°C). Charging below freezing causes lithium plating on the anode — metallic lithium deposits that can puncture the cell separator and permanently degrade capacity. Most quality rack batteries include a BMS low-temperature charging cutoff, but this is your last defense, not your first. See batteries for the full low-temperature charging warning and BMS verification steps.

Protecting your bank through winter:

Insulate the battery enclosure — Move batteries into the home's thermal envelope (basement, utility room, or insulated mechanical shed) where temperatures stay above 40°F (4°C). A battery in a heated space is also conveniently near your inverter, which reduces cable run length.
Add a battery heating pad — 12V or 24V heating pads designed for LiFePO4 batteries maintain minimum temperature with modest power draw (20–50W). Some rack batteries include integrated heating pads; check the spec sheet before assuming your batteries have this protection.
Accept reduced capacity in cold-storage situations — If your batteries must stay in an unheated space, recalculate your winter autonomy using the derated capacity at the coldest expected temperature, not the nameplate capacity. Build this into your load-shedding trigger points.
Allow batteries to warm before charging — After a cold night, the battery casing temperature may be below 32°F (0°C) even when the ambient temperature has risen above freezing. Wait until the BMS temperature sensor reads above 35°F (2°C) before initiating a charge from the generator.

Adapting consumption to the production curve

The key behavioral shift in seasonal energy management is aligning when you use power with when your system produces it — not just how much you use total. This is called load shifting, and in summer it is almost automatic; in winter it requires discipline.

Shift high-draw tasks into production windows. On a winter day with 4 PSH, your system may produce power from 9 a.m. to 3 p.m. — a 6-hour window. Running the clothes washer, water pump for tank filling, and laptop during that window draws directly from panels without touching the battery bank. Running the same loads at 10 p.m. draws entirely from batteries.

Batch cooking and food preservation. Electric cooking (Instant Pot, slow cooker) run during the solar window is essentially free. Moving dinner preparation to midday and using stored heat or a wood stove for evening meals is a significant behavioral adaptation that extends winter battery autonomy noticeably.

Defer non-time-sensitive loads. Clothes washing, vacuuming, workshop tool use — all of these have flexible timing. In winter, the rule is simple: if the sun is out and batteries are above 70% SoC, run discretionary loads. If the sun is down and SoC is below 60%, defer to tomorrow.

For a complete picture of how load timing interacts with system efficiency, see energy efficiency, which covers the DC vs. AC load strategy that reduces inverter losses, and whole-home off-grid which addresses load circuit design for whole-house systems.

Monthly energy audit template

A monthly audit takes less than 30 minutes and tells you whether your system is operating as designed. Do this on the first of each month using your monitoring system's historical data.

Data to collect:

Metric	Source	Record
Total kWh produced by solar	Charge controller or monitoring platform (e.g., Victron VRM)	kWh
Total kWh consumed by loads	Inverter/monitoring log	kWh
Generator runtime hours	Hour meter or monitoring log	Hours
Generator fuel consumed	Tank level difference	Gallons/liters
Battery ending SoC on worst day	Battery monitor history	%
Battery ending SoC on best day	Battery monitor history	%
Lowest overnight temperature at battery enclosure	Temperature sensor or local records	°F/°C

Calculations:

Solar self-sufficiency ratio = Total kWh produced ÷ Total kWh consumed. Target 90%+ in summer; accept 40–60% in deep winter with generator filling the gap.
Generator fuel efficiency = Generator fuel consumed ÷ Generator runtime hours. Compare to your generator's rated consumption at average load. A significant increase indicates engine wear, running at inefficient part-load, or more total runtime than planned.
Battery depth check = Review worst-day ending SoC. If the worst day ended below your 30% threshold, your winter plan has a gap — either consumption is higher than modeled, production is lower, or generator runtime was insufficient.
Year-over-year comparison = Compare this month's production to the same month last year. A decline in production without a change in panel count or tilt suggests soiling, shading from new vegetation, or panel degradation (typical LiFePO4 degradation is less than 2% per year at 3,000 cycles).

Seasonal targets by month:

Month	Target SoC floor (end of day)	Expected generator use
January	30–40%	High — 1.5–3 hrs/day
February	35–45%	Moderate-high — 1–2 hrs/day
March	50–60%	Low — 0–1 hr/day
April–September	70–90%	Minimal to none
October	55–65%	Low — 0–1 hr/day
November	40–50%	Moderate — 0.5–1.5 hrs/day
December	30–40%	High — 1.5–3 hrs/day

These targets assume a system sized for winter self-sufficiency with a 48-hour battery autonomy at critical loads. Adjust based on your actual PSH data and load profile.

Seasonal energy budgeting checklist

Your system's seasonal performance determines how comfortable and independent your off-grid life actually is. The winter planning work you do in September prevents the scramble in January. For whole-home load circuit design that supports load shedding without manual breaker switching, see whole-home off-grid. For generator integration, sizing, and maintenance scheduling that supports reliable winter backup, see generators.